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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 [thread overview] Message-ID: <bug-53117-4@http.gcc.gnu.org/bugzilla/> (raw) 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.
next reply other threads:[~2012-04-25 14:21 UTC|newest] Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top 2012-04-25 14:21 vermaelen.wouter at gmail dot com [this message] 2012-04-25 14:26 ` [Bug rtl-optimization/53117] " jakub at gcc dot gnu.org 2012-04-25 14:28 ` [Bug target/53117] " rguenth at gcc dot gnu.org 2012-04-25 15:31 ` vermaelen.wouter at gmail dot com 2021-12-15 1:43 ` pinskia at gcc dot gnu.org
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