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81 lines
3.5 KiB
ReStructuredText
81 lines
3.5 KiB
ReStructuredText
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==============================================
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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 do
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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 stored
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in a "list" structure that was reference counted. The advantage of this was that
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attributes could be manipulated with 'or's and 'and's. The disadvantage of this
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was that there was limited room for expansion, and virtually no support for
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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 attribute,
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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 next class is the AttributeSet class. This replaces the old AttributeList
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class. The AttributeSet stores a collection of Attribute objects for each kind
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of 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 at
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index "AttributeSet::ReturnIndex"; and the function's parameters' attributes are
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at indices 1, ..., n (where 'n' is the number of parameters). Most methods on
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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 remove
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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 encapsulation,
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and may be removed in a future release (i.e. 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 without
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having to create several different intermediate uniqued AttributeSet
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objects. The AttrBuilder class allows you to add and remove attributes at
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will. The attributes won't be uniqued until you call the appropriate
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"AttributeSet::get" method.
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An AttrBuilder object is *not* designed to be passed around by value. It should
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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)" c'tor. These are for backwards
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compatibility and may be removed in a future release (i.e. 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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