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From: "goswin-v-b at web dot de" <gcc-bugzilla@gcc.gnu.org> To: gcc-bugs@gcc.gnu.org Subject: [Bug c/105521] New: missed optimization in modulo arithmetic Date: Sun, 08 May 2022 04:08:22 +0000 [thread overview] Message-ID: <bug-105521-4@http.gcc.gnu.org/bugzilla/> (raw) https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105521 Bug ID: 105521 Summary: missed optimization in modulo arithmetic Product: gcc Version: 11.3.0 Status: UNCONFIRMED Severity: normal Priority: P3 Component: c Assignee: unassigned at gcc dot gnu.org Reporter: goswin-v-b at web dot de Target Milestone: --- I'm trying to compute (a*a)%n for uint64_t types on x86_64 using "gcc -O2 -W -Wall" like this: #include <stdint.h> #include <assert.h> uint64_t sqrmod(uint64_t a, uint64_t n) { assert(a < n); unsigned __int128 x = a; x *= a; return x % n; } I expected to get the following code: sqrmod: cmpq %rsi, %rdi jnb .L13 // assert(a < n) failure movq %rdi, %rax mul %rdi div %rsi movq %rdx, %rax ret The compiler does get the "mul" right but instead of the "div" it throws in a call to "__umodti3". The "__umodti3" function is horribly long code that will be worlds slower than a simple div. Note: The "asset(a < n);" should tell the compiler that the "div" instruction can not overflow and will not cause a #DivisionError. Without the assert the compiler could (conditionally) add "a %= n;" for the same effect. https://godbolt.org/z/cd57Wd4oo
next reply other threads:[~2022-05-08 4:08 UTC|newest] Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top 2022-05-08 4:08 goswin-v-b at web dot de [this message] 2022-05-08 4:38 ` [Bug target/105521] " pinskia at gcc dot gnu.org 2022-05-08 4:38 ` pinskia at gcc dot gnu.org 2022-05-08 4:40 ` pinskia at gcc dot gnu.org 2022-05-08 12:45 ` goswin-v-b at web dot de
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