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From: "andysem at mail dot ru" <gcc-bugzilla@gcc.gnu.org> To: gcc-bugs@gcc.gnu.org Subject: [Bug target/99563] New: Code miscompilation caused by _mm256_zeroupper() Date: Fri, 12 Mar 2021 16:36:55 +0000 [thread overview] Message-ID: <bug-99563-4@http.gcc.gnu.org/bugzilla/> (raw) https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99563 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 = 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 height) { __m128d mm_res = _mm_setzero_pd(); unsigned long block_count = static_cast< unsigned long >((width + block_size - 1) / block_size) * static_cast< unsigned long >((height + block_size - 1) / block_size); float res = 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 >= 16 && height >= block_size) { return compute_avx(data, width, height); } else { return compute_generic(data, width, height); } } $ g++ -O2 -march=sandybridge -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.
next reply other threads:[~2021-03-12 16:36 UTC|newest] Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top 2021-03-12 16:36 andysem at mail dot ru [this message] 2021-03-12 17:39 ` [Bug target/99563] [10/11 Regression] " jakub at gcc dot gnu.org 2021-03-15 12:51 ` jakub at gcc dot gnu.org 2021-03-16 10:17 ` cvs-commit at gcc dot gnu.org 2021-03-16 19:26 ` [Bug target/99563] [10 " jakub at gcc dot gnu.org 2021-03-16 20:12 ` andysem at mail dot ru 2021-03-19 23:30 ` cvs-commit at gcc dot gnu.org 2021-03-20 8:10 ` jakub at gcc dot gnu.org 2021-03-20 8:29 ` andysem at mail dot ru
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