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From: "goldstein.w.n at gmail dot com" <gcc-bugzilla@gcc.gnu.org> To: gcc-bugs@gcc.gnu.org Subject: [Bug c/96632] New: missed-optimization with conditionally unsetting bits in memory. Date: Sun, 16 Aug 2020 20:36:42 +0000 [thread overview] Message-ID: <bug-96632-4@http.gcc.gnu.org/bugzilla/> (raw) https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96632 Bug ID: 96632 Summary: missed-optimization with conditionally unsetting bits in memory. Product: gcc Version: 10.2.0 Status: UNCONFIRMED Severity: normal Priority: P3 Component: c Assignee: unassigned at gcc dot gnu.org Reporter: goldstein.w.n at gmail dot com Target Milestone: --- https://stackoverflow.com/questions/63432157/why-does-gcc-use-btq-in-conjunction-with-btcq-when-conditionally-set-a-bit-in-a Basically: void __attribute__((noinline)) cond_unset_bit(uint64_t * v, uint32_t b) { if(__builtin_expect(!!(*v & ((1UL) << b)), 1)) { *v ^= ((1UL) << b); } } Compiles to: cond_unset_bit(unsigned long*, unsigned int): movq (%rdi), %rax btq %rsi, %rax jnc .L6 btcq %rsi, %rax movq %rax, (%rdi) .L6: ret The btq instruction is unnecessary. cond_unset_bit(unsigned long*, unsigned int): movq (%rdi), %rax btcq %rsi, %rax jnc .L6 movq %rax, (%rdi) .L6: ret Accomplishes the same thing without the btq instruction.
next reply other threads:[~2020-08-16 20:36 UTC|newest] Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top 2020-08-16 20:36 goldstein.w.n at gmail dot com [this message] 2020-08-25 10:26 ` [Bug rtl-optimization/96632] " rguenth at gcc dot gnu.org 2021-08-14 9:47 ` pinskia at gcc dot gnu.org
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