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### 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.