Aggregate Return - GNU Compiler Collection (GCC) Internals

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15.10.9 How Large Values Are Returned

When a function value's mode is BLKmode (and in some other cases), the value is not returned according to TARGET_FUNCTION_VALUE (see Scalar Return). Instead, the caller passes the address of a block of memory in which the value should be stored. This address is called the structure value address.

This section describes how to control returning structure values in memory.

— Target Hook: bool TARGET_RETURN_IN_MEMORY (tree type, tree fntype)

This target hook should return a nonzero value to say to return the function value in memory, just as large structures are always returned. Here type will be the data type of the value, and fntype will be the type of the function doing the returning, or NULL for libcalls.
Note that values of mode BLKmode must be explicitly handled by this function. Also, the option -fpcc-struct-return takes effect regardless of this macro. On most systems, it is possible to leave the hook undefined; this causes a default definition to be used, whose value is the constant 1 for BLKmode values, and 0 otherwise.
Do not use this hook to indicate that structures and unions should always be returned in memory. You should instead use DEFAULT_PCC_STRUCT_RETURN to indicate this.

— Macro: DEFAULT_PCC_STRUCT_RETURN

Define this macro to be 1 if all structure and union return values must be in memory. Since this results in slower code, this should be defined only if needed for compatibility with other compilers or with an ABI. If you define this macro to be 0, then the conventions used for structure and union return values are decided by the TARGET_RETURN_IN_MEMORY target hook.
If not defined, this defaults to the value 1.

— Target Hook: rtx TARGET_STRUCT_VALUE_RTX (tree fndecl, int incoming)

This target hook should return the location of the structure value address (normally a mem or reg), or 0 if the address is passed as an “invisible” first argument. Note that fndecl may be NULL, for libcalls. You do not need to define this target hook if the address is always passed as an “invisible” first argument.
On some architectures the place where the structure value address is found by the called function is not the same place that the caller put it. This can be due to register windows, or it could be because the function prologue moves it to a different place. incoming is 1 or 2 when the location is needed in the context of the called function, and 0 in the context of the caller.
If incoming is nonzero and the address is to be found on the stack, return a mem which refers to the frame pointer. If incoming is 2, the result is being used to fetch the structure value address at the beginning of a function. If you need to emit adjusting code, you should do it at this point.

— Macro: PCC_STATIC_STRUCT_RETURN

Define this macro if the usual system convention on the target machine for returning structures and unions is for the called function to return the address of a static variable containing the value.
Do not define this if the usual system convention is for the caller to pass an address to the subroutine.
This macro has effect in -fpcc-struct-return mode, but it does nothing when you use -freg-struct-return mode.