llvm/docs/HowToUseAttributes.rst

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How To Use Attributes
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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 next class is the AttributeSet class. This 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. 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)" c'tor. These are for backwards
compatibility and may be removed in a future release (i.e. 4.0).

And that's basically it! A lot of functionality is hidden behind these classes,
but the interfaces are pretty straight forward.