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From: "pavel.morozkin at gmail dot com" <gcc-bugzilla@gcc.gnu.org> To: gcc-bugs@gcc.gnu.org Subject: [Bug rtl-optimization/111101] New: -finline-small-functions may invert FP arguments breaking FP bit accuracy in case of NaNs Date: Tue, 22 Aug 2023 13:30:05 +0000 [thread overview] Message-ID: <bug-111101-4@http.gcc.gnu.org/bugzilla/> (raw) https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111101 Bug ID: 111101 Summary: -finline-small-functions may invert FP arguments breaking FP bit accuracy in case of NaNs Product: gcc Version: 11.3.0 Status: UNCONFIRMED Severity: normal Priority: P3 Component: rtl-optimization Assignee: unassigned at gcc dot gnu.org Reporter: pavel.morozkin at gmail dot com Target Milestone: --- Notes: 1. This may not be a bug. 2. This may be a duplicate. 3. I don't have MRE. Brief: -finline-small-functions may invert FP arguments breaking FP bit accuracy in case of NaNs Demo: $ gcc t1.c -O1 -std=c11 && ./a r nan 7fe5ed65 r_ref nan 7fe5ed65 $ gcc t1.c -O1 -std=c11 -finline-small-functions && ./a r -nan fffffffe r_ref nan 7fe5ed65 Description: In my code I add two FP values (represented in "raw hex"): 0x7fa5ed65 (sNaN) with 0xfffffffe (qNaN). x86_64 instruction addss returns 0x7fe5ed65 (sNaN). However, under -finline-small-functions gcc, I guess, rewrites A+B to B+A, resulting in 0xfffffffe (qNaN), which breaks FP bit accuracy. I examined generated assembly code: -O1: add(x, y) x => ecx => ebp => xmm0 y => edx => edi => xmm1 addss xmm1, xmm0 (at&t syntax) -O1 -finline-small-functions: add(x, y) x => ecx => esi => xmm1 y => edx => ebx => xmm0 addss xmm1, xmm0 (at&t syntax) Here we see that in case of -finline-small-functions x and y are inverted. Notes: 1. Some software may rely on FP bit accuracy in case of NaNs (NaN boxing, etc.). 2. I'm not sure which "Component:" to select: rtl-optimization or tree-optimization.
next reply other threads:[~2023-08-22 13:30 UTC|newest] Thread overview: 2+ messages / expand[flat|nested] mbox.gz Atom feed top 2023-08-22 13:30 pavel.morozkin at gmail dot com [this message] 2023-08-22 14:06 ` [Bug rtl-optimization/111101] " amonakov at gcc dot gnu.org
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