[Bug target/114175] [13/14] RISC-V: Execution test failures on gcc.dg/c23-stdarg-6.c

["palmer at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

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.