GNU Compiler Collection (GCC) Internals: Varargs

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### 18.10 Implementing the Varargs Macros

GCC comes with an implementation of `<varargs.h>` and `<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.

ISO `<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.

However, `va_start` should not use this argument. The way to find the end of the named arguments is with the built-in functions described below.

Macro: __builtin_saveregs ()

  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 `va_start` must use `__builtin_saveregs`, unless you use `TARGET_SETUP_INCOMING_VARARGS` (see below) instead.

  On some machines, `__builtin_saveregs` is 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_saveregs` appears at the beginning of the function, as opposed to where the call to `__builtin_saveregs` is 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.

Macro: __builtin_next_arg (lastarg)

  This builtin 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_start` to initialize the pointer for fetching arguments from the stack. Also use it in `va_start` to verify that the second parameter lastarg is the last named argument of the current function.

Macro: __builtin_classify_type (object)

  Since each machine has its own conventions for which data types are passed in which kind of register, your implementation of `va_arg` has to embody these conventions. The easiest way to categorize the specified data type is to use `__builtin_classify_type` together with `sizeof` and `__alignof__`.

  `__builtin_classify_type` ignores 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 `__builtin_classify_type`.

These machine description macros help implement varargs:

Target Hook: rtx TARGET_EXPAND_BUILTIN_SAVEREGS (void)

  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 `__builtin_saveregs`.

Target Hook: void TARGET_SETUP_INCOMING_VARARGS (cumulative_args_t args_so_far, const function_arg_info &arg, int *pretend_args_size, int second_time)

  This target hook offers an alternative to using `__builtin_saveregs` and 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_ARGS` data structure, containing the values that are obtained after processing the named arguments. The argument arg describes the last of these named arguments.

  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_VARARGS` is 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_VARARGS` should not generate any instructions in this case.

Target Hook: bool TARGET_STRICT_ARGUMENT_NAMING (cumulative_args_t ca)

  Define this hook to return `true` if 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 `TARGET_FUNCTION_ARG` is 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 `false`, but `TARGET_PRETEND_OUTGOING_VARARGS_NAMED` 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 `false`.

Target Hook: void TARGET_CALL_ARGS (rtx, tree)

  While generating RTL for a function call, this target hook is invoked once for each argument passed to the function, either a register returned by `TARGET_FUNCTION_ARG` or a memory location. It is called just before the point where argument registers are stored. The type of the function to be called is also passed as the second argument; it is `NULL_TREE` for libcalls. The `TARGET_END_CALL_ARGS` hook is invoked just after the code to copy the return reg has been emitted. This functionality can be used to perform special setup of call argument registers if a target needs it. For functions without arguments, the hook is called once with `pc_rtx` passed instead of an argument register. Most ports do not need to implement anything for this hook.

Target Hook: void TARGET_END_CALL_ARGS (void)

  This target hook is invoked while generating RTL for a function call, just after the point where the return reg is copied into a pseudo. It signals that all the call argument and return registers for the just emitted call are now no longer in use. Most ports do not need to implement anything for this hook.

Target Hook: bool TARGET_PRETEND_OUTGOING_VARARGS_NAMED (cumulative_args_t ca)

  If you need to conditionally change ABIs so that one works with `TARGET_SETUP_INCOMING_VARARGS`, but the other works like neither `TARGET_SETUP_INCOMING_VARARGS` nor `TARGET_STRICT_ARGUMENT_NAMING` was defined, then define this hook to return `true` if `TARGET_SETUP_INCOMING_VARARGS` is used, `false` otherwise. Otherwise, you should not define this hook.

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