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* [Bug tree-optimization/112687] New: missed-optimization: switch statement does not simplify to it's expression @ 2023-11-23 21:43 goon.pri.low at gmail dot com 2023-11-24 8:04 ` [Bug tree-optimization/112687] " rguenth at gcc dot gnu.org 2024-01-26 11:59 ` fkastl at suse dot cz 0 siblings, 2 replies; 3+ messages in thread From: goon.pri.low at gmail dot com @ 2023-11-23 21:43 UTC (permalink / raw) To: gcc-bugs https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112687 Bug ID: 112687 Summary: missed-optimization: switch statement does not simplify to it's expression Product: gcc Version: 13.2.1 Status: UNCONFIRMED Severity: normal Priority: P3 Component: tree-optimization Assignee: unassigned at gcc dot gnu.org Reporter: goon.pri.low at gmail dot com Target Milestone: --- This following code: int unopt(int v) { switch (v & 3) { case 0: return 0; case 1: return 1; case 2: return 2; case 3: return 3; default: __builtin_unreachable(); } } unopt: mov eax, edi and eax, 3 lea edx, [rax-1] cmp edx, 3 mov edx, 0 cmovnb eax, edx ret Ought to be optimized to: int opt(int v) { return v & 3; } opt: mov eax, edi and eax, 3 ret Note: I decided to separate this problem from https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112645 since it was unrelated to the main bug, though Richard offers some initial insight into the problem. ^ permalink raw reply [flat|nested] 3+ messages in thread
* [Bug tree-optimization/112687] missed-optimization: switch statement does not simplify to it's expression 2023-11-23 21:43 [Bug tree-optimization/112687] New: missed-optimization: switch statement does not simplify to it's expression goon.pri.low at gmail dot com @ 2023-11-24 8:04 ` rguenth at gcc dot gnu.org 2024-01-26 11:59 ` fkastl at suse dot cz 1 sibling, 0 replies; 3+ messages in thread From: rguenth at gcc dot gnu.org @ 2023-11-24 8:04 UTC (permalink / raw) To: gcc-bugs https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112687 Richard Biener <rguenth at gcc dot gnu.org> changed: What |Removed |Added ---------------------------------------------------------------------------- Ever confirmed|0 |1 Last reconfirmed| |2023-11-24 Status|UNCONFIRMED |NEW CC| |fkastl at suse dot cz --- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> --- Confirmed. ^ permalink raw reply [flat|nested] 3+ messages in thread
* [Bug tree-optimization/112687] missed-optimization: switch statement does not simplify to it's expression 2023-11-23 21:43 [Bug tree-optimization/112687] New: missed-optimization: switch statement does not simplify to it's expression goon.pri.low at gmail dot com 2023-11-24 8:04 ` [Bug tree-optimization/112687] " rguenth at gcc dot gnu.org @ 2024-01-26 11:59 ` fkastl at suse dot cz 1 sibling, 0 replies; 3+ messages in thread From: fkastl at suse dot cz @ 2024-01-26 11:59 UTC (permalink / raw) To: gcc-bugs https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112687 --- Comment #2 from Filip Kastl <fkastl at suse dot cz> --- Created attachment 57222 --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=57222&action=edit WIP patch to fix the missed optimization, version 0 I'm working on a patch. The problem (as Richard stated in pr112645) is that the testcase gets optimized into switch (v & 3) { default: return 0; case 1: return 1; case 2: return 2; case 3: return 3; } so the information that there are only 4 possible values of the expression 'v & 3' is lost. I add capability for switch conversion to recover from this. With my patch, before processing the switch, switch conversion finds out if the default case only corresponds to one possible value of the index expression. If so, it creates a case for this value and marks default as unreachable. With the patch applied, functions opt() and unopt() compile into the same sequence of assembly instructions. I've attached the patch. Do you think this is a good approach? I'll also appreciate any suggestions/comments for the patch. ^ permalink raw reply [flat|nested] 3+ messages in thread
end of thread, other threads:[~2024-01-26 11:59 UTC | newest] Thread overview: 3+ messages (download: mbox.gz / follow: Atom feed) -- links below jump to the message on this page -- 2023-11-23 21:43 [Bug tree-optimization/112687] New: missed-optimization: switch statement does not simplify to it's expression goon.pri.low at gmail dot com 2023-11-24 8:04 ` [Bug tree-optimization/112687] " rguenth at gcc dot gnu.org 2024-01-26 11:59 ` fkastl at suse dot cz
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