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From: "vermaelen.wouter at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug rtl-optimization/53117] New: missed-optimization: worse code for 'x <= 0' compared to 'x < 0'
Date: Wed, 25 Apr 2012 14:21:00 -0000
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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.