17.11 Implementing the Varargs Macros
GCC comes with an implementation of
<stdarg.h> that work without change on machines that pass arguments
on the stack. Other machines require their own implementations of
varargs, and the two machine independent header files must have
conditionals to include it.
<stdarg.h> differs from traditional
<varargs.h> mainly in
the calling convention for
va_start. The traditional
implementation takes just one argument, which is the variable in which
to store the argument pointer. The ISO implementation of
va_start takes an additional second argument. The user is
supposed to write the last named argument of the function here.
va_start should not use this argument. The way to find
the end of the named arguments is with the built-in functions described
Use this built-in function to save the argument registers in memory so that the varargs mechanism can access them. Both ISO and traditional versions of
__builtin_saveregs, unless you use
TARGET_SETUP_INCOMING_VARARGS(see below) instead.
On some machines,
__builtin_saveregsis open-coded under the control of the target hook
TARGET_EXPAND_BUILTIN_SAVEREGS. On other machines, it calls a routine written in assembler language, found in libgcc2.c.
Code generated for the call to
__builtin_saveregsappears at the beginning of the function, as opposed to where the call to
__builtin_saveregsis written, regardless of what the code is. This is because the registers must be saved before the function starts to use them for its own purposes.
Use this built-in function to find the first anonymous arguments in registers.
In general, a machine may have several categories of registers used for arguments, each for a particular category of data types. (For example, on some machines, floating-point registers are used for floating-point arguments while other arguments are passed in the general registers.) To make non-varargs functions use the proper calling convention, you have defined the
CUMULATIVE_ARGSdata type to record how many registers in each category have been used so far
__builtin_args_infoaccesses the same data structure of type
CUMULATIVE_ARGSafter the ordinary argument layout is finished with it, with category specifying which word to access. Thus, the value indicates the first unused register in a given category.
Normally, you would use
__builtin_args_infoin the implementation of
va_start, accessing each category just once and storing the value in the
va_listobject. This is because
va_listwill have to update the values, and there is no way to alter the values accessed by
This is the equivalent of
__builtin_args_info, for stack arguments. It returns the address of the first anonymous stack argument, as type
void *. If
ARGS_GROW_DOWNWARD, it returns the address of the location above the first anonymous stack argument. Use it in
va_startto initialize the pointer for fetching arguments from the stack. Also use it in
va_startto verify that the second parameter lastarg is the last named argument of the current function.
Since each machine has its own conventions for which data types are passed in which kind of register, your implementation of
va_arghas to embody these conventions. The easiest way to categorize the specified data type is to use
__builtin_classify_typeignores the value of object, considering only its data type. It returns an integer describing what kind of type that is—integer, floating, pointer, structure, and so on.
The file typeclass.h defines an enumeration that you can use to interpret the values of
These machine description macros help implement varargs:
If defined, this hook produces the machine-specific code for a call to
__builtin_saveregs. This code will be moved to the very beginning of the function, before any parameter access are made. The return value of this function should be an RTX that contains the value to use as the return of
This target hook offers an alternative to using
__builtin_saveregsand defining the hook
TARGET_EXPAND_BUILTIN_SAVEREGS. Use it to store the anonymous register arguments into the stack so that all the arguments appear to have been passed consecutively on the stack. Once this is done, you can use the standard implementation of varargs that works for machines that pass all their arguments on the stack.
The argument args_so_far points to the
CUMULATIVE_ARGSdata structure, containing the values that are obtained after processing the named arguments. The arguments mode and type describe the last named argument—its machine mode and its data type as a tree node.
The target hook should do two things: first, push onto the stack all the argument registers not used for the named arguments, and second, store the size of the data thus pushed into the
int-valued variable pointed to by pretend_args_size. The value that you store here will serve as additional offset for setting up the stack frame.
Because you must generate code to push the anonymous arguments at compile time without knowing their data types,
TARGET_SETUP_INCOMING_VARARGSis only useful on machines that have just a single category of argument register and use it uniformly for all data types.
If the argument second_time is nonzero, it means that the arguments of the function are being analyzed for the second time. This happens for an inline function, which is not actually compiled until the end of the source file. The hook
TARGET_SETUP_INCOMING_VARARGSshould not generate any instructions in this case.
Define this hook to return
trueif the location where a function argument is passed depends on whether or not it is a named argument.
This hook controls how the named argument to
FUNCTION_ARGis set for varargs and stdarg functions. If this hook returns
true, the named argument is always true for named arguments, and false for unnamed arguments. If it returns
true, then all arguments are treated as named. Otherwise, all named arguments except the last are treated as named.
You need not define this hook if it always returns zero.
If you need to conditionally change ABIs so that one works with
TARGET_SETUP_INCOMING_VARARGS, but the other works like neither
TARGET_STRICT_ARGUMENT_NAMINGwas defined, then define this hook to return
falseotherwise. Otherwise, you should not define this hook.