https://gcc.gnu.org/bugzilla/show_bug.cgi?id=126476
--- Comment #4 from GCC Commits <cvs-commit at gcc dot gnu.org> --- The releases/gcc-16 branch has been updated by Jakub Jelinek <[email protected]>: https://gcc.gnu.org/g:ec33d63beddc5ba8fc7618792ccef13bd2fb8bc1 commit r16-9474-gec33d63beddc5ba8fc7618792ccef13bd2fb8bc1 Author: Jakub Jelinek <[email protected]> Date: Fri Jul 31 08:53:52 2026 +0200 match.pd: Fix up ((C << A) & D) != 0 simplification [PR126476] This simplification for power of two @1 and @2 folds to false (resp. to true for the == version) if @1 is larger than @2 (in unsigned comparison), because @1 & @2 is known to be zero (i.e. for shift count 0) and for shift count larger than that it will be zero too, either because @1 << @0 is even larger, or if @0 is too large @1 << @0 overflows to zero. This is the case of e.g. ((4 << x) & 2) != 0, which is always false. Now, this PR is about a different problem, if @1 is smaller than @2, say ((1 << x) & 256) != 0, but x has a very narrow type, say unsigned _BitInt(3), then the largest possible value of x is 7 and ((1 << 7) & 256) is still 0, 1 << 7 is 128 and so still smaller than 256. So, if c1 - c2 doesn't fit into the shift count type (resp. for the other case c2 - c1), it will be also always false (resp. true). Trying to improve it and using range of x (aka @0) is not needed, this simplification folds it into @0 != (c1 - c2) and so will be folded later. Just the case where c1 - c2 overflows is problematic because we've lost the details (unless we'd promote both operands or something). Another possible way to do this would be build_int_cst and check for the overflow flags, but I think this is shorter. 2026-07-31 Jakub Jelinek <[email protected]> PR tree-optimization/126476 * match.pd (((C << A) & D) != 0 -> A == 0, ((C << A) & D) == 0 -> A != 0): Fold to false/true if c1 - c2 resp. c2 - c1 doesn't fit into TREE_TYPE (@0). * gcc.dg/torture/bitint-103.c: New test. Reviewed-by: Richard Biener <[email protected]> (cherry picked from commit 5c62a2771f2c7d5301ee6456f1817098c2fea113)
