1724 lines
64 KiB
ReStructuredText
1724 lines
64 KiB
ReStructuredText
=======================================================
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Kaleidoscope: Extending the Language: Mutable Variables
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=======================================================
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.. contents::
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:local:
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Chapter 7 Introduction
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======================
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Welcome to Chapter 7 of the "`Implementing a language with
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LLVM <index.html>`_" tutorial. In chapters 1 through 6, we've built a
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very respectable, albeit simple, `functional programming
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language <http://en.wikipedia.org/wiki/Functional_programming>`_. In our
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journey, we learned some parsing techniques, how to build and represent
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an AST, how to build LLVM IR, and how to optimize the resultant code as
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well as JIT compile it.
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While Kaleidoscope is interesting as a functional language, the fact
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that it is functional makes it "too easy" to generate LLVM IR for it. In
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particular, a functional language makes it very easy to build LLVM IR
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directly in `SSA
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form <http://en.wikipedia.org/wiki/Static_single_assignment_form>`_.
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Since LLVM requires that the input code be in SSA form, this is a very
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nice property and it is often unclear to newcomers how to generate code
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for an imperative language with mutable variables.
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The short (and happy) summary of this chapter is that there is no need
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for your front-end to build SSA form: LLVM provides highly tuned and
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well tested support for this, though the way it works is a bit
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unexpected for some.
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Why is this a hard problem?
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===========================
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To understand why mutable variables cause complexities in SSA
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construction, consider this extremely simple C example:
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.. code-block:: c
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int G, H;
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int test(_Bool Condition) {
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int X;
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if (Condition)
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X = G;
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else
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X = H;
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return X;
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}
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In this case, we have the variable "X", whose value depends on the path
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executed in the program. Because there are two different possible values
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for X before the return instruction, a PHI node is inserted to merge the
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two values. The LLVM IR that we want for this example looks like this:
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.. code-block:: llvm
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@G = weak global i32 0 ; type of @G is i32*
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@H = weak global i32 0 ; type of @H is i32*
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define i32 @test(i1 %Condition) {
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entry:
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br i1 %Condition, label %cond_true, label %cond_false
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cond_true:
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%X.0 = load i32* @G
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br label %cond_next
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cond_false:
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%X.1 = load i32* @H
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br label %cond_next
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cond_next:
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%X.2 = phi i32 [ %X.1, %cond_false ], [ %X.0, %cond_true ]
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ret i32 %X.2
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}
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In this example, the loads from the G and H global variables are
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explicit in the LLVM IR, and they live in the then/else branches of the
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if statement (cond\_true/cond\_false). In order to merge the incoming
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values, the X.2 phi node in the cond\_next block selects the right value
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to use based on where control flow is coming from: if control flow comes
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from the cond\_false block, X.2 gets the value of X.1. Alternatively, if
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control flow comes from cond\_true, it gets the value of X.0. The intent
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of this chapter is not to explain the details of SSA form. For more
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information, see one of the many `online
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references <http://en.wikipedia.org/wiki/Static_single_assignment_form>`_.
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The question for this article is "who places the phi nodes when lowering
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assignments to mutable variables?". The issue here is that LLVM
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*requires* that its IR be in SSA form: there is no "non-ssa" mode for
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it. However, SSA construction requires non-trivial algorithms and data
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structures, so it is inconvenient and wasteful for every front-end to
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have to reproduce this logic.
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Memory in LLVM
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==============
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The 'trick' here is that while LLVM does require all register values to
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be in SSA form, it does not require (or permit) memory objects to be in
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SSA form. In the example above, note that the loads from G and H are
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direct accesses to G and H: they are not renamed or versioned. This
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differs from some other compiler systems, which do try to version memory
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objects. In LLVM, instead of encoding dataflow analysis of memory into
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the LLVM IR, it is handled with `Analysis
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Passes <../WritingAnLLVMPass.html>`_ which are computed on demand.
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With this in mind, the high-level idea is that we want to make a stack
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variable (which lives in memory, because it is on the stack) for each
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mutable object in a function. To take advantage of this trick, we need
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to talk about how LLVM represents stack variables.
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In LLVM, all memory accesses are explicit with load/store instructions,
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and it is carefully designed not to have (or need) an "address-of"
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operator. Notice how the type of the @G/@H global variables is actually
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"i32\*" even though the variable is defined as "i32". What this means is
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that @G defines *space* for an i32 in the global data area, but its
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*name* actually refers to the address for that space. Stack variables
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work the same way, except that instead of being declared with global
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variable definitions, they are declared with the `LLVM alloca
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instruction <../LangRef.html#alloca-instruction>`_:
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.. code-block:: llvm
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define i32 @example() {
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entry:
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%X = alloca i32 ; type of %X is i32*.
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...
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%tmp = load i32* %X ; load the stack value %X from the stack.
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%tmp2 = add i32 %tmp, 1 ; increment it
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store i32 %tmp2, i32* %X ; store it back
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...
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This code shows an example of how you can declare and manipulate a stack
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variable in the LLVM IR. Stack memory allocated with the alloca
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instruction is fully general: you can pass the address of the stack slot
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to functions, you can store it in other variables, etc. In our example
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above, we could rewrite the example to use the alloca technique to avoid
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using a PHI node:
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.. code-block:: llvm
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@G = weak global i32 0 ; type of @G is i32*
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@H = weak global i32 0 ; type of @H is i32*
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define i32 @test(i1 %Condition) {
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entry:
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%X = alloca i32 ; type of %X is i32*.
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br i1 %Condition, label %cond_true, label %cond_false
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cond_true:
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%X.0 = load i32* @G
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store i32 %X.0, i32* %X ; Update X
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br label %cond_next
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cond_false:
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%X.1 = load i32* @H
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store i32 %X.1, i32* %X ; Update X
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br label %cond_next
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cond_next:
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%X.2 = load i32* %X ; Read X
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ret i32 %X.2
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}
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With this, we have discovered a way to handle arbitrary mutable
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variables without the need to create Phi nodes at all:
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#. Each mutable variable becomes a stack allocation.
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#. Each read of the variable becomes a load from the stack.
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#. Each update of the variable becomes a store to the stack.
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#. Taking the address of a variable just uses the stack address
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directly.
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While this solution has solved our immediate problem, it introduced
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another one: we have now apparently introduced a lot of stack traffic
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for very simple and common operations, a major performance problem.
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Fortunately for us, the LLVM optimizer has a highly-tuned optimization
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pass named "mem2reg" that handles this case, promoting allocas like this
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into SSA registers, inserting Phi nodes as appropriate. If you run this
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example through the pass, for example, you'll get:
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.. code-block:: bash
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$ llvm-as < example.ll | opt -mem2reg | llvm-dis
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@G = weak global i32 0
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@H = weak global i32 0
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define i32 @test(i1 %Condition) {
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entry:
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br i1 %Condition, label %cond_true, label %cond_false
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cond_true:
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%X.0 = load i32* @G
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br label %cond_next
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cond_false:
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%X.1 = load i32* @H
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br label %cond_next
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cond_next:
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%X.01 = phi i32 [ %X.1, %cond_false ], [ %X.0, %cond_true ]
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ret i32 %X.01
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}
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The mem2reg pass implements the standard "iterated dominance frontier"
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algorithm for constructing SSA form and has a number of optimizations
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that speed up (very common) degenerate cases. The mem2reg optimization
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pass is the answer to dealing with mutable variables, and we highly
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recommend that you depend on it. Note that mem2reg only works on
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variables in certain circumstances:
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#. mem2reg is alloca-driven: it looks for allocas and if it can handle
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them, it promotes them. It does not apply to global variables or heap
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allocations.
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#. mem2reg only looks for alloca instructions in the entry block of the
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function. Being in the entry block guarantees that the alloca is only
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executed once, which makes analysis simpler.
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#. mem2reg only promotes allocas whose uses are direct loads and stores.
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If the address of the stack object is passed to a function, or if any
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funny pointer arithmetic is involved, the alloca will not be
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promoted.
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#. mem2reg only works on allocas of `first
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class <../LangRef.html#first-class-types>`_ values (such as pointers,
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scalars and vectors), and only if the array size of the allocation is
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1 (or missing in the .ll file). mem2reg is not capable of promoting
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structs or arrays to registers. Note that the "sroa" pass is
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more powerful and can promote structs, "unions", and arrays in many
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cases.
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All of these properties are easy to satisfy for most imperative
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languages, and we'll illustrate it below with Kaleidoscope. The final
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question you may be asking is: should I bother with this nonsense for my
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front-end? Wouldn't it be better if I just did SSA construction
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directly, avoiding use of the mem2reg optimization pass? In short, we
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strongly recommend that you use this technique for building SSA form,
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unless there is an extremely good reason not to. Using this technique
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is:
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- Proven and well tested: clang uses this technique
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for local mutable variables. As such, the most common clients of LLVM
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are using this to handle a bulk of their variables. You can be sure
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that bugs are found fast and fixed early.
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- Extremely Fast: mem2reg has a number of special cases that make it
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fast in common cases as well as fully general. For example, it has
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fast-paths for variables that are only used in a single block,
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variables that only have one assignment point, good heuristics to
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avoid insertion of unneeded phi nodes, etc.
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- Needed for debug info generation: `Debug information in
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LLVM <../SourceLevelDebugging.html>`_ relies on having the address of
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the variable exposed so that debug info can be attached to it. This
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technique dovetails very naturally with this style of debug info.
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If nothing else, this makes it much easier to get your front-end up and
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running, and is very simple to implement. Lets extend Kaleidoscope with
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mutable variables now!
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Mutable Variables in Kaleidoscope
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=================================
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Now that we know the sort of problem we want to tackle, lets see what
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this looks like in the context of our little Kaleidoscope language.
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We're going to add two features:
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#. The ability to mutate variables with the '=' operator.
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#. The ability to define new variables.
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While the first item is really what this is about, we only have
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variables for incoming arguments as well as for induction variables, and
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redefining those only goes so far :). Also, the ability to define new
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variables is a useful thing regardless of whether you will be mutating
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them. Here's a motivating example that shows how we could use these:
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::
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# Define ':' for sequencing: as a low-precedence operator that ignores operands
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# and just returns the RHS.
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def binary : 1 (x y) y;
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# Recursive fib, we could do this before.
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def fib(x)
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if (x < 3) then
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1
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else
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fib(x-1)+fib(x-2);
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# Iterative fib.
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def fibi(x)
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var a = 1, b = 1, c in
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(for i = 3, i < x in
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c = a + b :
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a = b :
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b = c) :
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b;
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# Call it.
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fibi(10);
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In order to mutate variables, we have to change our existing variables
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to use the "alloca trick". Once we have that, we'll add our new
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operator, then extend Kaleidoscope to support new variable definitions.
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Adjusting Existing Variables for Mutation
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=========================================
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The symbol table in Kaleidoscope is managed at code generation time by
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the '``named_values``' map. This map currently keeps track of the LLVM
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"Value\*" that holds the double value for the named variable. In order
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to support mutation, we need to change this slightly, so that it
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``named_values`` holds the *memory location* of the variable in
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question. Note that this change is a refactoring: it changes the
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structure of the code, but does not (by itself) change the behavior of
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the compiler. All of these changes are isolated in the Kaleidoscope code
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generator.
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At this point in Kaleidoscope's development, it only supports variables
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for two things: incoming arguments to functions and the induction
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variable of 'for' loops. For consistency, we'll allow mutation of these
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variables in addition to other user-defined variables. This means that
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these will both need memory locations.
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To start our transformation of Kaleidoscope, we'll change the
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``named_values`` map so that it maps to AllocaInst\* instead of Value\*.
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Once we do this, the C++ compiler will tell us what parts of the code we
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need to update:
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**Note:** the ocaml bindings currently model both ``Value*``'s and
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``AllocInst*``'s as ``Llvm.llvalue``'s, but this may change in the future
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to be more type safe.
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.. code-block:: ocaml
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let named_values:(string, llvalue) Hashtbl.t = Hashtbl.create 10
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Also, since we will need to create these alloca's, we'll use a helper
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function that ensures that the allocas are created in the entry block of
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the function:
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.. code-block:: ocaml
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(* Create an alloca instruction in the entry block of the function. This
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* is used for mutable variables etc. *)
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let create_entry_block_alloca the_function var_name =
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let builder = builder_at (instr_begin (entry_block the_function)) in
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build_alloca double_type var_name builder
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This funny looking code creates an ``Llvm.llbuilder`` object that is
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pointing at the first instruction of the entry block. It then creates an
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alloca with the expected name and returns it. Because all values in
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Kaleidoscope are doubles, there is no need to pass in a type to use.
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With this in place, the first functionality change we want to make is to
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variable references. In our new scheme, variables live on the stack, so
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code generating a reference to them actually needs to produce a load
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from the stack slot:
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.. code-block:: ocaml
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let rec codegen_expr = function
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...
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| Ast.Variable name ->
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let v = try Hashtbl.find named_values name with
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| Not_found -> raise (Error "unknown variable name")
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in
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(* Load the value. *)
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build_load v name builder
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As you can see, this is pretty straightforward. Now we need to update
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the things that define the variables to set up the alloca. We'll start
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with ``codegen_expr Ast.For ...`` (see the `full code listing <#id1>`_
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for the unabridged code):
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.. code-block:: ocaml
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| Ast.For (var_name, start, end_, step, body) ->
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let the_function = block_parent (insertion_block builder) in
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(* Create an alloca for the variable in the entry block. *)
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let alloca = create_entry_block_alloca the_function var_name in
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(* Emit the start code first, without 'variable' in scope. *)
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let start_val = codegen_expr start in
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(* Store the value into the alloca. *)
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ignore(build_store start_val alloca builder);
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...
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(* Within the loop, the variable is defined equal to the PHI node. If it
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* shadows an existing variable, we have to restore it, so save it
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* now. *)
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let old_val =
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try Some (Hashtbl.find named_values var_name) with Not_found -> None
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in
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Hashtbl.add named_values var_name alloca;
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...
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(* Compute the end condition. *)
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let end_cond = codegen_expr end_ in
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(* Reload, increment, and restore the alloca. This handles the case where
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* the body of the loop mutates the variable. *)
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let cur_var = build_load alloca var_name builder in
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let next_var = build_add cur_var step_val "nextvar" builder in
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ignore(build_store next_var alloca builder);
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...
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This code is virtually identical to the code `before we allowed mutable
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variables <OCamlLangImpl5.html#code-generation-for-the-for-loop>`_. The big difference is that
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we no longer have to construct a PHI node, and we use load/store to
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access the variable as needed.
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To support mutable argument variables, we need to also make allocas for
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them. The code for this is also pretty simple:
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.. code-block:: ocaml
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(* Create an alloca for each argument and register the argument in the symbol
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* table so that references to it will succeed. *)
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let create_argument_allocas the_function proto =
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let args = match proto with
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| Ast.Prototype (_, args) | Ast.BinOpPrototype (_, args, _) -> args
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in
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Array.iteri (fun i ai ->
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let var_name = args.(i) in
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(* Create an alloca for this variable. *)
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let alloca = create_entry_block_alloca the_function var_name in
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(* Store the initial value into the alloca. *)
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ignore(build_store ai alloca builder);
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(* Add arguments to variable symbol table. *)
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Hashtbl.add named_values var_name alloca;
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) (params the_function)
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For each argument, we make an alloca, store the input value to the
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function into the alloca, and register the alloca as the memory location
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for the argument. This method gets invoked by ``Codegen.codegen_func``
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right after it sets up the entry block for the function.
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The final missing piece is adding the mem2reg pass, which allows us to
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get good codegen once again:
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.. code-block:: ocaml
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let main () =
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...
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let the_fpm = PassManager.create_function Codegen.the_module in
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(* Set up the optimizer pipeline. Start with registering info about how the
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* target lays out data structures. *)
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DataLayout.add (ExecutionEngine.target_data the_execution_engine) the_fpm;
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(* Promote allocas to registers. *)
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add_memory_to_register_promotion the_fpm;
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(* Do simple "peephole" optimizations and bit-twiddling optzn. *)
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add_instruction_combining the_fpm;
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(* reassociate expressions. *)
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add_reassociation the_fpm;
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It is interesting to see what the code looks like before and after the
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mem2reg optimization runs. For example, this is the before/after code
|
|
for our recursive fib function. Before the optimization:
|
|
|
|
.. code-block:: llvm
|
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define double @fib(double %x) {
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entry:
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%x1 = alloca double
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store double %x, double* %x1
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%x2 = load double* %x1
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%cmptmp = fcmp ult double %x2, 3.000000e+00
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%booltmp = uitofp i1 %cmptmp to double
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%ifcond = fcmp one double %booltmp, 0.000000e+00
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br i1 %ifcond, label %then, label %else
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then: ; preds = %entry
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br label %ifcont
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else: ; preds = %entry
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%x3 = load double* %x1
|
|
%subtmp = fsub double %x3, 1.000000e+00
|
|
%calltmp = call double @fib(double %subtmp)
|
|
%x4 = load double* %x1
|
|
%subtmp5 = fsub double %x4, 2.000000e+00
|
|
%calltmp6 = call double @fib(double %subtmp5)
|
|
%addtmp = fadd double %calltmp, %calltmp6
|
|
br label %ifcont
|
|
|
|
ifcont: ; preds = %else, %then
|
|
%iftmp = phi double [ 1.000000e+00, %then ], [ %addtmp, %else ]
|
|
ret double %iftmp
|
|
}
|
|
|
|
Here there is only one variable (x, the input argument) but you can
|
|
still see the extremely simple-minded code generation strategy we are
|
|
using. In the entry block, an alloca is created, and the initial input
|
|
value is stored into it. Each reference to the variable does a reload
|
|
from the stack. Also, note that we didn't modify the if/then/else
|
|
expression, so it still inserts a PHI node. While we could make an
|
|
alloca for it, it is actually easier to create a PHI node for it, so we
|
|
still just make the PHI.
|
|
|
|
Here is the code after the mem2reg pass runs:
|
|
|
|
.. code-block:: llvm
|
|
|
|
define double @fib(double %x) {
|
|
entry:
|
|
%cmptmp = fcmp ult double %x, 3.000000e+00
|
|
%booltmp = uitofp i1 %cmptmp to double
|
|
%ifcond = fcmp one double %booltmp, 0.000000e+00
|
|
br i1 %ifcond, label %then, label %else
|
|
|
|
then:
|
|
br label %ifcont
|
|
|
|
else:
|
|
%subtmp = fsub double %x, 1.000000e+00
|
|
%calltmp = call double @fib(double %subtmp)
|
|
%subtmp5 = fsub double %x, 2.000000e+00
|
|
%calltmp6 = call double @fib(double %subtmp5)
|
|
%addtmp = fadd double %calltmp, %calltmp6
|
|
br label %ifcont
|
|
|
|
ifcont: ; preds = %else, %then
|
|
%iftmp = phi double [ 1.000000e+00, %then ], [ %addtmp, %else ]
|
|
ret double %iftmp
|
|
}
|
|
|
|
This is a trivial case for mem2reg, since there are no redefinitions of
|
|
the variable. The point of showing this is to calm your tension about
|
|
inserting such blatent inefficiencies :).
|
|
|
|
After the rest of the optimizers run, we get:
|
|
|
|
.. code-block:: llvm
|
|
|
|
define double @fib(double %x) {
|
|
entry:
|
|
%cmptmp = fcmp ult double %x, 3.000000e+00
|
|
%booltmp = uitofp i1 %cmptmp to double
|
|
%ifcond = fcmp ueq double %booltmp, 0.000000e+00
|
|
br i1 %ifcond, label %else, label %ifcont
|
|
|
|
else:
|
|
%subtmp = fsub double %x, 1.000000e+00
|
|
%calltmp = call double @fib(double %subtmp)
|
|
%subtmp5 = fsub double %x, 2.000000e+00
|
|
%calltmp6 = call double @fib(double %subtmp5)
|
|
%addtmp = fadd double %calltmp, %calltmp6
|
|
ret double %addtmp
|
|
|
|
ifcont:
|
|
ret double 1.000000e+00
|
|
}
|
|
|
|
Here we see that the simplifycfg pass decided to clone the return
|
|
instruction into the end of the 'else' block. This allowed it to
|
|
eliminate some branches and the PHI node.
|
|
|
|
Now that all symbol table references are updated to use stack variables,
|
|
we'll add the assignment operator.
|
|
|
|
New Assignment Operator
|
|
=======================
|
|
|
|
With our current framework, adding a new assignment operator is really
|
|
simple. We will parse it just like any other binary operator, but handle
|
|
it internally (instead of allowing the user to define it). The first
|
|
step is to set a precedence:
|
|
|
|
.. code-block:: ocaml
|
|
|
|
let main () =
|
|
(* Install standard binary operators.
|
|
* 1 is the lowest precedence. *)
|
|
Hashtbl.add Parser.binop_precedence '=' 2;
|
|
Hashtbl.add Parser.binop_precedence '<' 10;
|
|
Hashtbl.add Parser.binop_precedence '+' 20;
|
|
Hashtbl.add Parser.binop_precedence '-' 20;
|
|
...
|
|
|
|
Now that the parser knows the precedence of the binary operator, it
|
|
takes care of all the parsing and AST generation. We just need to
|
|
implement codegen for the assignment operator. This looks like:
|
|
|
|
.. code-block:: ocaml
|
|
|
|
let rec codegen_expr = function
|
|
begin match op with
|
|
| '=' ->
|
|
(* Special case '=' because we don't want to emit the LHS as an
|
|
* expression. *)
|
|
let name =
|
|
match lhs with
|
|
| Ast.Variable name -> name
|
|
| _ -> raise (Error "destination of '=' must be a variable")
|
|
in
|
|
|
|
Unlike the rest of the binary operators, our assignment operator doesn't
|
|
follow the "emit LHS, emit RHS, do computation" model. As such, it is
|
|
handled as a special case before the other binary operators are handled.
|
|
The other strange thing is that it requires the LHS to be a variable. It
|
|
is invalid to have "(x+1) = expr" - only things like "x = expr" are
|
|
allowed.
|
|
|
|
.. code-block:: ocaml
|
|
|
|
(* Codegen the rhs. *)
|
|
let val_ = codegen_expr rhs in
|
|
|
|
(* Lookup the name. *)
|
|
let variable = try Hashtbl.find named_values name with
|
|
| Not_found -> raise (Error "unknown variable name")
|
|
in
|
|
ignore(build_store val_ variable builder);
|
|
val_
|
|
| _ ->
|
|
...
|
|
|
|
Once we have the variable, codegen'ing the assignment is
|
|
straightforward: we emit the RHS of the assignment, create a store, and
|
|
return the computed value. Returning a value allows for chained
|
|
assignments like "X = (Y = Z)".
|
|
|
|
Now that we have an assignment operator, we can mutate loop variables
|
|
and arguments. For example, we can now run code like this:
|
|
|
|
::
|
|
|
|
# Function to print a double.
|
|
extern printd(x);
|
|
|
|
# Define ':' for sequencing: as a low-precedence operator that ignores operands
|
|
# and just returns the RHS.
|
|
def binary : 1 (x y) y;
|
|
|
|
def test(x)
|
|
printd(x) :
|
|
x = 4 :
|
|
printd(x);
|
|
|
|
test(123);
|
|
|
|
When run, this example prints "123" and then "4", showing that we did
|
|
actually mutate the value! Okay, we have now officially implemented our
|
|
goal: getting this to work requires SSA construction in the general
|
|
case. However, to be really useful, we want the ability to define our
|
|
own local variables, lets add this next!
|
|
|
|
User-defined Local Variables
|
|
============================
|
|
|
|
Adding var/in is just like any other other extensions we made to
|
|
Kaleidoscope: we extend the lexer, the parser, the AST and the code
|
|
generator. The first step for adding our new 'var/in' construct is to
|
|
extend the lexer. As before, this is pretty trivial, the code looks like
|
|
this:
|
|
|
|
.. code-block:: ocaml
|
|
|
|
type token =
|
|
...
|
|
(* var definition *)
|
|
| Var
|
|
|
|
...
|
|
|
|
and lex_ident buffer = parser
|
|
...
|
|
| "in" -> [< 'Token.In; stream >]
|
|
| "binary" -> [< 'Token.Binary; stream >]
|
|
| "unary" -> [< 'Token.Unary; stream >]
|
|
| "var" -> [< 'Token.Var; stream >]
|
|
...
|
|
|
|
The next step is to define the AST node that we will construct. For
|
|
var/in, it looks like this:
|
|
|
|
.. code-block:: ocaml
|
|
|
|
type expr =
|
|
...
|
|
(* variant for var/in. *)
|
|
| Var of (string * expr option) array * expr
|
|
...
|
|
|
|
var/in allows a list of names to be defined all at once, and each name
|
|
can optionally have an initializer value. As such, we capture this
|
|
information in the VarNames vector. Also, var/in has a body, this body
|
|
is allowed to access the variables defined by the var/in.
|
|
|
|
With this in place, we can define the parser pieces. The first thing we
|
|
do is add it as a primary expression:
|
|
|
|
.. code-block:: ocaml
|
|
|
|
(* primary
|
|
* ::= identifier
|
|
* ::= numberexpr
|
|
* ::= parenexpr
|
|
* ::= ifexpr
|
|
* ::= forexpr
|
|
* ::= varexpr *)
|
|
let rec parse_primary = parser
|
|
...
|
|
(* varexpr
|
|
* ::= 'var' identifier ('=' expression?
|
|
* (',' identifier ('=' expression)?)* 'in' expression *)
|
|
| [< 'Token.Var;
|
|
(* At least one variable name is required. *)
|
|
'Token.Ident id ?? "expected identifier after var";
|
|
init=parse_var_init;
|
|
var_names=parse_var_names [(id, init)];
|
|
(* At this point, we have to have 'in'. *)
|
|
'Token.In ?? "expected 'in' keyword after 'var'";
|
|
body=parse_expr >] ->
|
|
Ast.Var (Array.of_list (List.rev var_names), body)
|
|
|
|
...
|
|
|
|
and parse_var_init = parser
|
|
(* read in the optional initializer. *)
|
|
| [< 'Token.Kwd '='; e=parse_expr >] -> Some e
|
|
| [< >] -> None
|
|
|
|
and parse_var_names accumulator = parser
|
|
| [< 'Token.Kwd ',';
|
|
'Token.Ident id ?? "expected identifier list after var";
|
|
init=parse_var_init;
|
|
e=parse_var_names ((id, init) :: accumulator) >] -> e
|
|
| [< >] -> accumulator
|
|
|
|
Now that we can parse and represent the code, we need to support
|
|
emission of LLVM IR for it. This code starts out with:
|
|
|
|
.. code-block:: ocaml
|
|
|
|
let rec codegen_expr = function
|
|
...
|
|
| Ast.Var (var_names, body)
|
|
let old_bindings = ref [] in
|
|
|
|
let the_function = block_parent (insertion_block builder) in
|
|
|
|
(* Register all variables and emit their initializer. *)
|
|
Array.iter (fun (var_name, init) ->
|
|
|
|
Basically it loops over all the variables, installing them one at a
|
|
time. For each variable we put into the symbol table, we remember the
|
|
previous value that we replace in OldBindings.
|
|
|
|
.. code-block:: ocaml
|
|
|
|
(* Emit the initializer before adding the variable to scope, this
|
|
* prevents the initializer from referencing the variable itself, and
|
|
* permits stuff like this:
|
|
* var a = 1 in
|
|
* var a = a in ... # refers to outer 'a'. *)
|
|
let init_val =
|
|
match init with
|
|
| Some init -> codegen_expr init
|
|
(* If not specified, use 0.0. *)
|
|
| None -> const_float double_type 0.0
|
|
in
|
|
|
|
let alloca = create_entry_block_alloca the_function var_name in
|
|
ignore(build_store init_val alloca builder);
|
|
|
|
(* Remember the old variable binding so that we can restore the binding
|
|
* when we unrecurse. *)
|
|
|
|
begin
|
|
try
|
|
let old_value = Hashtbl.find named_values var_name in
|
|
old_bindings := (var_name, old_value) :: !old_bindings;
|
|
with Not_found > ()
|
|
end;
|
|
|
|
(* Remember this binding. *)
|
|
Hashtbl.add named_values var_name alloca;
|
|
) var_names;
|
|
|
|
There are more comments here than code. The basic idea is that we emit
|
|
the initializer, create the alloca, then update the symbol table to
|
|
point to it. Once all the variables are installed in the symbol table,
|
|
we evaluate the body of the var/in expression:
|
|
|
|
.. code-block:: ocaml
|
|
|
|
(* Codegen the body, now that all vars are in scope. *)
|
|
let body_val = codegen_expr body in
|
|
|
|
Finally, before returning, we restore the previous variable bindings:
|
|
|
|
.. code-block:: ocaml
|
|
|
|
(* Pop all our variables from scope. *)
|
|
List.iter (fun (var_name, old_value) ->
|
|
Hashtbl.add named_values var_name old_value
|
|
) !old_bindings;
|
|
|
|
(* Return the body computation. *)
|
|
body_val
|
|
|
|
The end result of all of this is that we get properly scoped variable
|
|
definitions, and we even (trivially) allow mutation of them :).
|
|
|
|
With this, we completed what we set out to do. Our nice iterative fib
|
|
example from the intro compiles and runs just fine. The mem2reg pass
|
|
optimizes all of our stack variables into SSA registers, inserting PHI
|
|
nodes where needed, and our front-end remains simple: no "iterated
|
|
dominance frontier" computation anywhere in sight.
|
|
|
|
Full Code Listing
|
|
=================
|
|
|
|
Here is the complete code listing for our running example, enhanced with
|
|
mutable variables and var/in support. To build this example, use:
|
|
|
|
.. code-block:: bash
|
|
|
|
# Compile
|
|
ocamlbuild toy.byte
|
|
# Run
|
|
./toy.byte
|
|
|
|
Here is the code:
|
|
|
|
\_tags:
|
|
::
|
|
|
|
<{lexer,parser}.ml>: use_camlp4, pp(camlp4of)
|
|
<*.{byte,native}>: g++, use_llvm, use_llvm_analysis
|
|
<*.{byte,native}>: use_llvm_executionengine, use_llvm_target
|
|
<*.{byte,native}>: use_llvm_scalar_opts, use_bindings
|
|
|
|
myocamlbuild.ml:
|
|
.. code-block:: ocaml
|
|
|
|
open Ocamlbuild_plugin;;
|
|
|
|
ocaml_lib ~extern:true "llvm";;
|
|
ocaml_lib ~extern:true "llvm_analysis";;
|
|
ocaml_lib ~extern:true "llvm_executionengine";;
|
|
ocaml_lib ~extern:true "llvm_target";;
|
|
ocaml_lib ~extern:true "llvm_scalar_opts";;
|
|
|
|
flag ["link"; "ocaml"; "g++"] (S[A"-cc"; A"g++"; A"-cclib"; A"-rdynamic"]);;
|
|
dep ["link"; "ocaml"; "use_bindings"] ["bindings.o"];;
|
|
|
|
token.ml:
|
|
.. code-block:: ocaml
|
|
|
|
(*===----------------------------------------------------------------------===
|
|
* Lexer Tokens
|
|
*===----------------------------------------------------------------------===*)
|
|
|
|
(* The lexer returns these 'Kwd' if it is an unknown character, otherwise one of
|
|
* these others for known things. *)
|
|
type token =
|
|
(* commands *)
|
|
| Def | Extern
|
|
|
|
(* primary *)
|
|
| Ident of string | Number of float
|
|
|
|
(* unknown *)
|
|
| Kwd of char
|
|
|
|
(* control *)
|
|
| If | Then | Else
|
|
| For | In
|
|
|
|
(* operators *)
|
|
| Binary | Unary
|
|
|
|
(* var definition *)
|
|
| Var
|
|
|
|
lexer.ml:
|
|
.. code-block:: ocaml
|
|
|
|
(*===----------------------------------------------------------------------===
|
|
* Lexer
|
|
*===----------------------------------------------------------------------===*)
|
|
|
|
let rec lex = parser
|
|
(* Skip any whitespace. *)
|
|
| [< ' (' ' | '\n' | '\r' | '\t'); stream >] -> lex stream
|
|
|
|
(* identifier: [a-zA-Z][a-zA-Z0-9] *)
|
|
| [< ' ('A' .. 'Z' | 'a' .. 'z' as c); stream >] ->
|
|
let buffer = Buffer.create 1 in
|
|
Buffer.add_char buffer c;
|
|
lex_ident buffer stream
|
|
|
|
(* number: [0-9.]+ *)
|
|
| [< ' ('0' .. '9' as c); stream >] ->
|
|
let buffer = Buffer.create 1 in
|
|
Buffer.add_char buffer c;
|
|
lex_number buffer stream
|
|
|
|
(* Comment until end of line. *)
|
|
| [< ' ('#'); stream >] ->
|
|
lex_comment stream
|
|
|
|
(* Otherwise, just return the character as its ascii value. *)
|
|
| [< 'c; stream >] ->
|
|
[< 'Token.Kwd c; lex stream >]
|
|
|
|
(* end of stream. *)
|
|
| [< >] -> [< >]
|
|
|
|
and lex_number buffer = parser
|
|
| [< ' ('0' .. '9' | '.' as c); stream >] ->
|
|
Buffer.add_char buffer c;
|
|
lex_number buffer stream
|
|
| [< stream=lex >] ->
|
|
[< 'Token.Number (float_of_string (Buffer.contents buffer)); stream >]
|
|
|
|
and lex_ident buffer = parser
|
|
| [< ' ('A' .. 'Z' | 'a' .. 'z' | '0' .. '9' as c); stream >] ->
|
|
Buffer.add_char buffer c;
|
|
lex_ident buffer stream
|
|
| [< stream=lex >] ->
|
|
match Buffer.contents buffer with
|
|
| "def" -> [< 'Token.Def; stream >]
|
|
| "extern" -> [< 'Token.Extern; stream >]
|
|
| "if" -> [< 'Token.If; stream >]
|
|
| "then" -> [< 'Token.Then; stream >]
|
|
| "else" -> [< 'Token.Else; stream >]
|
|
| "for" -> [< 'Token.For; stream >]
|
|
| "in" -> [< 'Token.In; stream >]
|
|
| "binary" -> [< 'Token.Binary; stream >]
|
|
| "unary" -> [< 'Token.Unary; stream >]
|
|
| "var" -> [< 'Token.Var; stream >]
|
|
| id -> [< 'Token.Ident id; stream >]
|
|
|
|
and lex_comment = parser
|
|
| [< ' ('\n'); stream=lex >] -> stream
|
|
| [< 'c; e=lex_comment >] -> e
|
|
| [< >] -> [< >]
|
|
|
|
ast.ml:
|
|
.. code-block:: ocaml
|
|
|
|
(*===----------------------------------------------------------------------===
|
|
* Abstract Syntax Tree (aka Parse Tree)
|
|
*===----------------------------------------------------------------------===*)
|
|
|
|
(* expr - Base type for all expression nodes. *)
|
|
type expr =
|
|
(* variant for numeric literals like "1.0". *)
|
|
| Number of float
|
|
|
|
(* variant for referencing a variable, like "a". *)
|
|
| Variable of string
|
|
|
|
(* variant for a unary operator. *)
|
|
| Unary of char * expr
|
|
|
|
(* variant for a binary operator. *)
|
|
| Binary of char * expr * expr
|
|
|
|
(* variant for function calls. *)
|
|
| Call of string * expr array
|
|
|
|
(* variant for if/then/else. *)
|
|
| If of expr * expr * expr
|
|
|
|
(* variant for for/in. *)
|
|
| For of string * expr * expr * expr option * expr
|
|
|
|
(* variant for var/in. *)
|
|
| Var of (string * expr option) array * expr
|
|
|
|
(* proto - This type represents the "prototype" for a function, which captures
|
|
* its name, and its argument names (thus implicitly the number of arguments the
|
|
* function takes). *)
|
|
type proto =
|
|
| Prototype of string * string array
|
|
| BinOpPrototype of string * string array * int
|
|
|
|
(* func - This type represents a function definition itself. *)
|
|
type func = Function of proto * expr
|
|
|
|
parser.ml:
|
|
.. code-block:: ocaml
|
|
|
|
(*===---------------------------------------------------------------------===
|
|
* Parser
|
|
*===---------------------------------------------------------------------===*)
|
|
|
|
(* binop_precedence - This holds the precedence for each binary operator that is
|
|
* defined *)
|
|
let binop_precedence:(char, int) Hashtbl.t = Hashtbl.create 10
|
|
|
|
(* precedence - Get the precedence of the pending binary operator token. *)
|
|
let precedence c = try Hashtbl.find binop_precedence c with Not_found -> -1
|
|
|
|
(* primary
|
|
* ::= identifier
|
|
* ::= numberexpr
|
|
* ::= parenexpr
|
|
* ::= ifexpr
|
|
* ::= forexpr
|
|
* ::= varexpr *)
|
|
let rec parse_primary = parser
|
|
(* numberexpr ::= number *)
|
|
| [< 'Token.Number n >] -> Ast.Number n
|
|
|
|
(* parenexpr ::= '(' expression ')' *)
|
|
| [< 'Token.Kwd '('; e=parse_expr; 'Token.Kwd ')' ?? "expected ')'" >] -> e
|
|
|
|
(* identifierexpr
|
|
* ::= identifier
|
|
* ::= identifier '(' argumentexpr ')' *)
|
|
| [< 'Token.Ident id; stream >] ->
|
|
let rec parse_args accumulator = parser
|
|
| [< e=parse_expr; stream >] ->
|
|
begin parser
|
|
| [< 'Token.Kwd ','; e=parse_args (e :: accumulator) >] -> e
|
|
| [< >] -> e :: accumulator
|
|
end stream
|
|
| [< >] -> accumulator
|
|
in
|
|
let rec parse_ident id = parser
|
|
(* Call. *)
|
|
| [< 'Token.Kwd '(';
|
|
args=parse_args [];
|
|
'Token.Kwd ')' ?? "expected ')'">] ->
|
|
Ast.Call (id, Array.of_list (List.rev args))
|
|
|
|
(* Simple variable ref. *)
|
|
| [< >] -> Ast.Variable id
|
|
in
|
|
parse_ident id stream
|
|
|
|
(* ifexpr ::= 'if' expr 'then' expr 'else' expr *)
|
|
| [< 'Token.If; c=parse_expr;
|
|
'Token.Then ?? "expected 'then'"; t=parse_expr;
|
|
'Token.Else ?? "expected 'else'"; e=parse_expr >] ->
|
|
Ast.If (c, t, e)
|
|
|
|
(* forexpr
|
|
::= 'for' identifier '=' expr ',' expr (',' expr)? 'in' expression *)
|
|
| [< 'Token.For;
|
|
'Token.Ident id ?? "expected identifier after for";
|
|
'Token.Kwd '=' ?? "expected '=' after for";
|
|
stream >] ->
|
|
begin parser
|
|
| [<
|
|
start=parse_expr;
|
|
'Token.Kwd ',' ?? "expected ',' after for";
|
|
end_=parse_expr;
|
|
stream >] ->
|
|
let step =
|
|
begin parser
|
|
| [< 'Token.Kwd ','; step=parse_expr >] -> Some step
|
|
| [< >] -> None
|
|
end stream
|
|
in
|
|
begin parser
|
|
| [< 'Token.In; body=parse_expr >] ->
|
|
Ast.For (id, start, end_, step, body)
|
|
| [< >] ->
|
|
raise (Stream.Error "expected 'in' after for")
|
|
end stream
|
|
| [< >] ->
|
|
raise (Stream.Error "expected '=' after for")
|
|
end stream
|
|
|
|
(* varexpr
|
|
* ::= 'var' identifier ('=' expression?
|
|
* (',' identifier ('=' expression)?)* 'in' expression *)
|
|
| [< 'Token.Var;
|
|
(* At least one variable name is required. *)
|
|
'Token.Ident id ?? "expected identifier after var";
|
|
init=parse_var_init;
|
|
var_names=parse_var_names [(id, init)];
|
|
(* At this point, we have to have 'in'. *)
|
|
'Token.In ?? "expected 'in' keyword after 'var'";
|
|
body=parse_expr >] ->
|
|
Ast.Var (Array.of_list (List.rev var_names), body)
|
|
|
|
| [< >] -> raise (Stream.Error "unknown token when expecting an expression.")
|
|
|
|
(* unary
|
|
* ::= primary
|
|
* ::= '!' unary *)
|
|
and parse_unary = parser
|
|
(* If this is a unary operator, read it. *)
|
|
| [< 'Token.Kwd op when op != '(' && op != ')'; operand=parse_expr >] ->
|
|
Ast.Unary (op, operand)
|
|
|
|
(* If the current token is not an operator, it must be a primary expr. *)
|
|
| [< stream >] -> parse_primary stream
|
|
|
|
(* binoprhs
|
|
* ::= ('+' primary)* *)
|
|
and parse_bin_rhs expr_prec lhs stream =
|
|
match Stream.peek stream with
|
|
(* If this is a binop, find its precedence. *)
|
|
| Some (Token.Kwd c) when Hashtbl.mem binop_precedence c ->
|
|
let token_prec = precedence c in
|
|
|
|
(* If this is a binop that binds at least as tightly as the current binop,
|
|
* consume it, otherwise we are done. *)
|
|
if token_prec < expr_prec then lhs else begin
|
|
(* Eat the binop. *)
|
|
Stream.junk stream;
|
|
|
|
(* Parse the primary expression after the binary operator. *)
|
|
let rhs = parse_unary stream in
|
|
|
|
(* Okay, we know this is a binop. *)
|
|
let rhs =
|
|
match Stream.peek stream with
|
|
| Some (Token.Kwd c2) ->
|
|
(* If BinOp binds less tightly with rhs than the operator after
|
|
* rhs, let the pending operator take rhs as its lhs. *)
|
|
let next_prec = precedence c2 in
|
|
if token_prec < next_prec
|
|
then parse_bin_rhs (token_prec + 1) rhs stream
|
|
else rhs
|
|
| _ -> rhs
|
|
in
|
|
|
|
(* Merge lhs/rhs. *)
|
|
let lhs = Ast.Binary (c, lhs, rhs) in
|
|
parse_bin_rhs expr_prec lhs stream
|
|
end
|
|
| _ -> lhs
|
|
|
|
and parse_var_init = parser
|
|
(* read in the optional initializer. *)
|
|
| [< 'Token.Kwd '='; e=parse_expr >] -> Some e
|
|
| [< >] -> None
|
|
|
|
and parse_var_names accumulator = parser
|
|
| [< 'Token.Kwd ',';
|
|
'Token.Ident id ?? "expected identifier list after var";
|
|
init=parse_var_init;
|
|
e=parse_var_names ((id, init) :: accumulator) >] -> e
|
|
| [< >] -> accumulator
|
|
|
|
(* expression
|
|
* ::= primary binoprhs *)
|
|
and parse_expr = parser
|
|
| [< lhs=parse_unary; stream >] -> parse_bin_rhs 0 lhs stream
|
|
|
|
(* prototype
|
|
* ::= id '(' id* ')'
|
|
* ::= binary LETTER number? (id, id)
|
|
* ::= unary LETTER number? (id) *)
|
|
let parse_prototype =
|
|
let rec parse_args accumulator = parser
|
|
| [< 'Token.Ident id; e=parse_args (id::accumulator) >] -> e
|
|
| [< >] -> accumulator
|
|
in
|
|
let parse_operator = parser
|
|
| [< 'Token.Unary >] -> "unary", 1
|
|
| [< 'Token.Binary >] -> "binary", 2
|
|
in
|
|
let parse_binary_precedence = parser
|
|
| [< 'Token.Number n >] -> int_of_float n
|
|
| [< >] -> 30
|
|
in
|
|
parser
|
|
| [< 'Token.Ident id;
|
|
'Token.Kwd '(' ?? "expected '(' in prototype";
|
|
args=parse_args [];
|
|
'Token.Kwd ')' ?? "expected ')' in prototype" >] ->
|
|
(* success. *)
|
|
Ast.Prototype (id, Array.of_list (List.rev args))
|
|
| [< (prefix, kind)=parse_operator;
|
|
'Token.Kwd op ?? "expected an operator";
|
|
(* Read the precedence if present. *)
|
|
binary_precedence=parse_binary_precedence;
|
|
'Token.Kwd '(' ?? "expected '(' in prototype";
|
|
args=parse_args [];
|
|
'Token.Kwd ')' ?? "expected ')' in prototype" >] ->
|
|
let name = prefix ^ (String.make 1 op) in
|
|
let args = Array.of_list (List.rev args) in
|
|
|
|
(* Verify right number of arguments for operator. *)
|
|
if Array.length args != kind
|
|
then raise (Stream.Error "invalid number of operands for operator")
|
|
else
|
|
if kind == 1 then
|
|
Ast.Prototype (name, args)
|
|
else
|
|
Ast.BinOpPrototype (name, args, binary_precedence)
|
|
| [< >] ->
|
|
raise (Stream.Error "expected function name in prototype")
|
|
|
|
(* definition ::= 'def' prototype expression *)
|
|
let parse_definition = parser
|
|
| [< 'Token.Def; p=parse_prototype; e=parse_expr >] ->
|
|
Ast.Function (p, e)
|
|
|
|
(* toplevelexpr ::= expression *)
|
|
let parse_toplevel = parser
|
|
| [< e=parse_expr >] ->
|
|
(* Make an anonymous proto. *)
|
|
Ast.Function (Ast.Prototype ("", [||]), e)
|
|
|
|
(* external ::= 'extern' prototype *)
|
|
let parse_extern = parser
|
|
| [< 'Token.Extern; e=parse_prototype >] -> e
|
|
|
|
codegen.ml:
|
|
.. code-block:: ocaml
|
|
|
|
(*===----------------------------------------------------------------------===
|
|
* Code Generation
|
|
*===----------------------------------------------------------------------===*)
|
|
|
|
open Llvm
|
|
|
|
exception Error of string
|
|
|
|
let context = global_context ()
|
|
let the_module = create_module context "my cool jit"
|
|
let builder = builder context
|
|
let named_values:(string, llvalue) Hashtbl.t = Hashtbl.create 10
|
|
let double_type = double_type context
|
|
|
|
(* Create an alloca instruction in the entry block of the function. This
|
|
* is used for mutable variables etc. *)
|
|
let create_entry_block_alloca the_function var_name =
|
|
let builder = builder_at context (instr_begin (entry_block the_function)) in
|
|
build_alloca double_type var_name builder
|
|
|
|
let rec codegen_expr = function
|
|
| Ast.Number n -> const_float double_type n
|
|
| Ast.Variable name ->
|
|
let v = try Hashtbl.find named_values name with
|
|
| Not_found -> raise (Error "unknown variable name")
|
|
in
|
|
(* Load the value. *)
|
|
build_load v name builder
|
|
| Ast.Unary (op, operand) ->
|
|
let operand = codegen_expr operand in
|
|
let callee = "unary" ^ (String.make 1 op) in
|
|
let callee =
|
|
match lookup_function callee the_module with
|
|
| Some callee -> callee
|
|
| None -> raise (Error "unknown unary operator")
|
|
in
|
|
build_call callee [|operand|] "unop" builder
|
|
| Ast.Binary (op, lhs, rhs) ->
|
|
begin match op with
|
|
| '=' ->
|
|
(* Special case '=' because we don't want to emit the LHS as an
|
|
* expression. *)
|
|
let name =
|
|
match lhs with
|
|
| Ast.Variable name -> name
|
|
| _ -> raise (Error "destination of '=' must be a variable")
|
|
in
|
|
|
|
(* Codegen the rhs. *)
|
|
let val_ = codegen_expr rhs in
|
|
|
|
(* Lookup the name. *)
|
|
let variable = try Hashtbl.find named_values name with
|
|
| Not_found -> raise (Error "unknown variable name")
|
|
in
|
|
ignore(build_store val_ variable builder);
|
|
val_
|
|
| _ ->
|
|
let lhs_val = codegen_expr lhs in
|
|
let rhs_val = codegen_expr rhs in
|
|
begin
|
|
match op with
|
|
| '+' -> build_add lhs_val rhs_val "addtmp" builder
|
|
| '-' -> build_sub lhs_val rhs_val "subtmp" builder
|
|
| '*' -> build_mul lhs_val rhs_val "multmp" builder
|
|
| '<' ->
|
|
(* Convert bool 0/1 to double 0.0 or 1.0 *)
|
|
let i = build_fcmp Fcmp.Ult lhs_val rhs_val "cmptmp" builder in
|
|
build_uitofp i double_type "booltmp" builder
|
|
| _ ->
|
|
(* If it wasn't a builtin binary operator, it must be a user defined
|
|
* one. Emit a call to it. *)
|
|
let callee = "binary" ^ (String.make 1 op) in
|
|
let callee =
|
|
match lookup_function callee the_module with
|
|
| Some callee -> callee
|
|
| None -> raise (Error "binary operator not found!")
|
|
in
|
|
build_call callee [|lhs_val; rhs_val|] "binop" builder
|
|
end
|
|
end
|
|
| Ast.Call (callee, args) ->
|
|
(* Look up the name in the module table. *)
|
|
let callee =
|
|
match lookup_function callee the_module with
|
|
| Some callee -> callee
|
|
| None -> raise (Error "unknown function referenced")
|
|
in
|
|
let params = params callee in
|
|
|
|
(* If argument mismatch error. *)
|
|
if Array.length params == Array.length args then () else
|
|
raise (Error "incorrect # arguments passed");
|
|
let args = Array.map codegen_expr args in
|
|
build_call callee args "calltmp" builder
|
|
| Ast.If (cond, then_, else_) ->
|
|
let cond = codegen_expr cond in
|
|
|
|
(* Convert condition to a bool by comparing equal to 0.0 *)
|
|
let zero = const_float double_type 0.0 in
|
|
let cond_val = build_fcmp Fcmp.One cond zero "ifcond" builder in
|
|
|
|
(* Grab the first block so that we might later add the conditional branch
|
|
* to it at the end of the function. *)
|
|
let start_bb = insertion_block builder in
|
|
let the_function = block_parent start_bb in
|
|
|
|
let then_bb = append_block context "then" the_function in
|
|
|
|
(* Emit 'then' value. *)
|
|
position_at_end then_bb builder;
|
|
let then_val = codegen_expr then_ in
|
|
|
|
(* Codegen of 'then' can change the current block, update then_bb for the
|
|
* phi. We create a new name because one is used for the phi node, and the
|
|
* other is used for the conditional branch. *)
|
|
let new_then_bb = insertion_block builder in
|
|
|
|
(* Emit 'else' value. *)
|
|
let else_bb = append_block context "else" the_function in
|
|
position_at_end else_bb builder;
|
|
let else_val = codegen_expr else_ in
|
|
|
|
(* Codegen of 'else' can change the current block, update else_bb for the
|
|
* phi. *)
|
|
let new_else_bb = insertion_block builder in
|
|
|
|
(* Emit merge block. *)
|
|
let merge_bb = append_block context "ifcont" the_function in
|
|
position_at_end merge_bb builder;
|
|
let incoming = [(then_val, new_then_bb); (else_val, new_else_bb)] in
|
|
let phi = build_phi incoming "iftmp" builder in
|
|
|
|
(* Return to the start block to add the conditional branch. *)
|
|
position_at_end start_bb builder;
|
|
ignore (build_cond_br cond_val then_bb else_bb builder);
|
|
|
|
(* Set a unconditional branch at the end of the 'then' block and the
|
|
* 'else' block to the 'merge' block. *)
|
|
position_at_end new_then_bb builder; ignore (build_br merge_bb builder);
|
|
position_at_end new_else_bb builder; ignore (build_br merge_bb builder);
|
|
|
|
(* Finally, set the builder to the end of the merge block. *)
|
|
position_at_end merge_bb builder;
|
|
|
|
phi
|
|
| Ast.For (var_name, start, end_, step, body) ->
|
|
(* Output this as:
|
|
* var = alloca double
|
|
* ...
|
|
* start = startexpr
|
|
* store start -> var
|
|
* goto loop
|
|
* loop:
|
|
* ...
|
|
* bodyexpr
|
|
* ...
|
|
* loopend:
|
|
* step = stepexpr
|
|
* endcond = endexpr
|
|
*
|
|
* curvar = load var
|
|
* nextvar = curvar + step
|
|
* store nextvar -> var
|
|
* br endcond, loop, endloop
|
|
* outloop: *)
|
|
|
|
let the_function = block_parent (insertion_block builder) in
|
|
|
|
(* Create an alloca for the variable in the entry block. *)
|
|
let alloca = create_entry_block_alloca the_function var_name in
|
|
|
|
(* Emit the start code first, without 'variable' in scope. *)
|
|
let start_val = codegen_expr start in
|
|
|
|
(* Store the value into the alloca. *)
|
|
ignore(build_store start_val alloca builder);
|
|
|
|
(* Make the new basic block for the loop header, inserting after current
|
|
* block. *)
|
|
let loop_bb = append_block context "loop" the_function in
|
|
|
|
(* Insert an explicit fall through from the current block to the
|
|
* loop_bb. *)
|
|
ignore (build_br loop_bb builder);
|
|
|
|
(* Start insertion in loop_bb. *)
|
|
position_at_end loop_bb builder;
|
|
|
|
(* Within the loop, the variable is defined equal to the PHI node. If it
|
|
* shadows an existing variable, we have to restore it, so save it
|
|
* now. *)
|
|
let old_val =
|
|
try Some (Hashtbl.find named_values var_name) with Not_found -> None
|
|
in
|
|
Hashtbl.add named_values var_name alloca;
|
|
|
|
(* Emit the body of the loop. This, like any other expr, can change the
|
|
* current BB. Note that we ignore the value computed by the body, but
|
|
* don't allow an error *)
|
|
ignore (codegen_expr body);
|
|
|
|
(* Emit the step value. *)
|
|
let step_val =
|
|
match step with
|
|
| Some step -> codegen_expr step
|
|
(* If not specified, use 1.0. *)
|
|
| None -> const_float double_type 1.0
|
|
in
|
|
|
|
(* Compute the end condition. *)
|
|
let end_cond = codegen_expr end_ in
|
|
|
|
(* Reload, increment, and restore the alloca. This handles the case where
|
|
* the body of the loop mutates the variable. *)
|
|
let cur_var = build_load alloca var_name builder in
|
|
let next_var = build_add cur_var step_val "nextvar" builder in
|
|
ignore(build_store next_var alloca builder);
|
|
|
|
(* Convert condition to a bool by comparing equal to 0.0. *)
|
|
let zero = const_float double_type 0.0 in
|
|
let end_cond = build_fcmp Fcmp.One end_cond zero "loopcond" builder in
|
|
|
|
(* Create the "after loop" block and insert it. *)
|
|
let after_bb = append_block context "afterloop" the_function in
|
|
|
|
(* Insert the conditional branch into the end of loop_end_bb. *)
|
|
ignore (build_cond_br end_cond loop_bb after_bb builder);
|
|
|
|
(* Any new code will be inserted in after_bb. *)
|
|
position_at_end after_bb builder;
|
|
|
|
(* Restore the unshadowed variable. *)
|
|
begin match old_val with
|
|
| Some old_val -> Hashtbl.add named_values var_name old_val
|
|
| None -> ()
|
|
end;
|
|
|
|
(* for expr always returns 0.0. *)
|
|
const_null double_type
|
|
| Ast.Var (var_names, body) ->
|
|
let old_bindings = ref [] in
|
|
|
|
let the_function = block_parent (insertion_block builder) in
|
|
|
|
(* Register all variables and emit their initializer. *)
|
|
Array.iter (fun (var_name, init) ->
|
|
(* Emit the initializer before adding the variable to scope, this
|
|
* prevents the initializer from referencing the variable itself, and
|
|
* permits stuff like this:
|
|
* var a = 1 in
|
|
* var a = a in ... # refers to outer 'a'. *)
|
|
let init_val =
|
|
match init with
|
|
| Some init -> codegen_expr init
|
|
(* If not specified, use 0.0. *)
|
|
| None -> const_float double_type 0.0
|
|
in
|
|
|
|
let alloca = create_entry_block_alloca the_function var_name in
|
|
ignore(build_store init_val alloca builder);
|
|
|
|
(* Remember the old variable binding so that we can restore the binding
|
|
* when we unrecurse. *)
|
|
begin
|
|
try
|
|
let old_value = Hashtbl.find named_values var_name in
|
|
old_bindings := (var_name, old_value) :: !old_bindings;
|
|
with Not_found -> ()
|
|
end;
|
|
|
|
(* Remember this binding. *)
|
|
Hashtbl.add named_values var_name alloca;
|
|
) var_names;
|
|
|
|
(* Codegen the body, now that all vars are in scope. *)
|
|
let body_val = codegen_expr body in
|
|
|
|
(* Pop all our variables from scope. *)
|
|
List.iter (fun (var_name, old_value) ->
|
|
Hashtbl.add named_values var_name old_value
|
|
) !old_bindings;
|
|
|
|
(* Return the body computation. *)
|
|
body_val
|
|
|
|
let codegen_proto = function
|
|
| Ast.Prototype (name, args) | Ast.BinOpPrototype (name, args, _) ->
|
|
(* Make the function type: double(double,double) etc. *)
|
|
let doubles = Array.make (Array.length args) double_type in
|
|
let ft = function_type double_type doubles in
|
|
let f =
|
|
match lookup_function name the_module with
|
|
| None -> declare_function name ft the_module
|
|
|
|
(* If 'f' conflicted, there was already something named 'name'. If it
|
|
* has a body, don't allow redefinition or reextern. *)
|
|
| Some f ->
|
|
(* If 'f' already has a body, reject this. *)
|
|
if block_begin f <> At_end f then
|
|
raise (Error "redefinition of function");
|
|
|
|
(* If 'f' took a different number of arguments, reject. *)
|
|
if element_type (type_of f) <> ft then
|
|
raise (Error "redefinition of function with different # args");
|
|
f
|
|
in
|
|
|
|
(* Set names for all arguments. *)
|
|
Array.iteri (fun i a ->
|
|
let n = args.(i) in
|
|
set_value_name n a;
|
|
Hashtbl.add named_values n a;
|
|
) (params f);
|
|
f
|
|
|
|
(* Create an alloca for each argument and register the argument in the symbol
|
|
* table so that references to it will succeed. *)
|
|
let create_argument_allocas the_function proto =
|
|
let args = match proto with
|
|
| Ast.Prototype (_, args) | Ast.BinOpPrototype (_, args, _) -> args
|
|
in
|
|
Array.iteri (fun i ai ->
|
|
let var_name = args.(i) in
|
|
(* Create an alloca for this variable. *)
|
|
let alloca = create_entry_block_alloca the_function var_name in
|
|
|
|
(* Store the initial value into the alloca. *)
|
|
ignore(build_store ai alloca builder);
|
|
|
|
(* Add arguments to variable symbol table. *)
|
|
Hashtbl.add named_values var_name alloca;
|
|
) (params the_function)
|
|
|
|
let codegen_func the_fpm = function
|
|
| Ast.Function (proto, body) ->
|
|
Hashtbl.clear named_values;
|
|
let the_function = codegen_proto proto in
|
|
|
|
(* If this is an operator, install it. *)
|
|
begin match proto with
|
|
| Ast.BinOpPrototype (name, args, prec) ->
|
|
let op = name.[String.length name - 1] in
|
|
Hashtbl.add Parser.binop_precedence op prec;
|
|
| _ -> ()
|
|
end;
|
|
|
|
(* Create a new basic block to start insertion into. *)
|
|
let bb = append_block context "entry" the_function in
|
|
position_at_end bb builder;
|
|
|
|
try
|
|
(* Add all arguments to the symbol table and create their allocas. *)
|
|
create_argument_allocas the_function proto;
|
|
|
|
let ret_val = codegen_expr body in
|
|
|
|
(* Finish off the function. *)
|
|
let _ = build_ret ret_val builder in
|
|
|
|
(* Validate the generated code, checking for consistency. *)
|
|
Llvm_analysis.assert_valid_function the_function;
|
|
|
|
(* Optimize the function. *)
|
|
let _ = PassManager.run_function the_function the_fpm in
|
|
|
|
the_function
|
|
with e ->
|
|
delete_function the_function;
|
|
raise e
|
|
|
|
toplevel.ml:
|
|
.. code-block:: ocaml
|
|
|
|
(*===----------------------------------------------------------------------===
|
|
* Top-Level parsing and JIT Driver
|
|
*===----------------------------------------------------------------------===*)
|
|
|
|
open Llvm
|
|
open Llvm_executionengine
|
|
|
|
(* top ::= definition | external | expression | ';' *)
|
|
let rec main_loop the_fpm the_execution_engine stream =
|
|
match Stream.peek stream with
|
|
| None -> ()
|
|
|
|
(* ignore top-level semicolons. *)
|
|
| Some (Token.Kwd ';') ->
|
|
Stream.junk stream;
|
|
main_loop the_fpm the_execution_engine stream
|
|
|
|
| Some token ->
|
|
begin
|
|
try match token with
|
|
| Token.Def ->
|
|
let e = Parser.parse_definition stream in
|
|
print_endline "parsed a function definition.";
|
|
dump_value (Codegen.codegen_func the_fpm e);
|
|
| Token.Extern ->
|
|
let e = Parser.parse_extern stream in
|
|
print_endline "parsed an extern.";
|
|
dump_value (Codegen.codegen_proto e);
|
|
| _ ->
|
|
(* Evaluate a top-level expression into an anonymous function. *)
|
|
let e = Parser.parse_toplevel stream in
|
|
print_endline "parsed a top-level expr";
|
|
let the_function = Codegen.codegen_func the_fpm e in
|
|
dump_value the_function;
|
|
|
|
(* JIT the function, returning a function pointer. *)
|
|
let result = ExecutionEngine.run_function the_function [||]
|
|
the_execution_engine in
|
|
|
|
print_string "Evaluated to ";
|
|
print_float (GenericValue.as_float Codegen.double_type result);
|
|
print_newline ();
|
|
with Stream.Error s | Codegen.Error s ->
|
|
(* Skip token for error recovery. *)
|
|
Stream.junk stream;
|
|
print_endline s;
|
|
end;
|
|
print_string "ready> "; flush stdout;
|
|
main_loop the_fpm the_execution_engine stream
|
|
|
|
toy.ml:
|
|
.. code-block:: ocaml
|
|
|
|
(*===----------------------------------------------------------------------===
|
|
* Main driver code.
|
|
*===----------------------------------------------------------------------===*)
|
|
|
|
open Llvm
|
|
open Llvm_executionengine
|
|
open Llvm_target
|
|
open Llvm_scalar_opts
|
|
|
|
let main () =
|
|
ignore (initialize_native_target ());
|
|
|
|
(* Install standard binary operators.
|
|
* 1 is the lowest precedence. *)
|
|
Hashtbl.add Parser.binop_precedence '=' 2;
|
|
Hashtbl.add Parser.binop_precedence '<' 10;
|
|
Hashtbl.add Parser.binop_precedence '+' 20;
|
|
Hashtbl.add Parser.binop_precedence '-' 20;
|
|
Hashtbl.add Parser.binop_precedence '*' 40; (* highest. *)
|
|
|
|
(* Prime the first token. *)
|
|
print_string "ready> "; flush stdout;
|
|
let stream = Lexer.lex (Stream.of_channel stdin) in
|
|
|
|
(* Create the JIT. *)
|
|
let the_execution_engine = ExecutionEngine.create Codegen.the_module in
|
|
let the_fpm = PassManager.create_function Codegen.the_module in
|
|
|
|
(* Set up the optimizer pipeline. Start with registering info about how the
|
|
* target lays out data structures. *)
|
|
DataLayout.add (ExecutionEngine.target_data the_execution_engine) the_fpm;
|
|
|
|
(* Promote allocas to registers. *)
|
|
add_memory_to_register_promotion the_fpm;
|
|
|
|
(* Do simple "peephole" optimizations and bit-twiddling optzn. *)
|
|
add_instruction_combination the_fpm;
|
|
|
|
(* reassociate expressions. *)
|
|
add_reassociation the_fpm;
|
|
|
|
(* Eliminate Common SubExpressions. *)
|
|
add_gvn the_fpm;
|
|
|
|
(* Simplify the control flow graph (deleting unreachable blocks, etc). *)
|
|
add_cfg_simplification the_fpm;
|
|
|
|
ignore (PassManager.initialize the_fpm);
|
|
|
|
(* Run the main "interpreter loop" now. *)
|
|
Toplevel.main_loop the_fpm the_execution_engine stream;
|
|
|
|
(* Print out all the generated code. *)
|
|
dump_module Codegen.the_module
|
|
;;
|
|
|
|
main ()
|
|
|
|
bindings.c
|
|
.. code-block:: c
|
|
|
|
#include <stdio.h>
|
|
|
|
/* putchard - putchar that takes a double and returns 0. */
|
|
extern double putchard(double X) {
|
|
putchar((char)X);
|
|
return 0;
|
|
}
|
|
|
|
/* printd - printf that takes a double prints it as "%f\n", returning 0. */
|
|
extern double printd(double X) {
|
|
printf("%f\n", X);
|
|
return 0;
|
|
}
|
|
|
|
`Next: Conclusion and other useful LLVM tidbits <OCamlLangImpl8.html>`_
|
|
|