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17.10.7 Passing Arguments in Registers

This section describes the macros which let you control how various types of arguments are passed in registers or how they are arranged in the stack.

— Macro: FUNCTION_ARG (cum, mode, type, named)

A C expression that controls whether a function argument is passed in a register, and which register.

The arguments are cum, which summarizes all the previous arguments; mode, the machine mode of the argument; type, the data type of the argument as a tree node or 0 if that is not known (which happens for C support library functions); and named, which is 1 for an ordinary argument and 0 for nameless arguments that correspond to `...' in the called function's prototype. type can be an incomplete type if a syntax error has previously occurred.

The value of the expression is usually either a reg RTX for the hard register in which to pass the argument, or zero to pass the argument on the stack.

For machines like the VAX and 68000, where normally all arguments are pushed, zero suffices as a definition.

The value of the expression can also be a parallel RTX. This is used when an argument is passed in multiple locations. The mode of the parallel should be the mode of the entire argument. The parallel holds any number of expr_list pairs; each one describes where part of the argument is passed. In each expr_list the first operand must be a reg RTX for the hard register in which to pass this part of the argument, and the mode of the register RTX indicates how large this part of the argument is. The second operand of the expr_list is a const_int which gives the offset in bytes into the entire argument of where this part starts. As a special exception the first expr_list in the parallel RTX may have a first operand of zero. This indicates that the entire argument is also stored on the stack.

The last time this macro is called, it is called with MODE == VOIDmode, and its result is passed to the call or call_value pattern as operands 2 and 3 respectively.

The usual way to make the ISO library stdarg.h work on a machine where some arguments are usually passed in registers, is to cause nameless arguments to be passed on the stack instead. This is done by making FUNCTION_ARG return 0 whenever named is 0.

You may use the hook targetm.calls.must_pass_in_stack in the definition of this macro to determine if this argument is of a type that must be passed in the stack. If REG_PARM_STACK_SPACE is not defined and FUNCTION_ARG returns nonzero for such an argument, the compiler will abort. If REG_PARM_STACK_SPACE is defined, the argument will be computed in the stack and then loaded into a register.

— Target Hook: bool TARGET_MUST_PASS_IN_STACK (enum machine_mode mode, const_tree type)

This target hook should return true if we should not pass type solely in registers. The file expr.h defines a definition that is usually appropriate, refer to expr.h for additional documentation.

— Macro: FUNCTION_INCOMING_ARG (cum, mode, type, named)

Define this macro if the target machine has “register windows”, so that the register in which a function sees an arguments is not necessarily the same as the one in which the caller passed the argument.

For such machines, FUNCTION_ARG computes the register in which the caller passes the value, and FUNCTION_INCOMING_ARG should be defined in a similar fashion to tell the function being called where the arguments will arrive.

If FUNCTION_INCOMING_ARG is not defined, FUNCTION_ARG serves both purposes.

— Target Hook: int TARGET_ARG_PARTIAL_BYTES (CUMULATIVE_ARGS *cum, enum machine_mode mode, tree type, bool named)

This target hook returns the number of bytes at the beginning of an argument that must be put in registers. The value must be zero for arguments that are passed entirely in registers or that are entirely pushed on the stack.

On some machines, certain arguments must be passed partially in registers and partially in memory. On these machines, typically the first few words of arguments are passed in registers, and the rest on the stack. If a multi-word argument (a double or a structure) crosses that boundary, its first few words must be passed in registers and the rest must be pushed. This macro tells the compiler when this occurs, and how many bytes should go in registers.

FUNCTION_ARG for these arguments should return the first register to be used by the caller for this argument; likewise FUNCTION_INCOMING_ARG, for the called function.

— Target Hook: bool TARGET_PASS_BY_REFERENCE (CUMULATIVE_ARGS *cum, enum machine_mode mode, const_tree type, bool named)

This target hook should return true if an argument at the position indicated by cum should be passed by reference. This predicate is queried after target independent reasons for being passed by reference, such as TREE_ADDRESSABLE (type).

If the hook returns true, a copy of that argument is made in memory and a pointer to the argument is passed instead of the argument itself. The pointer is passed in whatever way is appropriate for passing a pointer to that type.

— Target Hook: bool TARGET_CALLEE_COPIES (CUMULATIVE_ARGS *cum, enum machine_mode mode, const_tree type, bool named)

The function argument described by the parameters to this hook is known to be passed by reference. The hook should return true if the function argument should be copied by the callee instead of copied by the caller.

For any argument for which the hook returns true, if it can be determined that the argument is not modified, then a copy need not be generated.

The default version of this hook always returns false.


A C type for declaring a variable that is used as the first argument of FUNCTION_ARG and other related values. For some target machines, the type int suffices and can hold the number of bytes of argument so far.

There is no need to record in CUMULATIVE_ARGS anything about the arguments that have been passed on the stack. The compiler has other variables to keep track of that. For target machines on which all arguments are passed on the stack, there is no need to store anything in CUMULATIVE_ARGS; however, the data structure must exist and should not be empty, so use int.

— Macro: OVERRIDE_ABI_FORMAT (fndecl)

If defined, this macro is called before generating any code for a function, but after the cfun descriptor for the function has been created. The back end may use this macro to update cfun to reflect an ABI other than that which would normally be used by default. If the compiler is generating code for a compiler-generated function, fndecl may be NULL.

— Macro: INIT_CUMULATIVE_ARGS (cum, fntype, libname, fndecl, n_named_args)

A C statement (sans semicolon) for initializing the variable cum for the state at the beginning of the argument list. The variable has type CUMULATIVE_ARGS. The value of fntype is the tree node for the data type of the function which will receive the args, or 0 if the args are to a compiler support library function. For direct calls that are not libcalls, fndecl contain the declaration node of the function. fndecl is also set when INIT_CUMULATIVE_ARGS is used to find arguments for the function being compiled. n_named_args is set to the number of named arguments, including a structure return address if it is passed as a parameter, when making a call. When processing incoming arguments, n_named_args is set to −1.

When processing a call to a compiler support library function, libname identifies which one. It is a symbol_ref rtx which contains the name of the function, as a string. libname is 0 when an ordinary C function call is being processed. Thus, each time this macro is called, either libname or fntype is nonzero, but never both of them at once.

— Macro: INIT_CUMULATIVE_LIBCALL_ARGS (cum, mode, libname)

Like INIT_CUMULATIVE_ARGS but only used for outgoing libcalls, it gets a MODE argument instead of fntype, that would be NULL. indirect would always be zero, too. If this macro is not defined, INIT_CUMULATIVE_ARGS (cum, NULL_RTX, libname, 0) is used instead.

— Macro: INIT_CUMULATIVE_INCOMING_ARGS (cum, fntype, libname)

Like INIT_CUMULATIVE_ARGS but overrides it for the purposes of finding the arguments for the function being compiled. If this macro is undefined, INIT_CUMULATIVE_ARGS is used instead.

The value passed for libname is always 0, since library routines with special calling conventions are never compiled with GCC. The argument libname exists for symmetry with INIT_CUMULATIVE_ARGS.

— Macro: FUNCTION_ARG_ADVANCE (cum, mode, type, named)

A C statement (sans semicolon) to update the summarizer variable cum to advance past an argument in the argument list. The values mode, type and named describe that argument. Once this is done, the variable cum is suitable for analyzing the following argument with FUNCTION_ARG, etc.

This macro need not do anything if the argument in question was passed on the stack. The compiler knows how to track the amount of stack space used for arguments without any special help.

— Macro: FUNCTION_ARG_OFFSET (mode, type)

If defined, a C expression that is the number of bytes to add to the offset of the argument passed in memory. This is needed for the SPU, which passes char and short arguments in the preferred slot that is in the middle of the quad word instead of starting at the top.

— Macro: FUNCTION_ARG_PADDING (mode, type)

If defined, a C expression which determines whether, and in which direction, to pad out an argument with extra space. The value should be of type enum direction: either upward to pad above the argument, downward to pad below, or none to inhibit padding.

The amount of padding is always just enough to reach the next multiple of TARGET_FUNCTION_ARG_BOUNDARY; this macro does not control it.

This macro has a default definition which is right for most systems. For little-endian machines, the default is to pad upward. For big-endian machines, the default is to pad downward for an argument of constant size shorter than an int, and upward otherwise.


If defined, a C expression which determines whether the default implementation of va_arg will attempt to pad down before reading the next argument, if that argument is smaller than its aligned space as controlled by PARM_BOUNDARY. If this macro is not defined, all such arguments are padded down if BYTES_BIG_ENDIAN is true.

— Macro: BLOCK_REG_PADDING (mode, type, first)

Specify padding for the last element of a block move between registers and memory. first is nonzero if this is the only element. Defining this macro allows better control of register function parameters on big-endian machines, without using PARALLEL rtl. In particular, MUST_PASS_IN_STACK need not test padding and mode of types in registers, as there is no longer a "wrong" part of a register; For example, a three byte aggregate may be passed in the high part of a register if so required.

— Target Hook: unsigned int TARGET_FUNCTION_ARG_BOUNDARY (enum machine_mode mode, const_tree type)

This hook returns the alignment boundary, in bits, of an argument with the specified mode and type. The default hook returns PARM_BOUNDARY for all arguments.

— Macro: FUNCTION_ARG_REGNO_P (regno)

A C expression that is nonzero if regno is the number of a hard register in which function arguments are sometimes passed. This does not include implicit arguments such as the static chain and the structure-value address. On many machines, no registers can be used for this purpose since all function arguments are pushed on the stack.

— Target Hook: bool TARGET_SPLIT_COMPLEX_ARG (const_tree type)

This hook should return true if parameter of type type are passed as two scalar parameters. By default, GCC will attempt to pack complex arguments into the target's word size. Some ABIs require complex arguments to be split and treated as their individual components. For example, on AIX64, complex floats should be passed in a pair of floating point registers, even though a complex float would fit in one 64-bit floating point register.

The default value of this hook is NULL, which is treated as always false.

— Target Hook: tree TARGET_BUILD_BUILTIN_VA_LIST (void)

This hook returns a type node for va_list for the target. The default version of the hook returns void*.

— Target Hook: int TARGET_ENUM_VA_LIST_P (int idx, const char **pname, tree *ptree)

This target hook is used in function c_common_nodes_and_builtins to iterate through the target specific builtin types for va_list. The variable idx is used as iterator. pname has to be a pointer to a const char * and ptree a pointer to a tree typed variable. The arguments pname and ptree are used to store the result of this macro and are set to the name of the va_list builtin type and its internal type. If the return value of this macro is zero, then there is no more element. Otherwise the IDX should be increased for the next call of this macro to iterate through all types.

— Target Hook: tree TARGET_FN_ABI_VA_LIST (tree fndecl)

This hook returns the va_list type of the calling convention specified by fndecl. The default version of this hook returns va_list_type_node.

— Target Hook: tree TARGET_CANONICAL_VA_LIST_TYPE (tree type)

This hook returns the va_list type of the calling convention specified by the type of type. If type is not a valid va_list type, it returns NULL_TREE.

— Target Hook: tree TARGET_GIMPLIFY_VA_ARG_EXPR (tree valist, tree type, gimple_seq *pre_p, gimple_seq *post_p)

This hook performs target-specific gimplification of VA_ARG_EXPR. The first two parameters correspond to the arguments to va_arg; the latter two are as in gimplify.c:gimplify_expr.

— Target Hook: bool TARGET_VALID_POINTER_MODE (enum machine_mode mode)

Define this to return nonzero if the port can handle pointers with machine mode mode. The default version of this hook returns true for both ptr_mode and Pmode.

— Target Hook: bool TARGET_REF_MAY_ALIAS_ERRNO (struct ao_ref_s *ref)

Define this to return nonzero if the memory reference ref may alias with the system C library errno location. The default version of this hook assumes the system C library errno location is either a declaration of type int or accessed by dereferencing a pointer to int.

— Target Hook: bool TARGET_SCALAR_MODE_SUPPORTED_P (enum machine_mode mode)

Define this to return nonzero if the port is prepared to handle insns involving scalar mode mode. For a scalar mode to be considered supported, all the basic arithmetic and comparisons must work.

The default version of this hook returns true for any mode required to handle the basic C types (as defined by the port). Included here are the double-word arithmetic supported by the code in optabs.c.

— Target Hook: bool TARGET_VECTOR_MODE_SUPPORTED_P (enum machine_mode mode)

Define this to return nonzero if the port is prepared to handle insns involving vector mode mode. At the very least, it must have move patterns for this mode.

— Target Hook: bool TARGET_SMALL_REGISTER_CLASSES_FOR_MODE_P (enum machine_mode mode)

Define this to return nonzero for machine modes for which the port has small register classes. If this target hook returns nonzero for a given mode, the compiler will try to minimize the lifetime of registers in mode. The hook may be called with VOIDmode as argument. In this case, the hook is expected to return nonzero if it returns nonzero for any mode.

On some machines, it is risky to let hard registers live across arbitrary insns. Typically, these machines have instructions that require values to be in specific registers (like an accumulator), and reload will fail if the required hard register is used for another purpose across such an insn.

Passes before reload do not know which hard registers will be used in an instruction, but the machine modes of the registers set or used in the instruction are already known. And for some machines, register classes are small for, say, integer registers but not for floating point registers. For example, the AMD x86-64 architecture requires specific registers for the legacy x86 integer instructions, but there are many SSE registers for floating point operations. On such targets, a good strategy may be to return nonzero from this hook for INTEGRAL_MODE_P machine modes but zero for the SSE register classes.

The default version of this hook returns false for any mode. It is always safe to redefine this hook to return with a nonzero value. But if you unnecessarily define it, you will reduce the amount of optimizations that can be performed in some cases. If you do not define this hook to return a nonzero value when it is required, the compiler will run out of spill registers and print a fatal error message.

— Target Hook: unsigned int TARGET_FLAGS_REGNUM

If the target has a dedicated flags register, and it needs to use the post-reload comparison elimination pass, then this value should be set appropriately.