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* [Bug c/35776]  New: Simple loop isn't optimized well
@ 2008-03-31 17:04 yuri at tsoft dot com
  2008-04-01 11:06 ` [Bug tree-optimization/35776] " rguenth at gcc dot gnu dot org
  0 siblings, 1 reply; 2+ messages in thread
From: yuri at tsoft dot com @ 2008-03-31 17:04 UTC (permalink / raw)
  To: gcc-bugs

I have a C code:
void f();
void i(unsigned n) {
  while (n-- > 0) {
    f();
  }
}

Which when compiled with -O3 on i586 produces the assembly:
i:
        pushl   %ebp
        movl    %esp, %ebp
        pushl   %esi
        movl    8(%ebp), %esi
        pushl   %ebx
        testl   %esi, %esi
        je      .L5
        xorl    %ebx, %ebx
        .p2align 4,,7
.L4:
        addl    $1, %ebx
        call    f
        cmpl    %esi, %ebx
        jne     .L4
.L5:
        popl    %ebx
        popl    %esi
        popl    %ebp
        ret

And obviously shorter assembly with one less instruction in the loop is:
i:
        pushl   %ebp
        movl    %esp, %ebp
        pushl   %esi
        movl    8(%ebp), %esi
        testl   %esi, %esi
        je      .L5
        .p2align 4,,7
.L4:
        call    f
        dec     %esi
        jl      .L4
.L5:
        popl    %esi
        popl    %ebp
        ret

This is a very short and basic loop occurring in programs many times.
That's why this should be optimized very well.


-- 
           Summary: Simple loop isn't optimized well
           Product: gcc
           Version: 4.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: yuri at tsoft dot com


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=35776


^ permalink raw reply	[flat|nested] 2+ messages in thread
* [Bug tree-optimization/35776] Simple loop isn't optimized well
  2008-03-31 17:04 [Bug c/35776] New: Simple loop isn't optimized well yuri at tsoft dot com
@ 2008-04-01 11:06 ` rguenth at gcc dot gnu dot org
  0 siblings, 0 replies; 2+ messages in thread
From: rguenth at gcc dot gnu dot org @ 2008-04-01 11:06 UTC (permalink / raw)
  To: gcc-bugs



------- Comment #1 from rguenth at gcc dot gnu dot org  2008-04-01 11:05 -------
4.3 produces

.L5:
        call    f
        subl    $1, %ebx
        jne     .L5

thus fixed.


-- 

rguenth at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
          Component|c                           |tree-optimization
           Keywords|                            |missed-optimization
         Resolution|                            |FIXED
   Target Milestone|---                         |4.3.0


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=35776


^ permalink raw reply	[flat|nested] 2+ messages in thread
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2008-03-31 17:04 [Bug c/35776] New: Simple loop isn't optimized well yuri at tsoft dot com
2008-04-01 11:06 ` [Bug tree-optimization/35776] " rguenth at gcc dot gnu dot org

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