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From: "andysem at mail dot ru" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug target/99563] New: Code miscompilation caused by
 _mm256_zeroupper()
Date: Fri, 12 Mar 2021 16:36:55 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D99563

            Bug ID: 99563
           Summary: Code miscompilation caused by _mm256_zeroupper()
           Product: gcc
           Version: 10.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: andysem at mail dot ru
  Target Milestone: ---

Consider the following code:

#include <immintrin.h>

constexpr unsigned int block_size =3D 8u;

float compute_generic(const double* data, unsigned int width, unsigned int
height);

inline __attribute__((always_inline))
float compute_avx(const double* data, unsigned int width, unsigned int heig=
ht)
{
    __m128d mm_res =3D _mm_setzero_pd();
    unsigned long block_count =3D static_cast< unsigned long >((width +
block_size - 1) / block_size)
        * static_cast< unsigned long >((height + block_size - 1) / block_si=
ze);

    float res =3D static_cast< float >(_mm_cvtsd_f64(mm_res) / static_cast<
double >(block_count));

    _mm256_zeroupper();

    return res;
}

float compute(const double* data, unsigned int width, unsigned int height)
{
    if (width >=3D 16 && height >=3D block_size)
    {
        return compute_avx(data, width, height);
    }
    else
    {
        return compute_generic(data, width, height);
    }
}

$ g++ -O2 -march=3Dsandybridge -mno-vzeroupper -o test.o test.cpp

https://gcc.godbolt.org/z/dhr7an

The code compiles to:

compute(double const*, unsigned int, unsigned int):
        cmp     esi, 15
        jbe     .L2
        cmp     edx, 7
        jbe     .L2
        vzeroupper
        ret
.L2:
        jmp     compute_generic(double const*, unsigned int, unsigned int)

which leaves the result of compute() uninitialized if AVX path is taken. The
problem disappears if one of the following is done:

- -O2 is replaced with -O1
- -mno-vzeroupper is removed
- _mm256_zeroupper(); call is removed (the upper bits of vector registers is
left dirty, though)

This is a regression in gcc 10 branch and later, gcc 9.x compiles this
correctly.=