13.20 RTL Representation of Function-Call Insns

Insns that call subroutines have the RTL expression code call_insn. These insns must satisfy special rules, and their bodies must use a special RTL expression code, call.

A call expression has two operands, as follows:

(call (mem:fm addr) nbytes)

Here nbytes is an operand that represents the number of bytes of argument data being passed to the subroutine, fm is a machine mode (which must equal as the definition of the FUNCTION_MODE macro in the machine description) and addr represents the address of the subroutine.

For a subroutine that returns no value, the call expression as shown above is the entire body of the insn, except that the insn might also contain use or clobber expressions.

For a subroutine that returns a value whose mode is not BLKmode, the value is returned in a hard register. If this register’s number is r, then the body of the call insn looks like this:

(set (reg:m r)
     (call (mem:fm addr) nbytes))

This RTL expression makes it clear (to the optimizer passes) that the appropriate register receives a useful value in this insn.

When a subroutine returns a BLKmode value, it is handled by passing to the subroutine the address of a place to store the value. So the call insn itself does not “return” any value, and it has the same RTL form as a call that returns nothing.

On some machines, the call instruction itself clobbers some register, for example to contain the return address. call_insn insns on these machines should have a body which is a parallel that contains both the call expression and clobber expressions that indicate which registers are destroyed. Similarly, if the call instruction requires some register other than the stack pointer that is not explicitly mentioned in its RTL, a use subexpression should mention that register.

Functions that are called are assumed to modify all registers listed in the configuration macro CALL_USED_REGISTERS (see Register Basics) and, with the exception of const functions and library calls, to modify all of memory.

Insns containing just use expressions directly precede the call_insn insn to indicate which registers contain inputs to the function. Similarly, if registers other than those in CALL_USED_REGISTERS are clobbered by the called function, insns containing a single clobber follow immediately after the call to indicate which registers.