https://gcc.gnu.org/bugzilla/show_bug.cgi?id=118805
--- Comment #6 from GCC Commits <cvs-commit at gcc dot gnu.org> --- The master branch has been updated by Alexandre Oliva <aol...@gcc.gnu.org>: https://gcc.gnu.org/g:3768bedf7b5fcdd63a18871ecfce665ae1b8d87e commit r15-7597-g3768bedf7b5fcdd63a18871ecfce665ae1b8d87e Author: Alexandre Oliva <ol...@adacore.com> Date: Mon Feb 17 23:17:21 2025 -0300 [ifcombine] cope with signbit tests of extended values A compare with zero may be taken as a sign bit test by fold_truth_andor_for_ifcombine, but the operand may be extended from a narrower field. If the operand was narrower, the bitsize will reflect the narrowing conversion, but if it was wider, we'll only know whether the field is sign- or zero-extended from unsignedp, but we won't know whether it needed to be extended, because arg will have changed to the narrower variable when we get to the point in which we can compute the arg width. If it's sign-extended, we're testing the right bit, but if it's zero-extended, there isn't any bit we can test. Instead of punting and leaving the foldable compare to be figured out by another pass, arrange for the sign bit resulting from the widening zero-extension to be taken as zero, so that the modified compare will yield the desired result. While at that, avoid swapping the right-hand compare operands when we've already determined that it was a signbit test: it no use to even try. for gcc/ChangeLog PR tree-optimization/118805 * gimple-fold.cc (fold_truth_andor_for_combine): Detect and cope with zero-extension in signbit tests. Reject swapping right-compare operands if rsignbit. for gcc/testsuite/ChangeLog PR tree-optimization/118805 * gcc.dg/field-merge-26.c: New.
