[Bug rtl-optimization/53117] New: missed-optimization: worse code for 'x <= 0' compared to 'x < 0'

[Bug rtl-optimization/53117] New: missed-optimization: worse code for 'x <= 0' compared to 'x < 0'

[vermaelen.wouter at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53117

             Bug #: 53117
           Summary: missed-optimization: worse code for 'x <= 0' compared
                    to 'x < 0'
    Classification: Unclassified
           Product: gcc
           Version: 4.8.0
            Status: UNCONFIRMED
          Severity: minor
          Priority: P3
         Component: rtl-optimization
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: vermaelen.wouter@gmail.com


void f1(int* p) {
        p[1] -= 5;
        if (p[1] < 0) p[2] += 3;
}
void f2(int* p) {
        p[1] -= 5;
        if (p[1] <= 0) p[2] += 3;
}

The only difference between f1() and f2() is the comparison ('<' vs '<='). On
x86_64 (and x86) gcc revision trunk@186808 generates more efficient code for
f1() than for f2(). Here's the assembler output when compiled with -Os (but -O2
and -O3) show a similar difference:


0000000000000000 <_Z2f1Pi>:
   0:   83 6f 04 05             subl   $0x5,0x4(%rdi)
   4:   79 04                   jns    a <_Z2f1Pi+0xa>
   6:   83 47 08 03             addl   $0x3,0x8(%rdi)
   a:   c3                      retq   

000000000000000b <_Z2f2Pi>:
   b:   8b 47 04                mov    0x4(%rdi),%eax
   e:   83 e8 05                sub    $0x5,%eax
  11:   85 c0                   test   %eax,%eax
  13:   89 47 04                mov    %eax,0x4(%rdi)
  16:   7f 04                   jg     1c <_Z2f2Pi+0x11>
  18:   83 47 08 03             addl   $0x3,0x8(%rdi)
  1c:   c3                      retq 


gcc-4.6.1 generates the less efficient variant for both functions.

[Bug rtl-optimization/53117] missed-optimization: worse code for 'x <= 0' compared to 'x < 0'

[jakub at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53117

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jakub at gcc dot gnu.org

--- Comment #1 from Jakub Jelinek <jakub at gcc dot gnu.org> 2012-04-25 14:25:51 UTC ---
So, what are you proposing?  Testing of flags after the subtraction from memory
can't be used in the first case...

[Bug target/53117] missed-optimization: worse code for 'x <= 0' compared to 'x < 0'

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53117

Richard Guenther <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Target|                            |x86_64-*-*
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2012-04-25
          Component|rtl-optimization            |target
     Ever Confirmed|0                           |1

--- Comment #2 from Richard Guenther <rguenth at gcc dot gnu.org> 2012-04-25 14:28:09 UTC ---
Confirmed.

[Bug target/53117] missed-optimization: worse code for 'x <= 0' compared to 'x < 0'

[vermaelen.wouter at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53117

--- Comment #3 from Wouter Vermaelen <vermaelen.wouter at gmail dot com> 2012-04-25 15:30:42 UTC ---
@Jakub: At first I was puzzled by your comment. But after some investigation I
found out that this 'optimization' is indeed not possible when the subtraction
would underflow. So you can close this bug report as invalid.  (OTOH signed
overflow is anyway undefined behavior in C).

[Bug target/53117] missed-optimization: worse code for 'x <= 0' compared to 'x < 0'

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=53117

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Severity|minor                       |enhancement