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From: "nathan at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org> To: gcc-bugs@gcc.gnu.org Subject: [Bug tree-optimization/96633] New: missed optimization? Date: Sun, 16 Aug 2020 21:25:44 +0000 [thread overview] Message-ID: <bug-96633-4@http.gcc.gnu.org/bugzilla/> (raw) https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96633 Bug ID: 96633 Summary: missed optimization? Product: gcc Version: 10.0 Status: UNCONFIRMED Severity: normal Priority: P3 Component: tree-optimization Assignee: unassigned at gcc dot gnu.org Reporter: nathan at gcc dot gnu.org Target Milestone: --- Matt Godbolt's https://queue.acm.org/detail.cfm?id=3372264 has an example of optimizing on amd64: bool isWhitespace(char c) { return c == ' ' || c == '\r' || c == '\n' || c == '\t'; } GCC generates: xorl %eax, %eax cmpb $32, %dil ja .L1 movabsq $4294977024, %rax movl %edi, %ecx shrq %cl, %rax andl $1, %eax .L1: ret following an amazing comment on the ML, I've determined the following is abot 12% faster (and shorter too): movabsq $4294977024, %rax movl %edi, %ecx shrq %cl, %rax shr $6, %ecx andl $1, %eax shrq %cl, %rax ret We're dealing with chars in the range [-128,128), and x86's shift operator only considers the bottom 6 bits.
next reply other threads:[~2020-08-16 21:25 UTC|newest] Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top 2020-08-16 21:25 nathan at gcc dot gnu.org [this message] 2020-08-17 8:14 ` [Bug tree-optimization/96633] " marxin at gcc dot gnu.org 2020-08-17 12:07 ` amonakov at gcc dot gnu.org 2020-08-20 11:36 ` marxin at gcc dot gnu.org 2021-07-21 1:05 ` [Bug middle-end/96633] " pinskia at gcc dot gnu.org
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