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From: "andre.schackier at gmail dot com" <gcc-bugzilla@gcc.gnu.org> To: gcc-bugs@gcc.gnu.org Subject: [Bug regression/105126] New: Optimization regression gcc inserts not needed movsx when using switch statement Date: Fri, 01 Apr 2022 13:42:13 +0000 [thread overview] Message-ID: <bug-105126-4@http.gcc.gnu.org/bugzilla/> (raw) https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105126 Bug ID: 105126 Summary: Optimization regression gcc inserts not needed movsx when using switch statement Product: gcc Version: 12.0 Status: UNCONFIRMED Severity: normal Priority: P3 Component: regression Assignee: unassigned at gcc dot gnu.org Reporter: andre.schackier at gmail dot com Target Milestone: --- Given the following source code: ```cpp bool is_bin_0(const char c) { return (c == '0' || c == '1'); } bool is_bin_1(const char c) { switch (c) { case '0': case '1': return true; default: return false; } } ``` compiling with `-O3` gives the following output: ```asm is_bin_0(char): sub edi, 48 cmp dil, 1 setbe al ret is_bin_1(char): movsx edi, dil sub edi, 48 cmp edi, 1 setbe al ret ``` The version using a switch generates an extra movsx instruction which, as far is I understand the x86 CPU registers is not needed since DIL is the lower 8 bits of EDI anyways, but correct me if I'm wrong there. This also seems to be a regression introduced with GCC-8.1 since GCC-4.5 to GCC-7.5 generate the same assembly as for the first and second function and all version after that including trunk produce different outputs. [link to godbolt](https://godbolt.org/z/r3vhYbK6o)
next reply other threads:[~2022-04-01 13:42 UTC|newest] Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top 2022-04-01 13:42 andre.schackier at gmail dot com [this message] 2022-04-01 15:39 ` [Bug regression/105126] " marxin at gcc dot gnu.org 2022-04-04 7:11 ` [Bug middle-end/105126] [9/10/11/12 Regression] " rguenth at gcc dot gnu.org 2022-04-20 10:18 ` jakub at gcc dot gnu.org 2022-05-27 9:47 ` [Bug middle-end/105126] [10/11/12/13 " rguenth at gcc dot gnu.org 2022-06-28 10:48 ` jakub at gcc dot gnu.org 2022-11-28 18:21 ` pinskia at gcc dot gnu.org 2023-01-11 12:36 ` marxin at gcc dot gnu.org 2023-07-07 10:42 ` [Bug middle-end/105126] [11/12/13/14 " rguenth at gcc dot gnu.org
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