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From: "unlvsur at live dot com" <gcc-bugzilla@gcc.gnu.org> To: gcc-bugs@gcc.gnu.org Subject: [Bug rtl-optimization/107167] New: It looks like GCC wastes registers on trivial computations when result can be cached Date: Thu, 06 Oct 2022 05:33:59 +0000 [thread overview] Message-ID: <bug-107167-4@http.gcc.gnu.org/bugzilla/> (raw) https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107167 Bug ID: 107167 Summary: It looks like GCC wastes registers on trivial computations when result can be cached Product: gcc Version: 13.0 Status: UNCONFIRMED Severity: normal Priority: P3 Component: rtl-optimization Assignee: unassigned at gcc dot gnu.org Reporter: unlvsur at live dot com Target Milestone: --- I do not know whether it is a big issue or not with targets that provide tons of available registers (like aarch64 or loongarch64). However, this looks like a big issue for x86_64 which only provides 16 general purpose registers (plus %rsp is reserved, so 15 available registers) Take the example like this: https://godbolt.org/z/77rEsr1PG #include<bit> unsigned Sigma1(unsigned x) noexcept { return std::rotr(x,6)^std::rotr(x,11)^std::rotr(x,25); } GCC generates code like this to avoid dependencies. Sigma1m(unsigned int): movl %edi, %eax movl %edi, %edx roll $7, %edi rorl $6, %eax rorl $11, %edx xorl %edx, %eax xorl %edi, %eax ret However: mySigma1m(unsigned int): movl %edi, %eax rorl $6, %edi rorl $11, %eax xorl %edi, %eax rorl $19, %edi xorl %edi, %eax ret Saves one register in this task. That becomes a huge problem when tons of computation are involved where registers are in a position of shortage. 1st one also generates 1 more instruction and it can affect the code cache. Aggressively utilizing all registers may not give the best results. Local maximum =/= Global maximum. I don't know.
next reply other threads:[~2022-10-06 5:33 UTC|newest] Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top 2022-10-06 5:33 unlvsur at live dot com [this message] 2022-10-06 5:55 ` [Bug rtl-optimization/107167] " pinskia at gcc dot gnu.org 2022-10-06 6:00 ` unlvsur at live dot com 2022-10-06 6:03 ` unlvsur at live dot com 2022-10-06 6:05 ` unlvsur at live dot com 2022-10-06 6:47 ` pinskia at gcc dot gnu.org 2022-10-07 1:41 ` unlvsur at live dot com
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