https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95852
--- Comment #4 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:9febe9e4be7812519258ea3ed4f38bbc1a61624b commit r11-6580-g9febe9e4be7812519258ea3ed4f38bbc1a61624b Author: Jakub Jelinek <ja...@redhat.com> Date: Mon Jan 11 10:34:07 2021 +0100 widening_mul: Pattern recognize also signed multiplication with overflow check [PR95852] On top of the previous widening_mul patch, this one recognizes also (non-perfect) signed multiplication with overflow, like: int f5 (int x, int y, int *res) { *res = (unsigned) x * y; return x && (*res / x) != y; } The problem with such checks is that they invoke UB if x is -1 and y is INT_MIN during the division, but perhaps the code knows that those values won't appear. As that case is UB, we can do for that case whatever we want and handling that case as signed overflow is the best option. If x is a constant not equal to -1, then the checks are 100% correct though. Haven't tried to pattern match bullet-proof checks, because I really don't know if users would write it in real-world code like that, perhaps *res = (unsigned) x * y; return x && (x == -1 ? (*res / y) != x : (*res / x) != y); ? https://wiki.sei.cmu.edu/confluence/display/c/INT32-C.+Ensure+that+operations+on+signed+integers+do+not+result+in+overflow suggests to use twice as wide multiplication (perhaps we should handle that too, for both signed and unsigned), or some very large code with 4 different divisions nested in many conditionals, no way one can match all the possible variants thereof. 2021-01-11 Jakub Jelinek <ja...@redhat.com> PR tree-optimization/95852 * tree-ssa-math-opts.c (maybe_optimize_guarding_check): Change mul_stmts parameter type to vec<gimple *> &. Before cond_stmt allow in the bb any of the stmts in that vector, div_stmt and up to 3 cast stmts. (arith_cast_equal_p): New function. (arith_overflow_check_p): Add cast_stmt argument, handle signed multiply overflow checks. (match_arith_overflow): Adjust caller. Handle signed multiply overflow checks. * gcc.target/i386/pr95852-3.c: New test. * gcc.target/i386/pr95852-4.c: New test.
