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From: "gabravier at gmail dot com" <gcc-bugzilla@gcc.gnu.org> To: gcc-bugs@gcc.gnu.org Subject: [Bug target/98977] New: [x86] Failure to optimize consecutive sub flags usage Date: Fri, 05 Feb 2021 14:20:19 +0000 [thread overview] Message-ID: <bug-98977-4@http.gcc.gnu.org/bugzilla/> (raw) https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98977 Bug ID: 98977 Summary: [x86] Failure to optimize consecutive sub flags usage Product: gcc Version: 11.0 Status: UNCONFIRMED Severity: normal Priority: P3 Component: target Assignee: unassigned at gcc dot gnu.org Reporter: gabravier at gmail dot com Target Milestone: --- extern bool z, c; uint8_t f(uint8_t dest, uint8_t src) { u8 res = dest - src; z = !res; c = src > dest; return res; } With -O3, LLVM outputs this: f(unsigned char, unsigned char): mov eax, edi sub al, sil sete byte ptr [rip + z] setb byte ptr [rip + c] ret GCC outputs this: f(unsigned char, unsigned char): mov eax, edi sub al, sil sete BYTE PTR z[rip] cmp dil, sil setb BYTE PTR c[rip] ret It seems desirable to eliminate the `cmp`, unless there's some weird flag stall thing I'm not aware of.
next reply other threads:[~2021-02-05 14:20 UTC|newest] Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top 2021-02-05 14:20 gabravier at gmail dot com [this message] 2021-02-06 6:30 ` [Bug target/98977] " pinskia at gcc dot gnu.org 2021-12-23 21:13 ` pinskia at gcc dot gnu.org 2021-12-23 21:16 ` [Bug rtl-optimization/98977] " pinskia at gcc dot gnu.org 2021-12-30 9:34 ` [Bug rtl-optimization/98977] " crazylht at gmail dot com
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