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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.
Return an RTX indicating whether a function argument is passed in a register and if so, which register.
The arguments are ca, 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 true for an ordinary argument and false 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 return value 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.
The return value can be a const_int which means argument is
passed in a target specific slot with specified number. Target hooks
should be used to store or load argument in such case. See
TARGET_STORE_BOUNDS_FOR_ARG and TARGET_LOAD_BOUNDS_FOR_ARG
for more information.
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 hook 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 TARGET_FUNCTION_ARG return 0 whenever
named is false.
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 TARGET_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.
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.
Define this hook if the caller and callee on the target have different views of where arguments are passed. Also define this hook if there are functions that are never directly called, but are invoked by the hardware and which have nonstandard calling conventions.
In this case TARGET_FUNCTION_ARG computes the register in
which the caller passes the value, and
TARGET_FUNCTION_INCOMING_ARG should be defined in a similar
fashion to tell the function being called where the arguments will
arrive.
TARGET_FUNCTION_INCOMING_ARG can also return arbitrary address
computation using hard register, which can be forced into a register,
so that it can be used to pass special arguments.
If TARGET_FUNCTION_INCOMING_ARG is not defined,
TARGET_FUNCTION_ARG serves both purposes.
This hook should return 1 in case pseudo register should be created for pic_offset_table_rtx during function expand.
Perform a target dependent initialization of pic_offset_table_rtx. This hook is called at the start of register allocation.
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.
TARGET_FUNCTION_ARG for these arguments should return the first
register to be used by the caller for this argument; likewise
TARGET_FUNCTION_INCOMING_ARG, for the called function.
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.
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 TARGET_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.
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.
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.
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.
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.
This hook updates the summarizer variable pointed to by ca 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 TARGET_FUNCTION_ARG, etc.
This hook 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.
This hook returns the number of bytes to add to the offset of an
argument of type type and mode mode when 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. The default implementation returns 0.
This hook determines whether, and in which direction, to pad out
an argument of mode mode and type type. It returns
PAD_UPWARD to insert padding above the argument, PAD_DOWNWARD
to insert padding below the argument, or PAD_NONE to inhibit padding.
The amount of padding is not controlled by this hook, but by
TARGET_FUNCTION_ARG_ROUND_BOUNDARY. It is always just enough
to reach the next multiple of that boundary.
This hook has a default definition that 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.
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.
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.
Normally, the size of an argument is rounded up to PARM_BOUNDARY,
which is the default value for this hook. You can define this hook to
return a different value if an argument size must be rounded to a larger
value.
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.
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.
This hook returns a type node for va_list for the target.
The default version of the hook returns void*.
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.
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.
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.
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.
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.
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.
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.
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.
Return the mode that GCC should use for an array that has
nelems elements, with each element having mode mode.
Return no mode if the target has no special requirements. In the
latter case, GCC looks for an integer mode of the appropriate size
if available and uses BLKmode otherwise. Usually the search for the
integer mode is limited to MAX_FIXED_MODE_SIZE, but the
TARGET_ARRAY_MODE_SUPPORTED_P hook allows a larger mode to be
used in specific cases.
The main use of this hook is to specify that an array of vectors should also have a vector mode. The default implementation returns no mode.
Return true if GCC should try to use a scalar mode to store an array
of nelems elements, given that each element has mode mode.
Returning true here overrides the usual MAX_FIXED_MODE limit
and allows GCC to use any defined integer mode.
One use of this hook is to support vector load and store operations that operate on several homogeneous vectors. For example, ARM NEON has operations like:
int8x8x3_t vld3_s8 (const int8_t *)
where the return type is defined as:
typedef struct int8x8x3_t
{
int8x8_t val[3];
} int8x8x3_t;
If this hook allows val to have a scalar mode, then
int8x8x3_t can have the same mode. GCC can then store
int8x8x3_ts in registers rather than forcing them onto the stack.
Define this to return nonzero if libgcc provides support for the
floating-point mode mode, which is known to pass
TARGET_SCALAR_MODE_SUPPORTED_P. The default version of this
hook returns true for all of SFmode, DFmode,
XFmode and TFmode, if such modes exist.
Define this to return the machine mode to use for the type
_Floatn, if extended is false, or the type
_Floatnx, if extended is true. If such a type is not
supported, return opt_scalar_float_mode (). The default version of
this hook returns SFmode for _Float32, DFmode for
_Float64 and _Float32x and TFmode for
_Float128, if those modes exist and satisfy the requirements for
those types and pass TARGET_SCALAR_MODE_SUPPORTED_P and
TARGET_LIBGCC_FLOATING_MODE_SUPPORTED_P; for _Float64x, it
returns the first of XFmode and TFmode that exists and
satisfies the same requirements; for other types, it returns
opt_scalar_float_mode (). The hook is only called for values
of n and extended that are valid according to
ISO/IEC TS 18661-3:2015; that is, n is one of 32, 64, 128, or,
if extended is false, 16 or greater than 128 and a multiple of 32.
Define this to return true if the _Floatn and
_Floatnx built-in functions should implicitly enable the
built-in function without the __builtin_ prefix in addition to the
normal built-in function with the __builtin_ prefix. The default is
to only enable built-in functions without the __builtin_ prefix for
the GNU C langauge. In strict ANSI/ISO mode, the built-in function without
the __builtin_ prefix is not enabled. The argument FUNC is the
enum built_in_function id of the function to be enabled.
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.
Next: Scalar Return, Previous: Stack Arguments, Up: Stack and Calling [Contents][Index]