[Bug middle-end/42590] New: unnecessary stack frame set up

["andi-gcc at firstfloor dot org" <gcc-bugzilla at gcc dot gnu dot org>]

Continuation from 42586 and 42585 who handle other missed optimizations
for this test case.

>From one of the examples of
http://embed.cs.utah.edu/embarrassing/dec_09/harvest/gcc-head_llvm-gcc-head/

struct _fat_ptr
{
  unsigned char *curr;
  unsigned char *base;
  unsigned char *last_plus_one;
};
int Cyc_string_ungetc (int ignore, struct _fat_ptr *sptr);
int
Cyc_string_ungetc (int ignore, struct _fat_ptr *sptr)
{
  struct _fat_ptr *_T0;
  struct _fat_ptr *_T1;
  struct _fat_ptr _T2;
  int _T3;
  struct _fat_ptr _ans;
  int _change;

  {
    _T0 = sptr;
    _T1 = sptr;
    _T2 = *sptr;
    _T3 = -1;
    _ans = _T2;
    _change = -1;
    _ans.curr += 4294967295U;
    *sptr = _ans;
    return (0);
  }
}

generates 

hen compiled with -O2 -m32 on 4.5.0 20091219 generates

Cyc_string_ungetc:
        subl    $32, %esp
        movl    40(%esp), %eax
        movl    (%eax), %edx
        subl    $1, %edx
        movl    %edx, (%eax)
        xorl    %eax, %eax
        addl    $32, %esp
        ret


The stack frame manipulation is completely unnecessary.

A.Pinski commented on the other bug on this:

ell one thing is SRA does not do its job which is why there is useless stack
frame changes.

------- Comment #2 From Andrew Pinski 2010-01-03 06:27 [reply] -------
Oh and -Os is very bad looking :).

------- Comment #3 From Andrew Pinski 2010-01-03 06:30 [reply] -------
Oh in fact I think the reason is the SRA issue.  In that:
int
Cyc_string_ungetc (int ignore, struct _fat_ptr *sptr)
{
  sptr->curr += 4294967295U;
}
Works.

------- Comment #4 From Andrew Pinski 2010-01-03 06:33 [reply] -------
Hmm, 4.5 is worse off than 4.4 with respect of the stack space usage.

------- Comment #5 From Andrew Pinski 2010-01-03 06:35 [reply] -------
4.5 has:
  _T2 = *sptr_1(D);
  _T2$curr_14 = sptr_1(D)->curr;
  _ans = _T2;
  D.2697_7 = _T2$curr_14 + -1;
  *sptr_1(D) = _ans;
  sptr_1(D)->curr = D.2697_7;

While 4.4 does:
  _T2$base = sptr->base;
  D.1587 = sptr->curr + -1;
  sptr->last_plus_one = sptr->last_plus_one;
  sptr->base = _T2$base;
  sptr->curr = D.1587;

------- Comment #6 From Andrew Pinski 2010-01-03 06:38 [reply] -------
I think if we get the old SRA behavior back for this code, we will get this
optimization in 4.5 since we remove the sptr->base and the other unnecessary
store during PRE.


-- 
           Summary: unnecessary stack frame set up
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: middle-end
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: andi-gcc at firstfloor dot org
  GCC host triplet: x86_64-linux
GCC target triplet: x86_64-linux


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=42590