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