1=====================
2How To Use Attributes
3=====================
4
5.. contents::
6  :local:
7
8Introduction
9============
10
11Attributes in LLVM have changed in some fundamental ways.  It was necessary to
12do this to support expanding the attributes to encompass more than a handful of
13attributes --- e.g. command line options.  The old way of handling attributes
14consisted of representing them as a bit mask of values.  This bit mask was
15stored in a "list" structure that was reference counted.  The advantage of this
16was that attributes could be manipulated with 'or's and 'and's.  The
17disadvantage of this was that there was limited room for expansion, and
18virtually no support for attribute-value pairs other than alignment.
19
20In the new scheme, an ``Attribute`` object represents a single attribute that's
21uniqued.  You use the ``Attribute::get`` methods to create a new ``Attribute``
22object.  An attribute can be a single "enum" value (the enum being the
23``Attribute::AttrKind`` enum), a string representing a target-dependent
24attribute, or an attribute-value pair.  Some examples:
25
26* Target-independent: ``noinline``, ``zext``
27* Target-dependent: ``"no-sse"``, ``"thumb2"``
28* Attribute-value pair: ``"cpu" = "cortex-a8"``, ``align = 4``
29
30Note: for an attribute value pair, we expect a target-dependent attribute to
31have a string for the value.
32
33``Attribute``
34=============
35An ``Attribute`` object is designed to be passed around by value.
36
37Because attributes are no longer represented as a bit mask, you will need to
38convert any code which does treat them as a bit mask to use the new query
39methods on the Attribute class.
40
41``AttributeSet``
42================
43
44The ``AttributeSet`` class replaces the old ``AttributeList`` class.  The
45``AttributeSet`` stores a collection of Attribute objects for each kind of
46object that may have an attribute associated with it: the function as a
47whole, the return type, or the function's parameters.  A function's attributes
48are at index ``AttributeSet::FunctionIndex``; the return type's attributes are
49at index ``AttributeSet::ReturnIndex``; and the function's parameters'
50attributes are at indices 1, ..., n (where 'n' is the number of parameters).
51Most methods on the ``AttributeSet`` class take an index parameter.
52
53An ``AttributeSet`` is also a uniqued and immutable object.  You create an
54``AttributeSet`` through the ``AttributeSet::get`` methods.  You can add and
55remove attributes, which result in the creation of a new ``AttributeSet``.
56
57An ``AttributeSet`` object is designed to be passed around by value.
58
59Note: It is advised that you do *not* use the ``AttributeSet`` "introspection"
60methods (e.g. ``Raw``, ``getRawPointer``, etc.).  These methods break
61encapsulation, and may be removed in a future release (i.e. LLVM 4.0).
62
63``AttrBuilder``
64===============
65
66Lastly, we have a "builder" class to help create the ``AttributeSet`` object
67without having to create several different intermediate uniqued
68``AttributeSet`` objects.  The ``AttrBuilder`` class allows you to add and
69remove attributes at will.  The attributes won't be uniqued until you call the
70appropriate ``AttributeSet::get`` method.
71
72An ``AttrBuilder`` object is *not* designed to be passed around by value.  It
73should be passed by reference.
74
75Note: It is advised that you do *not* use the ``AttrBuilder::addRawValue()``
76method or the ``AttrBuilder(uint64_t Val)`` constructor.  These are for
77backwards compatibility and may be removed in a future release (i.e. LLVM 4.0).
78
79And that's basically it! A lot of functionality is hidden behind these classes,
80but the interfaces are pretty straight forward.
81
82