https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84485
--- Comment #3 from rsandifo at gcc dot gnu.org <rsandifo at gcc dot gnu.org> --- Author: rsandifo Date: Mon Mar 12 13:50:52 2018 New Revision: 258450 URL: https://gcc.gnu.org/viewcvs?rev=258450&root=gcc&view=rev Log: Don't vectorise zero-step rmw operations (PR 84485) GCC 6 and 7 would vectorise: void f (unsigned long incx, unsigned long incy, float *restrict dx, float *restrict dy) { unsigned long ix = 0, iy = 0; for (unsigned long i = 0; i < 512; ++i) { dy[iy] += dx[ix]; ix += incx; iy += incy; } } without first proving that incy is nonzero. This is a regression from GCC 5. It was fixed on trunk in r256644, which versioned the loop based on whether incy is zero, but that's obviously too invasive to backport. This patch instead bails out for possibly-zero steps in the place that trunk would try a check for zeroness. Also, the patch makes vect_analyze_data_ref_access check safelen as well as force_vectorize. 2018-03-12 Richard Sandiford <richard.sandif...@linaro.org> gcc/ PR tree-optimization/84485 * tree-vect-data-refs.c (vect_analyze_data_ref_dependence): Return true for zero dependence distances if the step might be zero, and if there is no metadata that guarantees correctness. (vect_analyze_data_ref_access): Check safelen as well as force_vectorize. gcc/testsuite/ PR tree-optimization/84485 * gcc.dg/vect/pr84485.c: New test. Added: branches/gcc-7-branch/gcc/testsuite/gcc.dg/vect/pr84485.c Modified: branches/gcc-7-branch/gcc/ChangeLog branches/gcc-7-branch/gcc/testsuite/ChangeLog branches/gcc-7-branch/gcc/tree-vect-data-refs.c