d/freebsd-dev
1
0
Fork 0
Code Issues Pull Requests Packages Projects Releases Wiki Activity
006a42a9cb
freebsd-dev/contrib/gcc/tree-ssa-copy.c
Alexander Kabaev 6b834ef156 GCC 4.2.0 release.
2007-05-19 01:19:51 +00:00

1163 lines
36 KiB
C
Raw Blame History

/* Copy propagation and SSA_NAME replacement support routines.
Copyright (C) 2004, 2005 Free Software Foundation, Inc.
This file is part of GCC.
GCC is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; either version 2, or (at your option)
any later version.
GCC is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU General Public License for more details.
You should have received a copy of the GNU General Public License
along with GCC; see the file COPYING. If not, write to
the Free Software Foundation, 51 Franklin Street, Fifth Floor,
Boston, MA 02110-1301, USA. */
#include "config.h"
#include "system.h"
#include "coretypes.h"
#include "tm.h"
#include "tree.h"
#include "flags.h"
#include "rtl.h"
#include "tm_p.h"
#include "ggc.h"
#include "basic-block.h"
#include "output.h"
#include "expr.h"
#include "function.h"
#include "diagnostic.h"
#include "timevar.h"
#include "tree-dump.h"
#include "tree-flow.h"
#include "tree-pass.h"
#include "tree-ssa-propagate.h"
#include "langhooks.h"
/* This file implements the copy propagation pass and provides a
handful of interfaces for performing const/copy propagation and
simple expression replacement which keep variable annotations
up-to-date.
We require that for any copy operation where the RHS and LHS have
a non-null memory tag the memory tag be the same. It is OK
for one or both of the memory tags to be NULL.
We also require tracking if a variable is dereferenced in a load or
store operation.
We enforce these requirements by having all copy propagation and
replacements of one SSA_NAME with a different SSA_NAME to use the
APIs defined in this file. */
/* Return true if we may propagate ORIG into DEST, false otherwise. */
bool
may_propagate_copy (tree dest, tree orig)
{
tree type_d = TREE_TYPE (dest);
tree type_o = TREE_TYPE (orig);
/* Do not copy between types for which we *do* need a conversion. */
if (!tree_ssa_useless_type_conversion_1 (type_d, type_o))
return false;
/* FIXME. GIMPLE is allowing pointer assignments and comparisons of
pointers that have different alias sets. This means that these
pointers will have different memory tags associated to them.
If we allow copy propagation in these cases, statements de-referencing
the new pointer will now have a reference to a different memory tag
with potentially incorrect SSA information.
This was showing up in libjava/java/util/zip/ZipFile.java with code
like:
struct java.io.BufferedInputStream *T.660;
struct java.io.BufferedInputStream *T.647;
struct java.io.InputStream *is;
struct java.io.InputStream *is.662;
[ ... ]
T.660 = T.647;
is = T.660; <-- This ought to be type-casted
is.662 = is;
Also, f/name.c exposed a similar problem with a COND_EXPR predicate
that was causing DOM to generate and equivalence with two pointers of
alias-incompatible types:
struct _ffename_space *n;
struct _ffename *ns;
[ ... ]
if (n == ns)
goto lab;
...
lab:
return n;
I think that GIMPLE should emit the appropriate type-casts. For the
time being, blocking copy-propagation in these cases is the safe thing
to do. */
if (TREE_CODE (dest) == SSA_NAME
&& TREE_CODE (orig) == SSA_NAME
&& POINTER_TYPE_P (type_d)
&& POINTER_TYPE_P (type_o))
{
tree mt_dest = var_ann (SSA_NAME_VAR (dest))->symbol_mem_tag;
tree mt_orig = var_ann (SSA_NAME_VAR (orig))->symbol_mem_tag;
if (mt_dest && mt_orig && mt_dest != mt_orig)
return false;
else if (!lang_hooks.types_compatible_p (type_d, type_o))
return false;
else if (get_alias_set (TREE_TYPE (type_d)) !=
get_alias_set (TREE_TYPE (type_o)))
return false;
/* Also verify flow-sensitive information is compatible. */
if (SSA_NAME_PTR_INFO (orig) && SSA_NAME_PTR_INFO (dest))
{
struct ptr_info_def *orig_ptr_info = SSA_NAME_PTR_INFO (orig);
struct ptr_info_def *dest_ptr_info = SSA_NAME_PTR_INFO (dest);
if (orig_ptr_info->name_mem_tag
&& dest_ptr_info->name_mem_tag
&& orig_ptr_info->pt_vars
&& dest_ptr_info->pt_vars
&& !bitmap_intersect_p (dest_ptr_info->pt_vars,
orig_ptr_info->pt_vars))
return false;
}
}
/* If the destination is a SSA_NAME for a virtual operand, then we have
some special cases to handle. */
if (TREE_CODE (dest) == SSA_NAME && !is_gimple_reg (dest))
{
/* If both operands are SSA_NAMEs referring to virtual operands, then
we can always propagate. */
if (TREE_CODE (orig) == SSA_NAME
&& !is_gimple_reg (orig))
return true;
/* We have a "copy" from something like a constant into a virtual
operand. Reject these. */
return false;
}
/* If ORIG flows in from an abnormal edge, it cannot be propagated. */
if (TREE_CODE (orig) == SSA_NAME
&& SSA_NAME_OCCURS_IN_ABNORMAL_PHI (orig))
return false;
/* If DEST is an SSA_NAME that flows from an abnormal edge, then it
cannot be replaced. */
if (TREE_CODE (dest) == SSA_NAME
&& SSA_NAME_OCCURS_IN_ABNORMAL_PHI (dest))
return false;
/* Anything else is OK. */
return true;
}
/* Similarly, but we know that we're propagating into an ASM_EXPR. */
bool
may_propagate_copy_into_asm (tree dest)
{
/* Hard register operands of asms are special. Do not bypass. */
return !(TREE_CODE (dest) == SSA_NAME
&& TREE_CODE (SSA_NAME_VAR (dest)) == VAR_DECL
&& DECL_HARD_REGISTER (SSA_NAME_VAR (dest)));
}
/* Given two SSA_NAMEs pointers ORIG and NEW such that we are copy
propagating NEW into ORIG, consolidate aliasing information so that
they both share the same memory tags. */
void
merge_alias_info (tree orig, tree new)
{
tree new_sym = SSA_NAME_VAR (new);
tree orig_sym = SSA_NAME_VAR (orig);
var_ann_t new_ann = var_ann (new_sym);
var_ann_t orig_ann = var_ann (orig_sym);
gcc_assert (POINTER_TYPE_P (TREE_TYPE (orig)));
gcc_assert (POINTER_TYPE_P (TREE_TYPE (new)));
#if defined ENABLE_CHECKING
gcc_assert (lang_hooks.types_compatible_p (TREE_TYPE (orig),
TREE_TYPE (new)));
/* If the pointed-to alias sets are different, these two pointers
would never have the same memory tag. In this case, NEW should
not have been propagated into ORIG. */
gcc_assert (get_alias_set (TREE_TYPE (TREE_TYPE (new_sym)))
== get_alias_set (TREE_TYPE (TREE_TYPE (orig_sym))));
#endif
/* Synchronize the symbol tags. If both pointers had a tag and they
are different, then something has gone wrong. Symbol tags can
always be merged because they are flow insensitive, all the SSA
names of the same base DECL share the same symbol tag. */
if (new_ann->symbol_mem_tag == NULL_TREE)
new_ann->symbol_mem_tag = orig_ann->symbol_mem_tag;
else if (orig_ann->symbol_mem_tag == NULL_TREE)
orig_ann->symbol_mem_tag = new_ann->symbol_mem_tag;
else
gcc_assert (new_ann->symbol_mem_tag == orig_ann->symbol_mem_tag);
/* Check that flow-sensitive information is compatible. Notice that
we may not merge flow-sensitive information here. This function
is called when propagating equivalences dictated by the IL, like
a copy operation P_i = Q_j, and from equivalences dictated by
control-flow, like if (P_i == Q_j).
In the former case, P_i and Q_j are equivalent in every block
dominated by the assignment, so their flow-sensitive information
is always the same. However, in the latter case, the pointers
P_i and Q_j are only equivalent in one of the sub-graphs out of
the predicate, so their flow-sensitive information is not the
same in every block dominated by the predicate.
Since we cannot distinguish one case from another in this
function, we can only make sure that if P_i and Q_j have
flow-sensitive information, they should be compatible. */
if (SSA_NAME_PTR_INFO (orig) && SSA_NAME_PTR_INFO (new))
{
struct ptr_info_def *orig_ptr_info = SSA_NAME_PTR_INFO (orig);
struct ptr_info_def *new_ptr_info = SSA_NAME_PTR_INFO (new);
/* Note that pointer NEW and ORIG may actually have different
pointed-to variables (e.g., PR 18291 represented in
testsuite/gcc.c-torture/compile/pr18291.c). However, since
NEW is being copy-propagated into ORIG, it must always be
true that the pointed-to set for pointer NEW is the same, or
a subset, of the pointed-to set for pointer ORIG. If this
isn't the case, we shouldn't have been able to do the
propagation of NEW into ORIG. */
if (orig_ptr_info->name_mem_tag
&& new_ptr_info->name_mem_tag
&& orig_ptr_info->pt_vars
&& new_ptr_info->pt_vars)
gcc_assert (bitmap_intersect_p (new_ptr_info->pt_vars,
orig_ptr_info->pt_vars));
}
}
/* Common code for propagate_value and replace_exp.
Replace use operand OP_P with VAL. FOR_PROPAGATION indicates if the
replacement is done to propagate a value or not. */
static void
replace_exp_1 (use_operand_p op_p, tree val,
bool for_propagation ATTRIBUTE_UNUSED)
{
tree op = USE_FROM_PTR (op_p);
#if defined ENABLE_CHECKING
gcc_assert (!(for_propagation
&& TREE_CODE (op) == SSA_NAME
&& TREE_CODE (val) == SSA_NAME
&& !may_propagate_copy (op, val)));
#endif
if (TREE_CODE (val) == SSA_NAME)
{
if (TREE_CODE (op) == SSA_NAME && POINTER_TYPE_P (TREE_TYPE (op)))
merge_alias_info (op, val);
SET_USE (op_p, val);
}
else
SET_USE (op_p, unsave_expr_now (val));
}
/* Propagate the value VAL (assumed to be a constant or another SSA_NAME)
into the operand pointed to by OP_P.
Use this version for const/copy propagation as it will perform additional
checks to ensure validity of the const/copy propagation. */
void
propagate_value (use_operand_p op_p, tree val)
{
replace_exp_1 (op_p, val, true);
}
/* Propagate the value VAL (assumed to be a constant or another SSA_NAME)
into the tree pointed to by OP_P.
Use this version for const/copy propagation when SSA operands are not
available. It will perform the additional checks to ensure validity of
the const/copy propagation, but will not update any operand information.
Be sure to mark the stmt as modified. */
void
propagate_tree_value (tree *op_p, tree val)
{
#if defined ENABLE_CHECKING
gcc_assert (!(TREE_CODE (val) == SSA_NAME
&& TREE_CODE (*op_p) == SSA_NAME
&& !may_propagate_copy (*op_p, val)));
#endif
if (TREE_CODE (val) == SSA_NAME)
{
if (TREE_CODE (*op_p) == SSA_NAME && POINTER_TYPE_P (TREE_TYPE (*op_p)))
merge_alias_info (*op_p, val);
*op_p = val;
}
else
*op_p = unsave_expr_now (val);
}
/* Replace *OP_P with value VAL (assumed to be a constant or another SSA_NAME).
Use this version when not const/copy propagating values. For example,
PRE uses this version when building expressions as they would appear
in specific blocks taking into account actions of PHI nodes. */
void
replace_exp (use_operand_p op_p, tree val)
{
replace_exp_1 (op_p, val, false);
}
/*---------------------------------------------------------------------------
Copy propagation
---------------------------------------------------------------------------*/
/* During propagation, we keep chains of variables that are copies of
one another. If variable X_i is a copy of X_j and X_j is a copy of
X_k, COPY_OF will contain:
COPY_OF[i].VALUE = X_j
COPY_OF[j].VALUE = X_k
COPY_OF[k].VALUE = X_k
After propagation, the copy-of value for each variable X_i is
converted into the final value by walking the copy-of chains and
updating COPY_OF[i].VALUE to be the last element of the chain. */
static prop_value_t *copy_of;
/* Used in set_copy_of_val to determine if the last link of a copy-of
chain has changed. */
static tree *cached_last_copy_of;
/* True if we are doing copy propagation on loads and stores. */
static bool do_store_copy_prop;
/* Return true if this statement may generate a useful copy. */
static bool
stmt_may_generate_copy (tree stmt)
{
tree lhs, rhs;
stmt_ann_t ann;
if (TREE_CODE (stmt) == PHI_NODE)
return !SSA_NAME_OCCURS_IN_ABNORMAL_PHI (PHI_RESULT (stmt));
if (TREE_CODE (stmt) != MODIFY_EXPR)
return false;
lhs = TREE_OPERAND (stmt, 0);
rhs = TREE_OPERAND (stmt, 1);
ann = stmt_ann (stmt);
/* If the statement has volatile operands, it won't generate a
useful copy. */
if (ann->has_volatile_ops)
return false;
/* If we are not doing store copy-prop, statements with loads and/or
stores will never generate a useful copy. */
if (!do_store_copy_prop
&& !ZERO_SSA_OPERANDS (stmt, SSA_OP_ALL_VIRTUALS))
return false;
/* Otherwise, the only statements that generate useful copies are
assignments whose RHS is just an SSA name that doesn't flow
through abnormal edges. */
return (do_store_copy_prop
&& TREE_CODE (lhs) == SSA_NAME)
|| (TREE_CODE (rhs) == SSA_NAME
&& !SSA_NAME_OCCURS_IN_ABNORMAL_PHI (rhs));
}
/* Return the copy-of value for VAR. */
static inline prop_value_t *
get_copy_of_val (tree var)
{
prop_value_t *val = &copy_of[SSA_NAME_VERSION (var)];
if (val->value == NULL_TREE
&& !stmt_may_generate_copy (SSA_NAME_DEF_STMT (var)))
{
/* If the variable will never generate a useful copy relation,
make it its own copy. */
val->value = var;
val->mem_ref = NULL_TREE;
}
return val;
}
/* Return last link in the copy-of chain for VAR. */
static tree
get_last_copy_of (tree var)
{
tree last;
int i;
/* Traverse COPY_OF starting at VAR until we get to the last
link in the chain. Since it is possible to have cycles in PHI
nodes, the copy-of chain may also contain cycles.
To avoid infinite loops and to avoid traversing lengthy copy-of
chains, we artificially limit the maximum number of chains we are
willing to traverse.
The value 5 was taken from a compiler and runtime library
bootstrap and a mixture of C and C++ code from various sources.
More than 82% of all copy-of chains were shorter than 5 links. */
#define LIMIT 5
last = var;
for (i = 0; i < LIMIT; i++)
{
tree copy = copy_of[SSA_NAME_VERSION (last)].value;
if (copy == NULL_TREE || copy == last)
break;
last = copy;
}
/* If we have reached the limit, then we are either in a copy-of
cycle or the copy-of chain is too long. In this case, just
return VAR so that it is not considered a copy of anything. */
return (i < LIMIT ? last : var);
}
/* Set FIRST to be the first variable in the copy-of chain for DEST.
If DEST's copy-of value or its copy-of chain has changed, return
true.
MEM_REF is the memory reference where FIRST is stored. This is
used when DEST is a non-register and we are copy propagating loads
and stores. */
static inline bool
set_copy_of_val (tree dest, tree first, tree mem_ref)
{
unsigned int dest_ver = SSA_NAME_VERSION (dest);
tree old_first, old_last, new_last;
/* Set FIRST to be the first link in COPY_OF[DEST]. If that
changed, return true. */
old_first = copy_of[dest_ver].value;
copy_of[dest_ver].value = first;
copy_of[dest_ver].mem_ref = mem_ref;
if (old_first != first)
return true;
/* If FIRST and OLD_FIRST are the same, we need to check whether the
copy-of chain starting at FIRST ends in a different variable. If
the copy-of chain starting at FIRST ends up in a different
variable than the last cached value we had for DEST, then return
true because DEST is now a copy of a different variable.
This test is necessary because even though the first link in the
copy-of chain may not have changed, if any of the variables in
the copy-of chain changed its final value, DEST will now be the
copy of a different variable, so we have to do another round of
propagation for everything that depends on DEST. */
old_last = cached_last_copy_of[dest_ver];
new_last = get_last_copy_of (dest);
cached_last_copy_of[dest_ver] = new_last;
return (old_last != new_last);
}
/* Dump the copy-of value for variable VAR to FILE. */
static void
dump_copy_of (FILE *file, tree var)
{
tree val;
sbitmap visited;
print_generic_expr (file, var, dump_flags);
if (TREE_CODE (var) != SSA_NAME)
return;
visited = sbitmap_alloc (num_ssa_names);
sbitmap_zero (visited);
SET_BIT (visited, SSA_NAME_VERSION (var));
fprintf (file, " copy-of chain: ");
val = var;
print_generic_expr (file, val, 0);
fprintf (file, " ");
while (copy_of[SSA_NAME_VERSION (val)].value)
{
fprintf (file, "-> ");
val = copy_of[SSA_NAME_VERSION (val)].value;
print_generic_expr (file, val, 0);
fprintf (file, " ");
if (TEST_BIT (visited, SSA_NAME_VERSION (val)))
break;
SET_BIT (visited, SSA_NAME_VERSION (val));
}
val = get_copy_of_val (var)->value;
if (val == NULL_TREE)
fprintf (file, "[UNDEFINED]");
else if (val != var)
fprintf (file, "[COPY]");
else
fprintf (file, "[NOT A COPY]");
sbitmap_free (visited);
}
/* Evaluate the RHS of STMT. If it produces a valid copy, set the LHS
value and store the LHS into *RESULT_P. If STMT generates more
than one name (i.e., STMT is an aliased store), it is enough to
store the first name in the V_MAY_DEF list into *RESULT_P. After
all, the names generated will be VUSEd in the same statements. */
static enum ssa_prop_result
copy_prop_visit_assignment (tree stmt, tree *result_p)
{
tree lhs, rhs;
prop_value_t *rhs_val;
lhs = TREE_OPERAND (stmt, 0);
rhs = TREE_OPERAND (stmt, 1);
gcc_assert (TREE_CODE (rhs) == SSA_NAME);
rhs_val = get_copy_of_val (rhs);
if (TREE_CODE (lhs) == SSA_NAME)
{
/* Straight copy between two SSA names. First, make sure that
we can propagate the RHS into uses of LHS. */
if (!may_propagate_copy (lhs, rhs))
return SSA_PROP_VARYING;
/* Notice that in the case of assignments, we make the LHS be a
copy of RHS's value, not of RHS itself. This avoids keeping
unnecessary copy-of chains (assignments cannot be in a cycle
like PHI nodes), speeding up the propagation process.
This is different from what we do in copy_prop_visit_phi_node.
In those cases, we are interested in the copy-of chains. */
*result_p = lhs;
if (set_copy_of_val (*result_p, rhs_val->value, rhs_val->mem_ref))
return SSA_PROP_INTERESTING;
else
return SSA_PROP_NOT_INTERESTING;
}
else if (stmt_makes_single_store (stmt))
{
/* Otherwise, set the names in V_MAY_DEF/V_MUST_DEF operands
to be a copy of RHS. */
ssa_op_iter i;
tree vdef;
bool changed;
/* This should only be executed when doing store copy-prop. */
gcc_assert (do_store_copy_prop);
/* Set the value of every VDEF to RHS_VAL. */
changed = false;
FOR_EACH_SSA_TREE_OPERAND (vdef, stmt, i, SSA_OP_VIRTUAL_DEFS)
changed |= set_copy_of_val (vdef, rhs_val->value, lhs);
/* Note that for propagation purposes, we are only interested in
visiting statements that load the exact same memory reference
stored here. Those statements will have the exact same list
of virtual uses, so it is enough to set the output of this
statement to be its first virtual definition. */
*result_p = first_vdef (stmt);
if (changed)
return SSA_PROP_INTERESTING;
else
return SSA_PROP_NOT_INTERESTING;
}
return SSA_PROP_VARYING;
}
/* Visit the COND_EXPR STMT. Return SSA_PROP_INTERESTING
if it can determine which edge will be taken. Otherwise, return
SSA_PROP_VARYING. */
static enum ssa_prop_result
copy_prop_visit_cond_stmt (tree stmt, edge *taken_edge_p)
{
enum ssa_prop_result retval;
tree cond;
cond = COND_EXPR_COND (stmt);
retval = SSA_PROP_VARYING;
/* The only conditionals that we may be able to compute statically
are predicates involving two SSA_NAMEs. */
if (COMPARISON_CLASS_P (cond)
&& TREE_CODE (TREE_OPERAND (cond, 0)) == SSA_NAME
&& TREE_CODE (TREE_OPERAND (cond, 1)) == SSA_NAME)
{
tree op0 = get_last_copy_of (TREE_OPERAND (cond, 0));
tree op1 = get_last_copy_of (TREE_OPERAND (cond, 1));
/* See if we can determine the predicate's value. */
if (dump_file && (dump_flags & TDF_DETAILS))
{
fprintf (dump_file, "Trying to determine truth value of ");
fprintf (dump_file, "predicate ");
print_generic_stmt (dump_file, cond, 0);
}
/* We can fold COND and get a useful result only when we have
the same SSA_NAME on both sides of a comparison operator. */
if (op0 == op1)
{
tree folded_cond = fold_binary (TREE_CODE (cond), boolean_type_node,
op0, op1);
if (folded_cond)
{
basic_block bb = bb_for_stmt (stmt);
*taken_edge_p = find_taken_edge (bb, folded_cond);
if (*taken_edge_p)
retval = SSA_PROP_INTERESTING;
}
}
}
if (dump_file && (dump_flags & TDF_DETAILS) && *taken_edge_p)
fprintf (dump_file, "\nConditional will always take edge %d->%d\n",
(*taken_edge_p)->src->index, (*taken_edge_p)->dest->index);
return retval;
}
/* Evaluate statement STMT. If the statement produces a new output
value, return SSA_PROP_INTERESTING and store the SSA_NAME holding
the new value in *RESULT_P.
If STMT is a conditional branch and we can determine its truth
value, set *TAKEN_EDGE_P accordingly.
If the new value produced by STMT is varying, return
SSA_PROP_VARYING. */
static enum ssa_prop_result
copy_prop_visit_stmt (tree stmt, edge *taken_edge_p, tree *result_p)
{
enum ssa_prop_result retval;
if (dump_file && (dump_flags & TDF_DETAILS))
{
fprintf (dump_file, "\nVisiting statement:\n");
print_generic_stmt (dump_file, stmt, dump_flags);
fprintf (dump_file, "\n");
}
if (TREE_CODE (stmt) == MODIFY_EXPR
&& TREE_CODE (TREE_OPERAND (stmt, 1)) == SSA_NAME
&& (do_store_copy_prop
|| TREE_CODE (TREE_OPERAND (stmt, 0)) == SSA_NAME))
{
/* If the statement is a copy assignment, evaluate its RHS to
see if the lattice value of its output has changed. */
retval = copy_prop_visit_assignment (stmt, result_p);
}
else if (TREE_CODE (stmt) == MODIFY_EXPR
&& TREE_CODE (TREE_OPERAND (stmt, 0)) == SSA_NAME
&& do_store_copy_prop
&& stmt_makes_single_load (stmt))
{
/* If the statement is a copy assignment with a memory load
on the RHS, see if we know the value of this load and
update the lattice accordingly. */
prop_value_t *val = get_value_loaded_by (stmt, copy_of);
if (val
&& val->mem_ref
&& is_gimple_reg (val->value)
&& operand_equal_p (val->mem_ref, TREE_OPERAND (stmt, 1), 0))
{
bool changed;
changed = set_copy_of_val (TREE_OPERAND (stmt, 0),
val->value, val->mem_ref);
if (changed)
{
*result_p = TREE_OPERAND (stmt, 0);
retval = SSA_PROP_INTERESTING;
}
else
retval = SSA_PROP_NOT_INTERESTING;
}
else
retval = SSA_PROP_VARYING;
}
else if (TREE_CODE (stmt) == COND_EXPR)
{
/* See if we can determine which edge goes out of a conditional
jump. */
retval = copy_prop_visit_cond_stmt (stmt, taken_edge_p);
}
else
retval = SSA_PROP_VARYING;
if (retval == SSA_PROP_VARYING)
{
tree def;
ssa_op_iter i;
/* Any other kind of statement is not interesting for constant
propagation and, therefore, not worth simulating. */
if (dump_file && (dump_flags & TDF_DETAILS))
fprintf (dump_file, "No interesting values produced.\n");
/* The assignment is not a copy operation. Don't visit this
statement again and mark all the definitions in the statement
to be copies of nothing. */
FOR_EACH_SSA_TREE_OPERAND (def, stmt, i, SSA_OP_ALL_DEFS)
set_copy_of_val (def, def, NULL_TREE);
}
return retval;
}
/* Visit PHI node PHI. If all the arguments produce the same value,
set it to be the value of the LHS of PHI. */
static enum ssa_prop_result
copy_prop_visit_phi_node (tree phi)
{
enum ssa_prop_result retval;
int i;
tree lhs;
prop_value_t phi_val = { 0, NULL_TREE, NULL_TREE };
lhs = PHI_RESULT (phi);
if (dump_file && (dump_flags & TDF_DETAILS))
{
fprintf (dump_file, "\nVisiting PHI node: ");
print_generic_expr (dump_file, phi, dump_flags);
fprintf (dump_file, "\n\n");
}
for (i = 0; i < PHI_NUM_ARGS (phi); i++)
{
prop_value_t *arg_val;
tree arg = PHI_ARG_DEF (phi, i);
edge e = PHI_ARG_EDGE (phi, i);
/* We don't care about values flowing through non-executable
edges. */
if (!(e->flags & EDGE_EXECUTABLE))
continue;
/* Constants in the argument list never generate a useful copy.
Similarly, names that flow through abnormal edges cannot be
used to derive copies. */
if (TREE_CODE (arg) != SSA_NAME || SSA_NAME_OCCURS_IN_ABNORMAL_PHI (arg))
{
phi_val.value = lhs;
break;
}
/* Avoid copy propagation from an inner into an outer loop.
Otherwise, this may move loop variant variables outside of
their loops and prevent coalescing opportunities. If the
value was loop invariant, it will be hoisted by LICM and
exposed for copy propagation. */
if (loop_depth_of_name (arg) > loop_depth_of_name (lhs))
{
phi_val.value = lhs;
break;
}
/* If the LHS appears in the argument list, ignore it. It is
irrelevant as a copy. */
if (arg == lhs || get_last_copy_of (arg) == lhs)
continue;
if (dump_file && (dump_flags & TDF_DETAILS))
{
fprintf (dump_file, "\tArgument #%d: ", i);
dump_copy_of (dump_file, arg);
fprintf (dump_file, "\n");
}
arg_val = get_copy_of_val (arg);
/* If the LHS didn't have a value yet, make it a copy of the
first argument we find. Notice that while we make the LHS be
a copy of the argument itself, we take the memory reference
from the argument's value so that we can compare it to the
memory reference of all the other arguments. */
if (phi_val.value == NULL_TREE)
{
phi_val.value = arg;
phi_val.mem_ref = arg_val->mem_ref;
continue;
}
/* If PHI_VAL and ARG don't have a common copy-of chain, then
this PHI node cannot be a copy operation. Also, if we are
copy propagating stores and these two arguments came from
different memory references, they cannot be considered
copies. */
if (get_last_copy_of (phi_val.value) != get_last_copy_of (arg)
|| (do_store_copy_prop
&& phi_val.mem_ref
&& arg_val->mem_ref
&& simple_cst_equal (phi_val.mem_ref, arg_val->mem_ref) != 1))
{
phi_val.value = lhs;
break;
}
}
if (phi_val.value && set_copy_of_val (lhs, phi_val.value, phi_val.mem_ref))
retval = (phi_val.value != lhs) ? SSA_PROP_INTERESTING : SSA_PROP_VARYING;
else
retval = SSA_PROP_NOT_INTERESTING;
if (dump_file && (dump_flags & TDF_DETAILS))
{
fprintf (dump_file, "\nPHI node ");
dump_copy_of (dump_file, lhs);
fprintf (dump_file, "\nTelling the propagator to ");
if (retval == SSA_PROP_INTERESTING)
fprintf (dump_file, "add SSA edges out of this PHI and continue.");
else if (retval == SSA_PROP_VARYING)
fprintf (dump_file, "add SSA edges out of this PHI and never visit again.");
else
fprintf (dump_file, "do nothing with SSA edges and keep iterating.");
fprintf (dump_file, "\n\n");
}
return retval;
}
/* Initialize structures used for copy propagation. PHIS_ONLY is true
if we should only consider PHI nodes as generating copy propagation
opportunities. */
static void
init_copy_prop (void)
{
basic_block bb;
copy_of = XNEWVEC (prop_value_t, num_ssa_names);
memset (copy_of, 0, num_ssa_names * sizeof (*copy_of));
cached_last_copy_of = XNEWVEC (tree, num_ssa_names);
memset (cached_last_copy_of, 0, num_ssa_names * sizeof (*cached_last_copy_of));
FOR_EACH_BB (bb)
{
block_stmt_iterator si;
tree phi, def;
int depth = bb->loop_depth;
for (si = bsi_start (bb); !bsi_end_p (si); bsi_next (&si))
{
tree stmt = bsi_stmt (si);
ssa_op_iter iter;
/* The only statements that we care about are those that may
generate useful copies. We also need to mark conditional
jumps so that their outgoing edges are added to the work
lists of the propagator.
Avoid copy propagation from an inner into an outer loop.
Otherwise, this may move loop variant variables outside of
their loops and prevent coalescing opportunities. If the
value was loop invariant, it will be hoisted by LICM and
exposed for copy propagation. */
if (stmt_ends_bb_p (stmt))
DONT_SIMULATE_AGAIN (stmt) = false;
else if (stmt_may_generate_copy (stmt)
&& loop_depth_of_name (TREE_OPERAND (stmt, 1)) <= depth)
DONT_SIMULATE_AGAIN (stmt) = false;
else
DONT_SIMULATE_AGAIN (stmt) = true;
/* Mark all the outputs of this statement as not being
the copy of anything. */
FOR_EACH_SSA_TREE_OPERAND (def, stmt, iter, SSA_OP_ALL_DEFS)
if (DONT_SIMULATE_AGAIN (stmt))
set_copy_of_val (def, def, NULL_TREE);
else
cached_last_copy_of[SSA_NAME_VERSION (def)] = def;
}
for (phi = phi_nodes (bb); phi; phi = PHI_CHAIN (phi))
{
def = PHI_RESULT (phi);
if (!do_store_copy_prop && !is_gimple_reg (def))
DONT_SIMULATE_AGAIN (phi) = true;
else
DONT_SIMULATE_AGAIN (phi) = false;
if (DONT_SIMULATE_AGAIN (phi))
set_copy_of_val (def, def, NULL_TREE);
else
cached_last_copy_of[SSA_NAME_VERSION (def)] = def;
}
}
}
/* Deallocate memory used in copy propagation and do final
substitution. */
static void
fini_copy_prop (void)
{
size_t i;
prop_value_t *tmp;
/* Set the final copy-of value for each variable by traversing the
copy-of chains. */
tmp = XNEWVEC (prop_value_t, num_ssa_names);
memset (tmp, 0, num_ssa_names * sizeof (*tmp));
for (i = 1; i < num_ssa_names; i++)
{
tree var = ssa_name (i);
if (var && copy_of[i].value && copy_of[i].value != var)
tmp[i].value = get_last_copy_of (var);
}
substitute_and_fold (tmp, false);
free (cached_last_copy_of);
free (copy_of);
free (tmp);
}
/* Main entry point to the copy propagator.
PHIS_ONLY is true if we should only consider PHI nodes as generating
copy propagation opportunities.
The algorithm propagates the value COPY-OF using ssa_propagate. For
every variable X_i, COPY-OF(X_i) indicates which variable is X_i created
from. The following example shows how the algorithm proceeds at a
high level:
1 a_24 = x_1
2 a_2 = PHI <a_24, x_1>
3 a_5 = PHI <a_2>
4 x_1 = PHI <x_298, a_5, a_2>
The end result should be that a_2, a_5, a_24 and x_1 are a copy of
x_298. Propagation proceeds as follows.
Visit #1: a_24 is copy-of x_1. Value changed.
Visit #2: a_2 is copy-of x_1. Value changed.
Visit #3: a_5 is copy-of x_1. Value changed.
Visit #4: x_1 is copy-of x_298. Value changed.
Visit #1: a_24 is copy-of x_298. Value changed.
Visit #2: a_2 is copy-of x_298. Value changed.
Visit #3: a_5 is copy-of x_298. Value changed.
Visit #4: x_1 is copy-of x_298. Stable state reached.
When visiting PHI nodes, we only consider arguments that flow
through edges marked executable by the propagation engine. So,
when visiting statement #2 for the first time, we will only look at
the first argument (a_24) and optimistically assume that its value
is the copy of a_24 (x_1).
The problem with this approach is that it may fail to discover copy
relations in PHI cycles. Instead of propagating copy-of
values, we actually propagate copy-of chains. For instance:
A_3 = B_1;
C_9 = A_3;
D_4 = C_9;
X_i = D_4;
In this code fragment, COPY-OF (X_i) = { D_4, C_9, A_3, B_1 }.
Obviously, we are only really interested in the last value of the
chain, however the propagator needs to access the copy-of chain
when visiting PHI nodes.
To represent the copy-of chain, we use the array COPY_CHAINS, which
holds the first link in the copy-of chain for every variable.
If variable X_i is a copy of X_j, which in turn is a copy of X_k,
the array will contain:
COPY_CHAINS[i] = X_j
COPY_CHAINS[j] = X_k
COPY_CHAINS[k] = X_k
Keeping copy-of chains instead of copy-of values directly becomes
important when visiting PHI nodes. Suppose that we had the
following PHI cycle, such that x_52 is already considered a copy of
x_53:
1 x_54 = PHI <x_53, x_52>
2 x_53 = PHI <x_898, x_54>
Visit #1: x_54 is copy-of x_53 (because x_52 is copy-of x_53)
Visit #2: x_53 is copy-of x_898 (because x_54 is a copy of x_53,
so it is considered irrelevant
as a copy).
Visit #1: x_54 is copy-of nothing (x_53 is a copy-of x_898 and
x_52 is a copy of x_53, so
they don't match)
Visit #2: x_53 is copy-of nothing
This problem is avoided by keeping a chain of copies, instead of
the final copy-of value. Propagation will now only keep the first
element of a variable's copy-of chain. When visiting PHI nodes,
arguments are considered equal if their copy-of chains end in the
same variable. So, as long as their copy-of chains overlap, we
know that they will be a copy of the same variable, regardless of
which variable that may be).
Propagation would then proceed as follows (the notation a -> b
means that a is a copy-of b):
Visit #1: x_54 = PHI <x_53, x_52>
x_53 -> x_53
x_52 -> x_53
Result: x_54 -> x_53. Value changed. Add SSA edges.
Visit #1: x_53 = PHI <x_898, x_54>
x_898 -> x_898
x_54 -> x_53
Result: x_53 -> x_898. Value changed. Add SSA edges.
Visit #2: x_54 = PHI <x_53, x_52>
x_53 -> x_898
x_52 -> x_53 -> x_898
Result: x_54 -> x_898. Value changed. Add SSA edges.
Visit #2: x_53 = PHI <x_898, x_54>
x_898 -> x_898
x_54 -> x_898
Result: x_53 -> x_898. Value didn't change. Stable state
Once the propagator stabilizes, we end up with the desired result
x_53 and x_54 are both copies of x_898. */
static void
execute_copy_prop (bool store_copy_prop)
{
do_store_copy_prop = store_copy_prop;
init_copy_prop ();
ssa_propagate (copy_prop_visit_stmt, copy_prop_visit_phi_node);
fini_copy_prop ();
}
static bool
gate_copy_prop (void)
{
return flag_tree_copy_prop != 0;
}
static unsigned int
do_copy_prop (void)
{
execute_copy_prop (false);
return 0;
}
struct tree_opt_pass pass_copy_prop =
{
"copyprop", /* name */
gate_copy_prop, /* gate */
do_copy_prop, /* execute */
NULL, /* sub */
NULL, /* next */
0, /* static_pass_number */
TV_TREE_COPY_PROP, /* tv_id */
PROP_ssa | PROP_alias | PROP_cfg, /* properties_required */
0, /* properties_provided */
0, /* properties_destroyed */
0, /* todo_flags_start */
TODO_cleanup_cfg
| TODO_dump_func
| TODO_ggc_collect
| TODO_verify_ssa
| TODO_update_ssa, /* todo_flags_finish */
0 /* letter */
};
static bool
gate_store_copy_prop (void)
{
/* STORE-COPY-PROP is enabled only with -ftree-store-copy-prop, but
when -fno-tree-store-copy-prop is specified, we should run
regular COPY-PROP. That's why the pass is enabled with either
flag. */
return flag_tree_store_copy_prop != 0 || flag_tree_copy_prop != 0;
}
static unsigned int
store_copy_prop (void)
{
/* If STORE-COPY-PROP is not enabled, we just run regular COPY-PROP. */
execute_copy_prop (flag_tree_store_copy_prop != 0);
return 0;
}
struct tree_opt_pass pass_store_copy_prop =
{
"store_copyprop", /* name */
gate_store_copy_prop, /* gate */
store_copy_prop, /* execute */
NULL, /* sub */
NULL, /* next */
0, /* static_pass_number */
TV_TREE_STORE_COPY_PROP, /* tv_id */
PROP_ssa | PROP_alias | PROP_cfg, /* properties_required */
0, /* properties_provided */
0, /* properties_destroyed */
0, /* todo_flags_start */
TODO_dump_func
| TODO_cleanup_cfg
| TODO_ggc_collect
| TODO_verify_ssa
| TODO_update_ssa, /* todo_flags_finish */
0 /* letter */
};
Reference in New Issue View Git Blame Copy Permalink