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From: "evan@coeus-group.com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug target/100927] New: [sse2] floating point to integer conversion
 functions incorrect results w/ NaN constants + optimization
Date: Sat, 05 Jun 2021 18:40:27 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D100927

            Bug ID: 100927
           Summary: [sse2] floating point to integer conversion functions
                    incorrect results w/ NaN constants + optimization
           Product: gcc
           Version: 11.1.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: evan@coeus-group.com
  Target Milestone: ---

_mm_cvttpd_epi32, _mm_cvttpd_pi32, _mm_cvttps_epi32, and _mm_cvttsd_si32 are
supposed to return INT32_MIN for NaN inputs.  However, when compiled with
optimization on GCC, if the values are known at compile time NaN inputs res=
ult
in 0 in the output.

Here is a quick test case, using _mm_cvttpd_epi32:


#include <xmmintrin.h>
#include <stdio.h>

int main(void) {
  static const double values[] =3D {
    __builtin_nan(""), -__builtin_nan("")
  };
  int32_t res[4];

  _mm_storeu_si128((__m128i*) res, _mm_cvttpd_epi32(_mm_loadu_pd(values)));

  for (int i =3D 0 ; i < 4 ; i++) {
    printf("%d\n", res[i]);
  }

  return 0;
}


Compile with `gcc -O1 -o test test.c` and you get all zeros, `gcc -O0 -o te=
st
test.c` and the first two elements of the result are INT32_MIN as they shou=
ld
be.  Changing the const to volatile (and adding -Wno-discarded-qualifiers)
"fixes" the issue.=