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From: "bartoldeman at users dot sourceforge.net" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug target/101683] New: Floating point exception for
 double->unsigned conversion on avx512 only
Date: Fri, 30 Jul 2021 02:30:04 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D101683

            Bug ID: 101683
           Summary: Floating point exception for double->unsigned
                    conversion on avx512 only
           Product: gcc
           Version: 10.3.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: bartoldeman at users dot sourceforge.net
  Target Milestone: ---

Created attachment 51222
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=3D51222&action=3Dedit
File to reproduce

For this code:

#define _GNU_SOURCE
#include <fenv.h>

int main(int argc, char **argv) {
    feenableexcept(FE_INVALID);
    double argcm10 =3D argc / -0.1;
    return (unsigned)(argcm10 < 0.0 ? 0 : argcm10);
}

$ gcc -O -march=3Dskylake-avx512 fpexcept.c -lm
$ ./a.out=20
Floating point exception


the instructions
        vcvttsd2usi     %xmm0, %eax
        vxorpd  %xmm1, %xmm1, %xmm1
        vucomisd        %xmm0, %xmm1
        movl    $0, %edx
        cmova   %edx, %eax
are generated just after the division, so the conversion happens before the
comparison.
"If a converted result cannot be represented in the destination format, the
floating-point invalid exception is raised, and if this exception is masked,
the integer value 2^w =E2=80=93 1 is returned, where w represents the numbe=
r of bits in
the destination format."

so when masked, for argcm10 =3D -10.0 the value 2^w-1 is discarded and all =
is
well, since it's < 0, but not when unmasked.

I can reproduce this issue with 9.3 as well, but not with 8.4 (the generated
code is correct for 8.4). I have not tried 11.1 yet.

Note: I found this issue with the UCX library when compiled with
-march=3Dskylake-avx512, this example is stripped down from:
https://github.com/openucx/ucx/blob/f5362f5e6f80d930b88c44c63b4d8d71cf91d21=
4/src/ucp/core/ucp_ep.c#L2699=