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