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 (i.e. LLVM 4.0).

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 (i.e. LLVM 4.0).

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