https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63605
Bug ID: 63605 Summary: wrong code at -O3 on x86_64-linux-gnu Product: gcc Version: 5.0 Status: UNCONFIRMED Severity: normal Priority: P3 Component: tree-optimization Assignee: unassigned at gcc dot gnu.org Reporter: su at cs dot ucdavis.edu The current gcc trunk (as well as 4.8.x and 4.9.x) miscompiles the following code on x86_64-linux at -O3 in both 32-bit and 64-bit modes. This is a regression from 4.7.x. The miscompilation seems to be caused by the tree vectorizer as -fno-tree-vectorize makes it disappear. $ gcc-trunk -v Using built-in specs. COLLECT_GCC=gcc-trunk COLLECT_LTO_WRAPPER=/usr/local/gcc-trunk/libexec/gcc/x86_64-unknown-linux-gnu/5.0.0/lto-wrapper Target: x86_64-unknown-linux-gnu Configured with: ../gcc-trunk/configure --prefix=/usr/local/gcc-trunk --enable-languages=c,c++ --disable-werror --enable-multilib Thread model: posix gcc version 5.0.0 20141018 (experimental) [trunk revision 216429] (GCC) $ gcc-trunk -O2 small.c; a.out 1 $ gcc-4.7 -O3 small.c; a.out 1 $ $ gcc-trunk -O3 small.c; a.out 0 $ -------------------------------- int printf (const char *, ...); int a, b[8] = { 2, 0, 0, 0, 0, 0, 0, 0 }, c[8]; int main () { int d; for (; a < 8; a++) { d = b[a] >> 1; c[a] = d != 0; } printf ("%d\n", c[0]); return 0; }