[Bug c/44262] New: va_list usage missed optimization.

[svfuerst at gmail dot com]

This function generates the following asm under -O3 in version 4.5

#include <stdarg.h>
void va_overload2(int p1, int p2);
void va_overload3(int p1, int p2, int p3);

void va_overload(int p1, int p2, ...)
{
        if (p2 == 7)
        {
                va_list v;
                va_start(v, p2);

                int p3 = va_arg(v, int);
                va_end(v);
                va_overload3(p1, p2, p3);

                return;
        }

        va_overload2(p1, p2);
}

Dump of assembler code for function va_overload:
   0x0000000000400520 <+0>:     sub    $0x58,%rsp
   0x0000000000400524 <+4>:     cmp    $0x7,%esi
   0x0000000000400527 <+7>:     mov    %rdx,0x30(%rsp)
   0x000000000040052c <+12>:    je     0x400540 <va_overload+32>
   0x000000000040052e <+14>:    callq  0x4004f0 <va_overload2>
   0x0000000000400533 <+19>:    add    $0x58,%rsp
   0x0000000000400537 <+23>:    retq   
   0x0000000000400538 <+24>:    nopl   0x0(%rax,%rax,1)
   0x0000000000400540 <+32>:    lea    0x60(%rsp),%rax
   0x0000000000400545 <+37>:    mov    0x30(%rsp),%edx
   0x0000000000400549 <+41>:    movl   $0x18,(%rsp)
   0x0000000000400550 <+48>:    mov    %rax,0x8(%rsp)
   0x0000000000400555 <+53>:    lea    0x20(%rsp),%rax
   0x000000000040055a <+58>:    mov    %rax,0x10(%rsp)
   0x000000000040055f <+63>:    callq  0x400500 <va_overload3>
   0x0000000000400564 <+68>:    add    $0x58,%rsp
   0x0000000000400568 <+72>:    retq

This could be replaced with the much more compact:
        cmp             $0x7, %esi
        je              1f
        jmp             va_overload2
1:      jmp             va_overload3
since the third parameter is passed in a register, and will still be there
after the comparison.  (Actually, if it were passed on the stack it still
wouldn't matter, because we can tail-call here.)


-- 
           Summary: va_list usage missed optimization.
           Product: gcc
           Version: 4.5.0
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: svfuerst at gmail dot com
 GCC build triplet: x86_64-linux
  GCC host triplet: x86_64-linux
GCC target triplet: x86_64-linux


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=44262