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From: "gabravier at gmail dot com" <gcc-bugzilla@gcc.gnu.org> To: gcc-bugs@gcc.gnu.org Subject: [Bug target/97887] New: Failure to optimize neg plus div to avoid using x87 floating point stack Date: Wed, 18 Nov 2020 09:35:52 +0000 [thread overview] Message-ID: <bug-97887-4@http.gcc.gnu.org/bugzilla/> (raw) https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97887 Bug ID: 97887 Summary: Failure to optimize neg plus div to avoid using x87 floating point stack Product: gcc Version: 11.0 Status: UNCONFIRMED Severity: normal Priority: P3 Component: target Assignee: unassigned at gcc dot gnu.org Reporter: gabravier at gmail dot com Target Milestone: --- float f(float a) { return -a / a; } On x86 -O3, LLVM outputs this: .LCPI0_0: .long 0x80000000 # float -0 .long 0x80000000 # float -0 .long 0x80000000 # float -0 .long 0x80000000 # float -0 f(float): movaps xmm1, xmmword ptr [rip + .LCPI0_0] # xmm1 = [-0.0E+0,-0.0E+0,-0.0E+0,-0.0E+0] xorps xmm1, xmm0 divss xmm1, xmm0 movaps xmm0, xmm1 ret GCC outputs this: f(float): movss DWORD PTR [rsp-4], xmm0 fld DWORD PTR [rsp-4] movaps xmm1, xmm0 fchs fstp DWORD PTR [rsp-4] movss xmm0, DWORD PTR [rsp-4] divss xmm0, xmm1 ret I'm *pretty sure* that loading the value into the x87 stack (especially mixed with SSE instructions) is much slower than using SSE instructions for this.
next reply other threads:[~2020-11-18 9:35 UTC|newest] Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top 2020-11-18 9:35 gabravier at gmail dot com [this message] 2020-11-18 10:33 ` [Bug target/97887] [10/11 Regression] " rguenth at gcc dot gnu.org 2020-11-18 10:33 ` rguenth at gcc dot gnu.org 2020-11-18 10:42 ` rguenth at gcc dot gnu.org 2020-11-18 12:11 ` ubizjak at gmail dot com 2020-11-18 13:54 ` rguenth at gcc dot gnu.org 2020-11-18 13:56 ` ubizjak at gmail dot com 2020-11-18 13:58 ` rguenth at gcc dot gnu.org 2020-11-18 15:09 ` ubizjak at gmail dot com 2020-11-19 9:00 ` rguenth at gcc dot gnu.org
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