https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110753
--- Comment #4 from CVS Commits <cvs-commit at gcc dot gnu.org> --- The master branch has been updated by Aldy Hernandez <al...@gcc.gnu.org>: https://gcc.gnu.org/g:53fcd3bdbc9104a8acdc79134d8d272d7eead0fd commit r14-3326-g53fcd3bdbc9104a8acdc79134d8d272d7eead0fd Author: Aldy Hernandez <al...@redhat.com> Date: Fri Aug 18 12:41:46 2023 +0200 [irange] Return FALSE if updated bitmask is unchanged [PR110753] The mask/value pair we track in the irange is a bit fickle in that it can sometimes contradict the bitmask inherent in the range. This can happen when a series of calculations yield a combination such as: [3, 1000] MASK 0xfffffffe VALUE 0x0 The mask/value above implies that the lowest bit is a known 0, which would exclude the 3 in the range. At one time we tried keeping mask and ranges 100% consistent, but the performance penalty was too high (5% in VRP). Also, it's unclear whether the intersection of two incompatible known bits should make the whole range undefined, or just the contradicting bits. This is all documented in irange::get_bitmask(). We could revisit both of these assumptions in the future. In this testcase IPA ends up with a range where the lower 2 bits are expected to be 0, but the range is [1,1]. [irange] long int [1, 1] MASK 0xfffffffffffffffc VALUE 0x0 This causes irange::union_bitmask() to think an update occurred, when no semantic change happened, thus triggering an assert in IPA-cp. We could get rid of the assert, but it's cleaner to make irange::{union,intersect}_bitmask always tell the truth. Beside, the ranger's cache also depends on union being truthful. PR ipa/110753 gcc/ChangeLog: * value-range.cc (irange::union_bitmask): Return FALSE if updated bitmask is semantically equivalent to the original mask. (irange::intersect_bitmask): Same. (irange::get_bitmask): Add comment. gcc/testsuite/ChangeLog: * gcc.dg/tree-ssa/pr110753.c: New test.
