https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98302
--- Comment #13 from CVS Commits <cvs-commit at gcc dot gnu.org> --- The master branch has been updated by Richard Sandiford <rsand...@gcc.gnu.org>: https://gcc.gnu.org/g:58a12b0eadac62e691fcf7325ab2bc2c93d46b61 commit r11-6381-g58a12b0eadac62e691fcf7325ab2bc2c93d46b61 Author: Richard Sandiford <richard.sandif...@arm.com> Date: Thu Dec 31 16:51:34 2020 +0000 vect: Avoid generating out-of-range shifts [PR98302] In this testcase we end up with: unsigned long long x = ...; char y = (char) (x << 37); The overwidening pattern realised that only the low 8 bits of x << 37 are needed, but then tried to turn that into: unsigned long long x = ...; char y = (char) x << 37; which gives an out-of-range shift. In this case y can simply be replaced by zero, but as the comment in the patch says, it's kind-of awkward to do that in the middle of vectorisation. Most of the overwidening stuff is about keeping operations as narrow as possible, which is important for vectorisation but could be counter-productive for scalars (especially on RISC targets). In contrast, optimising y to zero in the above feels like an independent optimisation that would benefit scalar code and that should happen before vectorisation. gcc/ PR tree-optimization/98302 * tree-vect-patterns.c (vect_determine_precisions_from_users): Make sure that the precision remains greater than the shift count. gcc/testsuite/ PR tree-optimization/98302 * gcc.dg/vect/pr98302.c: New test.
