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* [Bug c/50272] New: A case that PRE optimization hurts performance @ 2011-09-02 5:08 jiangning.liu at arm dot com 2011-09-02 5:12 ` [Bug c/50272] " jiangning.liu at arm dot com ` (6 more replies) 0 siblings, 7 replies; 8+ messages in thread From: jiangning.liu at arm dot com @ 2011-09-02 5:08 UTC (permalink / raw) To: gcc-bugs http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50272 Bug #: 50272 Summary: A case that PRE optimization hurts performance Classification: Unclassified Product: gcc Version: 4.7.0 Status: UNCONFIRMED Severity: normal Priority: P3 Component: c AssignedTo: unassigned@gcc.gnu.org ReportedBy: jiangning.liu@arm.com For the following simple test case, PRE optimization hoists computation (s!=1) into the default branch of the switch statement, and finally causes very poor code generation. This problem occurs in both X86 and ARM, and I believe it is also a problem for other targets. int f(char *t) { int s=0; while (*t && s != 1) { switch (s) { case 0: s = 2; break; case 2: s = 1; break; default: if (*t == '-') s = 1; break; } t++; } return s; } Taking X86 as an example, with option "-O2" you may find 52 instructions generated like below, 00000000 <f>: 0: 55 push %ebp 1: 31 c0 xor %eax,%eax 3: 89 e5 mov %esp,%ebp 5: 57 push %edi 6: 56 push %esi 7: 53 push %ebx 8: 8b 55 08 mov 0x8(%ebp),%edx b: 0f b6 0a movzbl (%edx),%ecx e: 84 c9 test %cl,%cl 10: 74 50 je 62 <f+0x62> 12: 83 c2 01 add $0x1,%edx 15: 85 c0 test %eax,%eax 17: 75 23 jne 3c <f+0x3c> 19: 8d b4 26 00 00 00 00 lea 0x0(%esi,%eiz,1),%esi 20: 0f b6 0a movzbl (%edx),%ecx 23: 84 c9 test %cl,%cl 25: 0f 95 c0 setne %al 28: 89 c7 mov %eax,%edi 2a: b8 02 00 00 00 mov $0x2,%eax 2f: 89 fb mov %edi,%ebx 31: 83 c2 01 add $0x1,%edx 34: 84 db test %bl,%bl 36: 74 2a je 62 <f+0x62> 38: 85 c0 test %eax,%eax 3a: 74 e4 je 20 <f+0x20> 3c: 83 f8 02 cmp $0x2,%eax 3f: 74 1f je 60 <f+0x60> 41: 80 f9 2d cmp $0x2d,%cl 44: 74 22 je 68 <f+0x68> 46: 0f b6 0a movzbl (%edx),%ecx 49: 83 f8 01 cmp $0x1,%eax 4c: 0f 95 c3 setne %bl 4f: 89 df mov %ebx,%edi 51: 84 c9 test %cl,%cl 53: 0f 95 c3 setne %bl 56: 89 de mov %ebx,%esi 58: 21 f7 and %esi,%edi 5a: eb d3 jmp 2f <f+0x2f> 5c: 8d 74 26 00 lea 0x0(%esi,%eiz,1),%esi 60: b0 01 mov $0x1,%al 62: 5b pop %ebx 63: 5e pop %esi 64: 5f pop %edi 65: 5d pop %ebp 66: c3 ret 67: 90 nop 68: b8 01 00 00 00 mov $0x1,%eax 6d: 5b pop %ebx 6e: 5e pop %esi 6f: 5f pop %edi 70: 5d pop %ebp 71: c3 ret But with command line option "-O2 -fno-tree-pre", there are only 12 instructions generated, and the code would be very clean like below, 00000000 <f>: 0: 55 push %ebp 1: 31 c0 xor %eax,%eax 3: 89 e5 mov %esp,%ebp 5: 8b 55 08 mov 0x8(%ebp),%edx 8: 80 3a 00 cmpb $0x0,(%edx) b: 74 0e je 1b <f+0x1b> d: 80 7a 01 00 cmpb $0x0,0x1(%edx) 11: b0 02 mov $0x2,%al 13: ba 01 00 00 00 mov $0x1,%edx 18: 0f 45 c2 cmovne %edx,%eax 1b: 5d pop %ebp 1c: c3 ret ^ permalink raw reply [flat|nested] 8+ messages in thread
* [Bug c/50272] A case that PRE optimization hurts performance 2011-09-02 5:08 [Bug c/50272] New: A case that PRE optimization hurts performance jiangning.liu at arm dot com @ 2011-09-02 5:12 ` jiangning.liu at arm dot com 2011-09-02 6:29 ` [Bug tree-optimization/50272] " pinskia at gcc dot gnu.org ` (5 subsequent siblings) 6 siblings, 0 replies; 8+ messages in thread From: jiangning.liu at arm dot com @ 2011-09-02 5:12 UTC (permalink / raw) To: gcc-bugs http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50272 --- Comment #1 from Jiangning Liu <jiangning.liu at arm dot com> 2011-09-02 05:11:38 UTC --- Richard gave some analysis at http://gcc.gnu.org/ml/gcc/2011-08/msg00037.html ^ permalink raw reply [flat|nested] 8+ messages in thread
* [Bug tree-optimization/50272] A case that PRE optimization hurts performance 2011-09-02 5:08 [Bug c/50272] New: A case that PRE optimization hurts performance jiangning.liu at arm dot com 2011-09-02 5:12 ` [Bug c/50272] " jiangning.liu at arm dot com @ 2011-09-02 6:29 ` pinskia at gcc dot gnu.org 2011-09-02 9:28 ` rguenth at gcc dot gnu.org ` (4 subsequent siblings) 6 siblings, 0 replies; 8+ messages in thread From: pinskia at gcc dot gnu.org @ 2011-09-02 6:29 UTC (permalink / raw) To: gcc-bugs http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50272 Andrew Pinski <pinskia at gcc dot gnu.org> changed: What |Removed |Added ---------------------------------------------------------------------------- Keywords| |missed-optimization Component|c |tree-optimization --- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> 2011-09-02 06:29:18 UTC --- I recognize this loop, it is part of coremarks. Anyways confirmed and it happens on MIPS64 too. ^ permalink raw reply [flat|nested] 8+ messages in thread
* [Bug tree-optimization/50272] A case that PRE optimization hurts performance 2011-09-02 5:08 [Bug c/50272] New: A case that PRE optimization hurts performance jiangning.liu at arm dot com 2011-09-02 5:12 ` [Bug c/50272] " jiangning.liu at arm dot com 2011-09-02 6:29 ` [Bug tree-optimization/50272] " pinskia at gcc dot gnu.org @ 2011-09-02 9:28 ` rguenth at gcc dot gnu.org 2011-09-08 20:41 ` hp at gcc dot gnu.org ` (3 subsequent siblings) 6 siblings, 0 replies; 8+ messages in thread From: rguenth at gcc dot gnu.org @ 2011-09-02 9:28 UTC (permalink / raw) To: gcc-bugs http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50272 --- Comment #3 from Richard Guenther <rguenth at gcc dot gnu.org> 2011-09-02 09:28:09 UTC --- Bah, stupid benchmarks ;) ^ permalink raw reply [flat|nested] 8+ messages in thread
* [Bug tree-optimization/50272] A case that PRE optimization hurts performance 2011-09-02 5:08 [Bug c/50272] New: A case that PRE optimization hurts performance jiangning.liu at arm dot com ` (2 preceding siblings ...) 2011-09-02 9:28 ` rguenth at gcc dot gnu.org @ 2011-09-08 20:41 ` hp at gcc dot gnu.org 2012-03-29 17:56 ` vhaisman at gmail dot com ` (2 subsequent siblings) 6 siblings, 0 replies; 8+ messages in thread From: hp at gcc dot gnu.org @ 2011-09-08 20:41 UTC (permalink / raw) To: gcc-bugs http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50272 Hans-Peter Nilsson <hp at gcc dot gnu.org> changed: What |Removed |Added ---------------------------------------------------------------------------- Status|UNCONFIRMED |NEW Last reconfirmed| |2011-09-08 CC| |hp at gcc dot gnu.org Ever Confirmed|0 |1 ^ permalink raw reply [flat|nested] 8+ messages in thread
* [Bug tree-optimization/50272] A case that PRE optimization hurts performance 2011-09-02 5:08 [Bug c/50272] New: A case that PRE optimization hurts performance jiangning.liu at arm dot com ` (3 preceding siblings ...) 2011-09-08 20:41 ` hp at gcc dot gnu.org @ 2012-03-29 17:56 ` vhaisman at gmail dot com 2012-03-30 3:48 ` liujiangning at gcc dot gnu.org 2021-07-26 20:54 ` pinskia at gcc dot gnu.org 6 siblings, 0 replies; 8+ messages in thread From: vhaisman at gmail dot com @ 2012-03-29 17:56 UTC (permalink / raw) To: gcc-bugs http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50272 --- Comment #4 from Václav Zeman <vhaisman at gmail dot com> 2012-03-29 17:51:51 UTC --- This is still a problem in version 4.7.0 20120225. ^ permalink raw reply [flat|nested] 8+ messages in thread
* [Bug tree-optimization/50272] A case that PRE optimization hurts performance 2011-09-02 5:08 [Bug c/50272] New: A case that PRE optimization hurts performance jiangning.liu at arm dot com ` (4 preceding siblings ...) 2012-03-29 17:56 ` vhaisman at gmail dot com @ 2012-03-30 3:48 ` liujiangning at gcc dot gnu.org 2021-07-26 20:54 ` pinskia at gcc dot gnu.org 6 siblings, 0 replies; 8+ messages in thread From: liujiangning at gcc dot gnu.org @ 2012-03-30 3:48 UTC (permalink / raw) To: gcc-bugs http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50272 Jiangning Liu <liujiangning at gcc dot gnu.org> changed: What |Removed |Added ---------------------------------------------------------------------------- CC| |liujiangning at gcc dot | |gnu.org --- Comment #5 from Jiangning Liu <liujiangning at gcc dot gnu.org> 2012-03-30 03:41:42 UTC --- (In reply to comment #4) > This is still a problem in version 4.7.0 20120225. http://gcc.gnu.org/ml/gcc/2011-09/msg00342.html Jeff already gave comments here. The proposed solution is path sensitive optimization. It seems it's hard to solve this problem in short time. ^ permalink raw reply [flat|nested] 8+ messages in thread
* [Bug tree-optimization/50272] A case that PRE optimization hurts performance 2011-09-02 5:08 [Bug c/50272] New: A case that PRE optimization hurts performance jiangning.liu at arm dot com ` (5 preceding siblings ...) 2012-03-30 3:48 ` liujiangning at gcc dot gnu.org @ 2021-07-26 20:54 ` pinskia at gcc dot gnu.org 6 siblings, 0 replies; 8+ messages in thread From: pinskia at gcc dot gnu.org @ 2021-07-26 20:54 UTC (permalink / raw) To: gcc-bugs https://gcc.gnu.org/bugzilla/show_bug.cgi?id=50272 --- Comment #6 from Andrew Pinski <pinskia at gcc dot gnu.org> --- In GCC 5+ we can get rid of the loop fully (in the reduced testcase). ^ permalink raw reply [flat|nested] 8+ messages in thread
end of thread, other threads:[~2021-07-26 20:54 UTC | newest] Thread overview: 8+ messages (download: mbox.gz / follow: Atom feed) -- links below jump to the message on this page -- 2011-09-02 5:08 [Bug c/50272] New: A case that PRE optimization hurts performance jiangning.liu at arm dot com 2011-09-02 5:12 ` [Bug c/50272] " jiangning.liu at arm dot com 2011-09-02 6:29 ` [Bug tree-optimization/50272] " pinskia at gcc dot gnu.org 2011-09-02 9:28 ` rguenth at gcc dot gnu.org 2011-09-08 20:41 ` hp at gcc dot gnu.org 2012-03-29 17:56 ` vhaisman at gmail dot com 2012-03-30 3:48 ` liujiangning at gcc dot gnu.org 2021-07-26 20:54 ` pinskia at gcc dot gnu.org
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