https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111307
Bug ID: 111307
Summary: RFE: builtin to construct va_list
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: equinox-gccbugs at diac24 dot net
Target Milestone: ---
A common pattern of defining a pair of varargs functions looks like:
void somefuncv(char *spec, va_list ap)
{
/* ... */
}
void somefunc(char *spec, ...)
{
va_list ap
va_start(ap, spec);
sumefuncv(spec, ap);
va_end(ap);
}
somefunc("%d", 1234); /* => quite a bit of wasted moving bits around */
However, the resulting code for calling somefunc() is suboptimal; the arguments
are arranged as specified by the psABI, only to then be rearranged into a
va_list by somefunc (which can be rather complex since somefunc has no clue
about type, number, or even floating-point calling conventions of its
arguments.)
It would be very helpful if GCC added a builtin function to create a va_list
for a given "...", with a (pseudo-)prototype like "__builtin_va_list
__builtin_va_construct(...)". This could be used to directly put together a
va_list at the caller's location, e.g.:
#define somefunc(spec, ...) somefuncv(spec,
__builtin_va_construct(__VA_ARGS__))
somefunc("%d", 1234); /* => minimal va_list to carry a single int */
Thus the round-trip through the psABI could be avoided, with the caller
constructing a minimal (since it knows all the args) va_list.
(This builtin would also make inlining varargs functions much less relevant,
since there are no more actual varargs functions in the latter example...)
