82 lines
3.5 KiB
ReStructuredText
82 lines
3.5 KiB
ReStructuredText
|
=====================
|
||
|
|
How To Use Attributes
|
||
|
|
=====================
|
||
|
|
|
||
|
|
.. contents::
|
||
|
|
:local:
|
||
|
|
|
||
|
|
Introduction
|
||
|
|
============
|
||
|
|
|
||
|
|
Attributes in LLVM have changed in some fundamental ways. It was necessary to
|
||
|
|
do this to support expanding the attributes to encompass more than a handful of
|
||
|
|
attributes --- e.g. command line options. The old way of handling attributes
|
||
|
|
consisted of representing them as a bit mask of values. This bit mask was
|
||
|
|
stored in a "list" structure that was reference counted. The advantage of this
|
||
|
|
was that attributes could be manipulated with 'or's and 'and's. The
|
||
|
|
disadvantage of this was that there was limited room for expansion, and
|
||
|
|
virtually no support for attribute-value pairs other than alignment.
|
||
|
|
|
||
|
|
In the new scheme, an ``Attribute`` object represents a single attribute that's
|
||
|
|
uniqued. You use the ``Attribute::get`` methods to create a new ``Attribute``
|
||
|
|
object. An attribute can be a single "enum" value (the enum being the
|
||
|
|
``Attribute::AttrKind`` enum), a string representing a target-dependent
|
||
|
|
attribute, or an attribute-value pair. Some examples:
|
||
|
|
|
||
|
|
* Target-independent: ``noinline``, ``zext``
|
||
|
|
* Target-dependent: ``"no-sse"``, ``"thumb2"``
|
||
|
|
* Attribute-value pair: ``"cpu" = "cortex-a8"``, ``align = 4``
|
||
|
|
|
||
|
|
Note: for an attribute value pair, we expect a target-dependent attribute to
|
||
|
|
have a string for the value.
|
||
|
|
|
||
|
|
``Attribute``
|
||
|
|
=============
|
||
|
|
An ``Attribute`` object is designed to be passed around by value.
|
||
|
|
|
||
|
|
Because attributes are no longer represented as a bit mask, you will need to
|
||
|
|
convert any code which does treat them as a bit mask to use the new query
|
||
|
|
methods on the Attribute class.
|
||
|
|
|
||
|
|
``AttributeSet``
|
||
|
|
================
|
||
|
|
|
||
|
|
The ``AttributeSet`` class replaces the old ``AttributeList`` class. The
|
||
|
|
``AttributeSet`` stores a collection of Attribute objects for each kind of
|
||
|
|
object that may have an attribute associated with it: the function as a
|
||
|
|
whole, the return type, or the function's parameters. A function's attributes
|
||
|
|
are at index ``AttributeSet::FunctionIndex``; the return type's attributes are
|
||
|
|
at index ``AttributeSet::ReturnIndex``; and the function's parameters'
|
||
|
|
attributes are at indices 1, ..., n (where 'n' is the number of parameters).
|
||
|
|
Most methods on the ``AttributeSet`` class take an index parameter.
|
||
|
|
|
||
|
|
An ``AttributeSet`` is also a uniqued and immutable object. You create an
|
||
|
|
``AttributeSet`` through the ``AttributeSet::get`` methods. You can add and
|
||
|
|
remove attributes, which result in the creation of a new ``AttributeSet``.
|
||
|
|
|
||
|
|
An ``AttributeSet`` object is designed to be passed around by value.
|
||
|
|
|
||
|
|
Note: It is advised that you do *not* use the ``AttributeSet`` "introspection"
|
||
|
|
methods (e.g. ``Raw``, ``getRawPointer``, etc.). These methods break
|
||
|
|
encapsulation, and may be removed in a future release (i.e. LLVM 4.0).
|
||
|
|
|
||
|
|
``AttrBuilder``
|
||
|
|
===============
|
||
|
|
|
||
|
|
Lastly, we have a "builder" class to help create the ``AttributeSet`` object
|
||
|
|
without having to create several different intermediate uniqued
|
||
|
|
``AttributeSet`` objects. The ``AttrBuilder`` class allows you to add and
|
||
|
|
remove attributes at will. The attributes won't be uniqued until you call the
|
||
|
|
appropriate ``AttributeSet::get`` method.
|
||
|
|
|
||
|
|
An ``AttrBuilder`` object is *not* designed to be passed around by value. It
|
||
|
|
should be passed by reference.
|
||
|
|
|
||
|
|
Note: It is advised that you do *not* use the ``AttrBuilder::addRawValue()``
|
||
|
|
method or the ``AttrBuilder(uint64_t Val)`` constructor. These are for
|
||
|
|
backwards compatibility and may be removed in a future release (i.e. LLVM 4.0).
|
||
|
|
|
||
|
|
And that's basically it! A lot of functionality is hidden behind these classes,
|
||
|
|
but the interfaces are pretty straight forward.
|
||
|
|
|