https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109031
--- Comment #30 from CVS Commits <cvs-commit at gcc dot gnu.org> --- The master branch has been updated by Roger Sayle <sa...@gcc.gnu.org>: https://gcc.gnu.org/g:eb1d8df792f990574cbb695b55c92ee2684fc96b commit r13-6621-geb1d8df792f990574cbb695b55c92ee2684fc96b Author: Roger Sayle <ro...@nextmovesoftware.com> Date: Sun Mar 12 22:52:41 2023 +0000 PR middle-end/109031: Fix final value replacement from narrower IVs. This patch fixes a P1 regression, a problem with my February 2022 patch to improve folding for final value replacement: https://gcc.gnu.org/pipermail/gcc-patches/2022-February/590618.html The motivation for the original patch is that because we know the number of loop iterations can't be negative, final value expressions such as (int) ((unsigned int) x + 4294967295) + 1 can be simplified to x, as this is effectively ((x - 1) + 1) without overflow. The bug/oversight is that using integer_all_onesp to check for the implicit tree constant -1 it didn't consider that the inner (unsigned) type might be narrower than hthe outer result type. For the case in the PR, (int)((unsigned char)x + 255) + 1 gets simplified to (int)x, but when x is originally zero, the correct result should be 256. The fix is to check that the inner type's precision (the width of the subtraction) is at least as wide as the result type (that of the addition). I've also added a test for signed types, but without -fwrapv this invokes undefined behaviour, and with -fwrapv it doesn't exhibit the problem in the PR. 2023-03-12 Roger Sayle <ro...@nextmovesoftware.com> gcc/ChangeLog PR middle-end/109031 * tree-chrec.cc (chrec_apply): When folding "{a, +, a} (x-1)", ensure that the type of x is as wide or wider than the type of a. gcc/testsuite/ChangeLog PR middle-end/109031 * gcc.dg/tree-ssa/pr109031-1.c: New test case. * gcc.dg/tree-ssa/pr109031-2.c: Likewise.
