https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83239
--- Comment #23 from Alexandre Oliva <aoliva at gcc dot gnu.org> --- Author: aoliva Date: Wed Dec 19 06:51:41 2018 New Revision: 267252 URL: https://gcc.gnu.org/viewcvs?rev=267252&root=gcc&view=rev Log: [PR86153] simplify more overflow tests in VRP PR 86153 was originally filed when changes to the C++11's implementation of vector resize(size_type) limited inlining that were required for testsuite/g++.dg/pr83239.C to verify that we did not issue an undesired warning. That was worked by increasing the limit for inlining, but that in turn caused the C++98 implementation of vector resize, that is significantly different, to also be fully inlined, and that happened to issue the very warnings the test was meant to verify we did NOT issue. The reason we issued the warnings was that we failed to optimize out some parts of _M_fill_insert, used by the C++98 version of vector resize, although the call of _M_fill_insert was guarded by a test that could never pass: test testcase only calls resize when the vector size is >= 3, to decrement the size by two. The limitation we hit in VRP was that the compared values could pass as an overflow test, if the vector size was 0 or 1 (we knew it wasn't), but even with dynamic ranges we failed to decide that the test result could be determined at compile time, even though after the test we introduced ASSERT_EXPRs that required a condition known to be false from earlier ones. I pondered turning ASSERT_EXPRs that show impossible conditions into traps, to enable subsequent instructions to be optimized, but I ended up finding an earlier spot in which an overflow test that would have introduced the impossible ASSERT_EXPR can have its result deduced from earlier known ranges and resolved to the other path. Although such overflow tests could be uniformly simplified to compares against a constant, the original code would only perform such simplifications when the test could be resolved to an equality test against zero. I've thus avoided introducing compares against other constants, and instead added code that will only simplify overflow tests that weren't simplified before when the condition can be evaluated at compile time. for gcc/ChangeLog PR testsuite/86153 PR middle-end/83239 * vr-values.c (vr_values::vrp_evaluate_conditional_warnv_with_ops): Extend simplification of overflow tests to cover cases in which we can determine the result of the comparison. for gcc/testsuite/ChangeLog PR testsuite/86153 PR middle-end/83239 * gcc.dg/vrp-overflow-1.c: New. Added: trunk/gcc/testsuite/gcc.dg/vrp-overflow-1.c Modified: trunk/gcc/ChangeLog trunk/gcc/testsuite/ChangeLog trunk/gcc/vr-values.c
