[Bug target/63596] New: Saving of GPR/FPRs for stdarg even though the variable argument is not used

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63596

            Bug ID: 63596
           Summary: Saving of GPR/FPRs for stdarg even though the variable
                    argument is not used
           Product: gcc
           Version: 4.9.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: pinskia at gcc dot gnu.org
            Target: aarch64

Take a function like:
int f(int a, ...)
{
  return a;
}

Currently with the top of the trunk we get:
f:
    sub    sp, sp, #192
    stp    x1, x2, [sp, 136]
    stp    x3, x4, [sp, 152]
    stp    x5, x6, [sp, 168]
    str    x7, [sp, 184]
    str    q0, [sp]
    str    q1, [sp, 16]
    str    q2, [sp, 32]
    str    q3, [sp, 48]
    str    q4, [sp, 64]
    str    q5, [sp, 80]
    str    q6, [sp, 96]
    str    q7, [sp, 112]
    add    sp, sp, 192
    ret
--- CUT ---
But we can optimize this down to just 
f:
    ret
--- CUT ---
The .stdarg debug dump says:
f: va_list escapes 0, needs to save 0 GPR units and 0 FPR units.

But nowhere in aarch64.c uses cfun->va_list_gpr_size or cfun->va_list_fpr_size
to figure out how many registers need to be saved.