public inbox for gcc-bugs@sourceware.org help / color / mirror / Atom feed
From: "redbeard0531 at gmail dot com" <gcc-bugzilla@gcc.gnu.org> To: gcc-bugs@gcc.gnu.org Subject: [Bug target/105496] New: Comparison optimizations result in unnecessary cmp instructions Date: Thu, 05 May 2022 14:18:53 +0000 [thread overview] Message-ID: <bug-105496-4@http.gcc.gnu.org/bugzilla/> (raw) https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105496 Bug ID: 105496 Summary: Comparison optimizations result in unnecessary cmp instructions Product: gcc Version: unknown Status: UNCONFIRMED Severity: normal Priority: P3 Component: target Assignee: unassigned at gcc dot gnu.org Reporter: redbeard0531 at gmail dot com Target Milestone: --- https://godbolt.org/z/1zdYsaqEj Consider these equivalent functions: int test1(int x) { if (x <= 10) return 123; if (x == 11) return 456; return 789; } int test2(int x) { if (x < 11) return 123; if (x == 11) return 456; return 789; } In test2 it is very clear that you can do a single cmp of x with 11 then use different flag bits to choose your case. In test1 it is less clear, but because x<=10 and x<11 are equivalent, you could always transform one to the other. Clang seems to do this correctly and transforms test1 into test2 and only emits a single cmp instruction in each. For some reason, not only does gcc miss this optimization, it seems to go the other direction and transform test2 into test1, emitting 2 cmp instructions for both! test1(int): mov eax, 123 cmp edi, 10 jle .L1 cmp edi, 11 mov eax, 456 mov edx, 789 cmovne eax, edx .L1: ret test2(int): mov eax, 123 cmp edi, 10 jle .L6 cmp edi, 11 mov eax, 456 mov edx, 789 cmovne eax, edx .L6: ret Observed with at least -O2 and -O3. I initially observed this for code where each if generated an actual branch rather than a cmov, but when I reduced the example, the cmov was generated. I'm not sure if this should be a middle-end or target specific optimization, since ideally it would be smart on all targets that use comparison flags, even if there are some targets that don't. Is there ever a down side to trying to make two adjacent comparisons compare the same number?
next reply other threads:[~2022-05-05 14:18 UTC|newest] Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top 2022-05-05 14:18 redbeard0531 at gmail dot com [this message] 2022-05-06 6:06 ` [Bug target/105496] " rguenth at gcc dot gnu.org 2023-11-22 0:31 ` pinskia at gcc dot gnu.org
Reply instructions: You may reply publicly to this message via plain-text email using any one of the following methods: * Save the following mbox file, import it into your mail client, and reply-to-all from there: mbox Avoid top-posting and favor interleaved quoting: https://en.wikipedia.org/wiki/Posting_style#Interleaved_style * Reply using the --to, --cc, and --in-reply-to switches of git-send-email(1): git send-email \ --in-reply-to=bug-105496-4@http.gcc.gnu.org/bugzilla/ \ --to=gcc-bugzilla@gcc.gnu.org \ --cc=gcc-bugs@gcc.gnu.org \ /path/to/YOUR_REPLY https://kernel.org/pub/software/scm/git/docs/git-send-email.html * If your mail client supports setting the In-Reply-To header via mailto: links, try the mailto: linkBe sure your reply has a Subject: header at the top and a blank line before the message body.
This is a public inbox, see mirroring instructions for how to clone and mirror all data and code used for this inbox; as well as URLs for read-only IMAP folder(s) and NNTP newsgroup(s).