https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70251
Bug ID: 70251 Summary: Wrong code with -O3 -march=skylake-avx512. Product: gcc Version: 6.0 Status: UNCONFIRMED Severity: normal Priority: P3 Component: c++ Assignee: unassigned at gcc dot gnu.org Reporter: vsevolod.livinskij at frtk dot ru Target Milestone: --- Created attachment 37985 --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=37985&action=edit Reproducer. Test case produces incorrect result with -O3 -march=skylake-avx512. It gives different result for every launch. Everything works fine with -O2 option. Output: > g++ repr.cpp -o out -O3 -march=skylake-avx512; sde -skx -- ./out 12033857864019976279 > g++ repr.cpp -o out -O2 -march=skylake-avx512; sde -skx -- ./out 5785906989299578598 GCC version: > g++ -v Using built-in specs. COLLECT_GCC=g++ COLLECT_LTO_WRAPPER=/export/users/vlivinsk/gcc-trunk/bin/libexec/gcc/x86_64-pc-linux-gnu/6.0.0/lto-wrapper Target: x86_64-pc-linux-gnu Configured with: /export/users/vlivinsk/gcc-trunk/gcc/configure --with-arch=corei7 --with-cpu=corei7 --enable-clocale=gnu --with-system-zlib --enable-shared --with-demangler-in-ld --enable-cloog-backend=isl --with-fpmath=sse --enable-checking=release --enable-languages=c,c++,lto --with-gmp=/export/users/vlivinsk/gcc-trunk/gmp-6.1.0/bin --with-mpfr=/export/users/vlivinsk/gcc-trunk/mpfr-3.1.3/bin --with-mpc=/export/users/vlivinsk/gcc-trunk/mpc-1.0.3/bin --prefix=/export/users/vlivinsk/gcc-trunk/bin Thread model: posix gcc version 6.0.0 20160315 (experimental) (Revision=234226) Test: #include <iostream> void hash(unsigned long long int &seed, unsigned long long int const &v) { seed ^= v + 0x9e3779b9 + (seed<<6) + (seed>>2); } unsigned int a [100]; signed char b [100]; signed char c [100]; void init () { for (int i = 0; i < 100; ++i) { a [i] = 1000L; b [i] = 10; c [i] = 5; } } void foo () { for (int i = 0; i < 100; ++i) b [i] = (!b [i] ^ (a [i] >= b [i])) + c [i]; } unsigned long long int checksum () { unsigned long long int seed = 0ULL; for (int i = 0; i < 100; ++i) hash(seed, b [i]); return seed; } int main () { init (); foo (); std::cout << checksum () << std::endl; return 0; }