https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104196
--- Comment #11 from CVS Commits <cvs-commit at gcc dot gnu.org> --- The master branch has been updated by Jakub Jelinek <ja...@gcc.gnu.org>: https://gcc.gnu.org/g:82c8ff79d06cc7d389c72f94d4443c509cf85313 commit r12-6888-g82c8ff79d06cc7d389c72f94d4443c509cf85313 Author: Jakub Jelinek <ja...@redhat.com> Date: Thu Jan 27 10:47:00 2022 +0100 reassoc: Fix up inter-bb range optimization [PR104196] As mentioned in the PR, reassoc1 miscompiles following testcase. We have: if (a.1_1 >= 0) goto <bb 5>; [59.00%] else goto <bb 4>; [41.00%] <bb 4> [local count: 440234144]: _3 = -2147483647 - a.1_1; _9 = a.1_1 != -2147479551; _4 = _3 == 1; _8 = _4 | _9; if (_8 != 0) goto <bb 5>; [34.51%] else goto <bb 3>; [65.49%] and the inter-bb range test optimization treats it as: if ((a.1_1 >= 0) | (-2147483647 - a.1_1 == 1) | (a.1_1 != -2147479551)) goto bb5; else goto bb3; and recognizes that a.1_1 >= 0 is redundant with a.1_1 != -2147479551 and so will optimize it into: if (0 | (-2147483647 - a.1_1 == 1) | (a.1_1 != -2147479551)) goto bb5; else goto bb3; When merging 2 comparisons, we use update_range_test which picks one of the comparisons as the one holding the result (currently always the RANGE one rather than all the OTHERRANGE* ones) and adjusts the others to be true or false. The problem with doing that is that means the _3 = -2147483647 - a.1_1; stmt with undefined behavior on overflow used to be conditional before but now is unconditional. reassoc performs a no_side_effect_bb check which among other checks punts on gimple_has_side_effects and gimple_assign_rhs_could_trap_p stmts as well as ones that have uses of their lhs outside of the same bb, but it doesn't punt for this potential signed overflow case. Now, for this testcase, it can be fixed in update_range_test by being smarter and choosing the other comparison to modify. This is achieved by storing into oe->id index of the bb with GIMPLE_COND the comparison feeds into not just for the cases where the comparison is the GIMPLE_COND itself, but in all cases, and then finding oe->id that isn't dominated by others. If we find such, use that oe for the merge test and if successful, swap range->idx and swap_with->idx. So for the above case we optimize it into: if ((a.1_1 != -2147479551) | (-2147483647 - a.1_1 == 1) | 0) goto bb5; else goto bb3; instead. Unfortunately, this doesn't work in all cases, optimize_range_tests_to_bit_test and optimize_range_tests_cmp_bitwise optimizations use non-NULL seq to update_range_test and they carefully choose a particular comparison because the sequence then uses SSA_NAMEs that may be defined only in their blocks. For that case, the patch keeps using the chosen comparison but if the merge is successful, rewrites stmts with signed overflow behavior into defined overflow. For this I ran into a problem, rewrite_to_defined_overflow returns a sequence that includes the original stmt with modified arguments, this means it needs to be gsi_remove first. Unfortunately, gsi_remove regardless of the remove_permanently argument creates debug temps for the lhs, which I think is quite undesirable here. So I've added an argument (default to false) to rewrite_to_defined_overflow to do the modification in place without the need to remove the stmt. 2022-01-27 Jakub Jelinek <ja...@redhat.com> PR tree-optimization/104196 * gimple-fold.h (rewrite_to_defined_overflow): Add IN_PLACE argument. * gimple-fold.cc (rewrite_to_defined_overflow): Likewise. If true, return NULL and emit needed stmts before and after stmt. * tree-ssa-reassoc.cc (update_range_test): For inter-bb range opt pick as operand_entry that will hold the merged test the one feeding earliest condition, ensure that by swapping range->idx with some other range's idx if needed. If seq is non-NULL, don't actually swap it but instead rewrite stmts with undefined overflow in between the two locations. (maybe_optimize_range_tests): Set ops[]->id to bb->index with the corresponding condition even if they have non-NULL ops[]->op. Formatting fix. * gcc.c-torture/execute/pr104196.c: New test.
