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14.9.8 How Scalar Function Values Are Returned

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

— Macro: FUNCTION_VALUE (valtype, func)

A C expression to create an RTX representing the place where a function returns a value of data type valtype. valtype is a tree node representing a data type. Write TYPE_MODE (valtype) to get the machine mode used to represent that type. On many machines, only the mode 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 FUNCTION_ARG for an explanation of the parallel form.

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.

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

— Macro: FUNCTION_OUTGOING_VALUE (valtype, func)

Define this macro if the target machine has “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, FUNCTION_VALUE computes the register in which the caller will see the value. FUNCTION_OUTGOING_VALUE should be defined in a similar fashion to tell the function where to put the value.

If FUNCTION_OUTGOING_VALUE is not defined, FUNCTION_VALUE serves both purposes.

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

— Macro: LIBCALL_VALUE (mode)

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

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.

The definition of LIBRARY_VALUE need not be concerned aggregate data types, because none of the library functions returns such types.

— 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.

— 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_RETURN_IN_MSB (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 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, FUNCTION_VALUE should provide an SImode rtx.