| # How To Use Attributes |
| |
| ## 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. |
| |
| ## `AttributeList` |
| |
| The `AttributeList` 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 `AttributeList::FunctionIndex`; the return type's attributes are at |
| index `AttributeList::ReturnIndex`; and the function's parameters' attributes |
| are at indices 1, ..., n (where 'n' is the number of parameters). Most methods |
| on the `AttributeList` class take an index parameter. |
| |
| An `AttributeList` is also a uniqued and immutable object. You create an |
| `AttributeList` through the `AttributeList::get` methods. You can add and |
| remove attributes, which result in the creation of a new `AttributeList`. |
| |
| An `AttributeList` object is designed to be passed around by value. |
| |
| Note: It is advised that you do *not* use the `AttributeList` "introspection" |
| methods (e.g. `Raw`, `getRawPointer`, etc.). These methods break |
| encapsulation, and may be removed in a future release. |
| |
| ## `AttrBuilder` |
| |
| Lastly, we have a "builder" class to help create the `AttributeList` object |
| without having to create several different intermediate uniqued |
| `AttributeList` objects. The `AttrBuilder` class allows you to add and |
| remove attributes at will. The attributes won't be uniqued until you call the |
| appropriate `AttributeList::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)` constructor. These are for |
| backwards compatibility and may be removed in a future release. |
| |
| And that's basically it! A lot of functionality is hidden behind these classes, |
| but the interfaces are pretty straight forward. |
| |
