https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90248
--- Comment #20 from CVS Commits <cvs-commit at gcc dot gnu.org> --- The releases/gcc-9 branch has been updated by Jakub Jelinek <ja...@gcc.gnu.org>: https://gcc.gnu.org/g:45a6eae129c6fee387a3bb7075181d8509fa6e2a commit r9-9407-g45a6eae129c6fee387a3bb7075181d8509fa6e2a Author: Jakub Jelinek <ja...@redhat.com> Date: Fri Jan 22 11:50:18 2021 +0100 match.pd: Replace incorrect simplifications into copysign [PR90248] In the PR Andrew said he has implemented a simplification that has been added to LLVM, but that actually is not true, what is in there are X * (X cmp 0.0 ? +-1.0 : -+1.0) simplifications into +-abs(X) but what has been added into GCC are (X cmp 0.0 ? +-1.0 : -+1.0) simplifications into copysign(1, +-X) and then X * copysign (1, +-X) into +-abs (X). The problem is with the (X cmp 0.0 ? +-1.0 : -+1.0) simplifications, they don't work correctly when X is zero. E.g. (X > 0.0 ? 1.0 : -1.0) is -1.0 when X is either -0.0 or 0.0, but copysign will make it return 1.0 for 0.0 and -1.0 only for -0.0. (X >= 0.0 ? 1.0 : -1.0) is 1.0 when X is either -0.0 or 0.0, but copysign will make it return still 1.0 for 0.0 and -1.0 for -0.0. The simplifications were guarded on !HONOR_SIGNED_ZEROS, but as discussed in the PR, that option doesn't mean that -0.0 will not ever appear as operand of some operation, it is hard to guarantee that without compiler adding canonicalizations of -0.0 to 0.0 after most of the operations and thus making it very slow, but that the user asserts that he doesn't care if the result of operations will be 0.0 or -0.0. Not to mention that some of the transformations are incorrect even for positive 0.0. So, instead of those simplifications this patch recognizes patterns where those ?: expressions are multiplied by X, directly into +-abs. That works fine even for 0.0 and -0.0 (as long as we don't care about whether the result is exactly 0.0 or -0.0 in those cases), because whether the result of copysign is -1.0 or 1.0 doesn't matter when it is multiplied by 0.0 or -0.0. As a follow-up, maybe we should add the simplification mentioned in the PR, in particular doing copysign by hand through VIEW_CONVERT_EXPR <int, float_X> < 0 ? -float_constant : float_constant into copysign (float_constant, float_X). But I think that would need to be done in phiopt. 2021-01-22 Jakub Jelinek <ja...@redhat.com> PR tree-optimization/90248 * match.pd (X cmp 0.0 ? 1.0 : -1.0 -> copysign(1, +-X), X cmp 0.0 ? -1.0 : +1.0 -> copysign(1, -+X)): Remove simplifications. (X * (X cmp 0.0 ? 1.0 : -1.0) -> +-abs(X), X * (X cmp 0.0 ? -1.0 : 1.0) -> +-abs(X)): New simplifications. * gcc.dg/tree-ssa/copy-sign-1.c: Don't expect any copysign builtins. * gcc.dg/pr90248.c: New test. (cherry picked from commit dd92986ea6d2d363146e1726817a84910453fdc8)
