Functions

A function is represented by a FUNCTION_DECL node. It stores the basic pieces of the function such as body, parameters, and return type as well as information on the surrounding context, visibility, and linkage.

Function Basics

A function has four core parts: the name, the parameters, the result, and the body. The following macros and functions access these parts of a FUNCTION_DECL as well as other basic features:

DECL_NAME

This macro returns the unqualified name of the function, as an IDENTIFIER_NODE. For an instantiation of a function template, the DECL_NAME is the unqualified name of the template, not something like f<int>. The value of DECL_NAME is undefined when used on a constructor, destructor, overloaded operator, or type-conversion operator, or any function that is implicitly generated by the compiler. See below for macros that can be used to distinguish these cases.

DECL_ASSEMBLER_NAME

This macro returns the mangled name of the function, also an IDENTIFIER_NODE. This name does not contain leading underscores on systems that prefix all identifiers with underscores. The mangled name is computed in the same way on all platforms; if special processing is required to deal with the object file format used on a particular platform, it is the responsibility of the back end to perform those modifications. (Of course, the back end should not modify DECL_ASSEMBLER_NAME itself.)

Using DECL_ASSEMBLER_NAME will cause additional memory to be allocated (for the mangled name of the entity) so it should be used only when emitting assembly code. It should not be used within the optimizers to determine whether or not two declarations are the same, even though some of the existing optimizers do use it in that way. These uses will be removed over time.

DECL_ARGUMENTS

This macro returns the PARM_DECL for the first argument to the function. Subsequent PARM_DECL nodes can be obtained by following the TREE_CHAIN links.

DECL_RESULT

This macro returns the RESULT_DECL for the function.

DECL_SAVED_TREE

This macro returns the complete body of the function.

TREE_TYPE

This macro returns the FUNCTION_TYPE or METHOD_TYPE for the function.

DECL_INITIAL

A function that has a definition in the current translation unit will have a non- NULL DECL_INITIAL. However, back ends should not make use of the particular value given by DECL_INITIAL.

It should contain a tree of BLOCK nodes that mirrors the scopes that variables are bound in the function. Each block contains a list of decls declared in a basic block, a pointer to a chain of blocks at the next lower scope level, then a pointer to the next block at the same level and a backpointer to the parent BLOCK or FUNCTION_DECL. So given a function as follows:

void foo()
{
  int a;
  {
    int b;
  }
  int c;
}

you would get the following:

tree foo = FUNCTION_DECL;
tree decl_a = VAR_DECL;
tree decl_b = VAR_DECL;
tree decl_c = VAR_DECL;
tree block_a = BLOCK;
tree block_b = BLOCK;
tree block_c = BLOCK;
BLOCK_VARS(block_a) = decl_a;
BLOCK_SUBBLOCKS(block_a) = block_b;
BLOCK_CHAIN(block_a) = block_c;
BLOCK_SUPERCONTEXT(block_a) = foo;
BLOCK_VARS(block_b) = decl_b;
BLOCK_SUPERCONTEXT(block_b) = block_a;
BLOCK_VARS(block_c) = decl_c;
BLOCK_SUPERCONTEXT(block_c) = foo;
DECL_INITIAL(foo) = block_a;

Function Properties

To determine the scope of a function, you can use the DECL_CONTEXT macro. This macro will return the class (either a RECORD_TYPE or a UNION_TYPE) or namespace (a NAMESPACE_DECL) of which the function is a member. For a virtual function, this macro returns the class in which the function was actually defined, not the base class in which the virtual declaration occurred.

In C, the DECL_CONTEXT for a function maybe another function. This representation indicates that the GNU nested function extension is in use. For details on the semantics of nested functions, see the GCC Manual. The nested function can refer to local variables in its containing function. Such references are not explicitly marked in the tree structure; back ends must look at the DECL_CONTEXT for the referenced VAR_DECL. If the DECL_CONTEXT for the referenced VAR_DECL is not the same as the function currently being processed, and neither DECL_EXTERNAL nor TREE_STATIC hold, then the reference is to a local variable in a containing function, and the back end must take appropriate action.

DECL_EXTERNAL

This predicate holds if the function is undefined.

TREE_PUBLIC

This predicate holds if the function has external linkage.

TREE_STATIC

This predicate holds if the function has been defined.

TREE_THIS_VOLATILE

This predicate holds if the function does not return normally.

TREE_READONLY

This predicate holds if the function can only read its arguments.

DECL_PURE_P

This predicate holds if the function can only read its arguments, but may also read global memory.

DECL_VIRTUAL_P

This predicate holds if the function is virtual.

DECL_ARTIFICIAL

This macro holds if the function was implicitly generated by the compiler, rather than explicitly declared. In addition to implicitly generated class member functions, this macro holds for the special functions created to implement static initialization and destruction, to compute run-time type information, and so forth.

DECL_FUNCTION_SPECIFIC_TARGET

This macro returns a tree node that holds the target options that are to be used to compile this particular function or NULL_TREE if the function is to be compiled with the target options specified on the command line.

DECL_FUNCTION_SPECIFIC_OPTIMIZATION

This macro returns a tree node that holds the optimization options that are to be used to compile this particular function or NULL_TREE if the function is to be compiled with the optimization options specified on the command line.