GNU Compiler Collection (GCC) Internals: Scalar Return

GNU Compiler Collection (GCC) Internals: Scalar Return

17.9.8 How Scalar Function Values Are Returned

This section discusses the macros that control returning scalars as values—values that can fit in registers.

Target Hook: rtx TARGET_FUNCTION_VALUE (const_tree ret_type, const_tree fn_decl_or_type, bool outgoing)

Define this to return an RTX representing the place where a function returns or receives a value of data type ret_type, a tree node representing a data type. fn_decl_or_type is a tree node representing FUNCTION_DECL or FUNCTION_TYPE of a function being called. If outgoing is false, the hook should compute the register in which the caller will see the return value. Otherwise, the hook should return an RTX representing the place where a function returns a value.

On many machines, only TYPE_MODE (ret_type) is relevant. (Actually, on most machines, scalar values are returned in the same place regardless of mode.) The value of the expression is usually a reg RTX for the hard register where the return value is stored. The value can also be a parallel RTX, if the return value is in multiple places. See TARGET_FUNCTION_ARG for an explanation of the parallel form. Note that the callee will populate every location specified in the parallel, but if the first element of the parallel contains the whole return value, callers will use that element as the canonical location and ignore the others. The m68k port uses this type of parallel to return pointers in both ‘%a0’ (the canonical location) and ‘%d0’.

If TARGET_PROMOTE_FUNCTION_RETURN returns true, you must apply the same promotion rules specified in PROMOTE_MODE if valtype is a scalar type.

If the precise function being called is known, func is a tree node (FUNCTION_DECL) for it; otherwise, func is a null pointer. This makes it possible to use a different value-returning convention for specific functions when all their calls are known.

Some target machines have “register windows” so that the register in which a function returns its value is not the same as the one in which the caller sees the value. For such machines, you should return different RTX depending on outgoing.

TARGET_FUNCTION_VALUE is not used for return values with aggregate data types, because these are returned in another way. See TARGET_STRUCT_VALUE_RTX and related macros, below.

Macro: FUNCTION_VALUE (valtype, func): This macro has been deprecated. Use TARGET_FUNCTION_VALUE for a new target instead.

Macro: LIBCALL_VALUE (mode)

A C expression to create an RTX representing the place where a library function returns a value of mode mode.

Note that “library function” in this context means a compiler support routine, used to perform arithmetic, whose name is known specially by the compiler and was not mentioned in the C code being compiled.

Target Hook: rtx TARGET_LIBCALL_VALUE (machine_mode mode, const_rtx fun)

Define this hook if the back-end needs to know the name of the libcall function in order to determine where the result should be returned.

The mode of the result is given by mode and the name of the called library function is given by fun. The hook should return an RTX representing the place where the library function result will be returned.

If this hook is not defined, then LIBCALL_VALUE will be used.

Macro: FUNCTION_VALUE_REGNO_P (regno)

A C expression that is nonzero if regno is the number of a hard register in which the values of called function may come back.

A register whose use for returning values is limited to serving as the second of a pair (for a value of type double, say) need not be recognized by this macro. So for most machines, this definition suffices:

#define FUNCTION_VALUE_REGNO_P(N) ((N) == 0)

If the machine has register windows, so that the caller and the called function use different registers for the return value, this macro should recognize only the caller’s register numbers.

This macro has been deprecated. Use TARGET_FUNCTION_VALUE_REGNO_P for a new target instead.

Target Hook: bool TARGET_FUNCTION_VALUE_REGNO_P (const unsigned int regno)

A target hook that return true if regno is the number of a hard register in which the values of called function may come back.

A register whose use for returning values is limited to serving as the second of a pair (for a value of type double, say) need not be recognized by this target hook.

If the machine has register windows, so that the caller and the called function use different registers for the return value, this target hook should recognize only the caller’s register numbers.

If this hook is not defined, then FUNCTION_VALUE_REGNO_P will be used.

Macro: APPLY_RESULT_SIZE: Define this macro if ‘untyped_call’ and ‘untyped_return’ need more space than is implied by FUNCTION_VALUE_REGNO_P for saving and restoring an arbitrary return value.

Target Hook: bool TARGET_OMIT_STRUCT_RETURN_REG: Normally, when a function returns a structure by memory, the address is passed as an invisible pointer argument, but the compiler also arranges to return the address from the function like it would a normal pointer return value. Define this to true if that behavior is undesirable on your target.

Target Hook: bool TARGET_RETURN_IN_MSB (const_tree type)

This hook should return true if values of type type are returned at the most significant end of a register (in other words, if they are padded at the least significant end). You can assume that type is returned in a register; the caller is required to check this.

Note that the register provided by TARGET_FUNCTION_VALUE must be able to hold the complete return value. For example, if a 1-, 2- or 3-byte structure is returned at the most significant end of a 4-byte register, TARGET_FUNCTION_VALUE should provide an SImode rtx.