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From: "david at westcontrol dot com" <gcc-bugzilla@gcc.gnu.org> To: gcc-bugs@gcc.gnu.org Subject: [Bug c/103660] New: Sub-optimal code with relational operators Date: Sat, 11 Dec 2021 13:14:51 +0000 [thread overview] Message-ID: <bug-103660-4@http.gcc.gnu.org/bugzilla/> (raw) https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103660 Bug ID: 103660 Summary: Sub-optimal code with relational operators Product: gcc Version: unknown Status: UNCONFIRMED Severity: normal Priority: P3 Component: c Assignee: unassigned at gcc dot gnu.org Reporter: david at westcontrol dot com Target Milestone: --- I recently looked at some of gcc's "if-conversions" and other optimisations of expressions involving relational operators - something people might use when trying to write branchless code. I know that it is often best to write in the clearest way, including branches, and let the compiler handle the optimisation. But people do try to do this kind of thing by hand. I tested 6 examples of ways to write a simple "min" function: int min1(int a, int b) { if (a < b) return a; else return b; } int min2(int a, int b) { return (a < b) ? a : b; } int min3(int a, int b) { return (a < b) * a | (a >= b) * b; } int min4(int a, int b) { return (a < b) * a + (a >= b) * b; } int min5(int a, int b) { const int c = a < b; return c * a + (1 - c) * b; } int min6(int a, int b) { const bool c = a < b; return c * a + !c * b; } gcc happily optimises the first two versions. For the next two, it uses conditional moves for each half of the expression, then combines them with "or" or "add". For version 5, it generates two multiply instructions, and version 6 is even worse in trunk (gcc 12). This last one is a regression - gcc 11 generates the same code for version 6 as for version 4 (not optimal, but not as bad). For comparison, clang 5+ generates optimal code for all versions. I have tried a number of different targets (godbolt is wonderful for this stuff), with similar results.
next reply other threads:[~2021-12-11 13:14 UTC|newest] Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top 2021-12-11 13:14 david at westcontrol dot com [this message] 2021-12-12 23:58 ` [Bug tree-optimization/103660] " pinskia at gcc dot gnu.org 2023-08-23 3:45 ` pinskia at gcc dot gnu.org 2023-08-23 4:45 ` pinskia at gcc dot gnu.org 2023-08-23 4:49 ` pinskia at gcc dot gnu.org
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