https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114175
--- Comment #15 from palmer at gcc dot gnu.org ---
It's a little easier to see from the float version of the code.
$ cat gcc/testsuite/gcc.dg/c23-stdarg-6.c
/* Test C23 variadic functions with no named parameters, or last named
parameter with a declaration not allowed in C17. Execution tests. */
/* { dg-do run } */
/* { dg-options "-std=c23 -pedantic-errors" } */
#include <stdarg.h>
#include <stdio.h>
extern void abort (void);
extern void exit (int);
struct s { char c[1000]; };
struct s
f (...)
{
va_list ap;
va_start (ap);
int r = va_arg (ap, double);
va_end (ap);
struct s ret = {};
ret.c[0] = r;
ret.c[999] = 42;
return ret;
}
int
main ()
{
struct s x = f (1.0);
fprintf(stderr, "%d\n", x.c[0]);
if (x.c[0] != 1)
abort ();
exit (0);
}
$ riscv64-unknown-linux-gnu-gcc gcc/testsuite/gcc.dg/c23-stdarg-6.c -o test
-std=c2x -static -O3
$ riscv64-unknown-linux-gnu-objdump -d test
...
0000000000010412 <main>:
...
1042e: 850a mv a0,sp
...
10438: 112000ef jal 1054a <f>
...
000000000001054a <f>:
1054a: f20507d3 fmv.d.x fa5,a0
The psABI says
A callee with variadic arguments is responsible for copying the contents
of registers used to pass variadic arguments to the vararg save area,
which must be contiguous with arguments passed on the stack.
which I'm taking to mean the "1.0" is meant to be passed in a register. It
also says
Values are returned in the same manner as a first named argument of the
same type would be passed. If such an argument would have been passed by
reference, the caller allocates memory for the return value, and passes
the address as an implicit first parameter.
So I think we're screwing up both ends of this one: the caller is passing the
return struct in a0 (losing the first arg), which the callee is obtaining the
first argument from a0 (losing the return struct).
That all very much seems like a backend bug to me.
