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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
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X-Bugzilla-Product: gcc
X-Bugzilla-Component: rtl-optimization
X-Bugzilla-Version: 11.3.0
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X-Bugzilla-Severity: normal
X-Bugzilla-Who: pavel.morozkin at gmail dot com
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D111101

            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=20
 in case of NaNs

Demo:
$ gcc t1.c -O1 -std=3Dc11 && ./a
r     nan 7fe5ed65
r_ref nan 7fe5ed65

$ gcc t1.c -O1 -std=3Dc11 -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 =3D> ecx =3D> ebp =3D> xmm0
y =3D> edx =3D> edi =3D> xmm1
addss xmm1, xmm0 (at&t syntax)

-O1 -finline-small-functions:
add(x, y)
x =3D> ecx =3D> esi =3D> xmm1
y =3D> edx =3D> ebx =3D> 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.=