[Bug c/24233] New: -fomit-frame-pointer causes really dire code to be generated

["arjanv at redhat dot com" <gcc-bugzilla at gcc dot gnu dot org>]

The testcase (taken from zlib) below
int huft_build(void)
{
  unsigned int c[16];
  unsigned int *p;
  p = c;

#define C0 *p++ = 0;
#define C2 C0 C0 C0 C0
#define C4 C2 C2 C2 C2
  C4

 /* stop optimisation from throwing all the above code away */
  __asm__ __volatile__("": : :"memory");
  return *p;
}

yields really bad code on x86 when using the following compiler flags
 gcc  -O2 -fomit-frame-pointer -march=i686 -c inftrees.c -o inftrees.o

snippet from objdump output:
   3:   31 c0                   xor    %eax,%eax
   5:   89 44 24 04             mov    %eax,0x4(%esp)
   9:   31 c0                   xor    %eax,%eax
   b:   31 c9                   xor    %ecx,%ecx
   d:   89 44 24 08             mov    %eax,0x8(%esp)
  11:   31 c0                   xor    %eax,%eax
  13:   31 d2                   xor    %edx,%edx
  15:   89 44 24 0c             mov    %eax,0xc(%esp)
  19:   31 c0                   xor    %eax,%eax
  1b:   89 44 24 10             mov    %eax,0x10(%esp)
  1f:   31 c0                   xor    %eax,%eax
  21:   89 44 24 1c             mov    %eax,0x1c(%esp)
  25:   31 c0                   xor    %eax,%eax
  27:   89 44 24 20             mov    %eax,0x20(%esp)
  2b:   31 c0                   xor    %eax,%eax
  2d:   89 44 24 24             mov    %eax,0x24(%esp)
  31:   31 c0                   xor    %eax,%eax
  33:   89 44 24 28             mov    %eax,0x28(%esp)
  37:   31 c0                   xor    %eax,%eax


notice how it zeroes %eax after each write, instead of remembering eax was
already zero!

without -fomit-framepointer sane code is generated:
   6:   c7 45 c0 00 00 00 00    movl   $0x0,0xffffffc0(%ebp)
   d:   c7 45 c4 00 00 00 00    movl   $0x0,0xffffffc4(%ebp)
  14:   c7 45 c8 00 00 00 00    movl   $0x0,0xffffffc8(%ebp)
  1b:   c7 45 cc 00 00 00 00    movl   $0x0,0xffffffcc(%ebp)
etc


-- 
           Summary: -fomit-frame-pointer causes really dire code to be
                    generated
           Product: gcc
           Version: 4.0.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: arjanv at redhat dot com
  GCC host triplet: i686-gnu-linux


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=24233