https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107107
Bug ID: 107107 Summary: Wrong codegen from TBAA when stores to distinct same-mode types are collapsed? Product: gcc Version: unknown Status: UNCONFIRMED Severity: normal Priority: P3 Component: middle-end Assignee: unassigned at gcc dot gnu.org Reporter: bugdal at aerifal dot cx Target Milestone: --- Created attachment 53646 --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=53646&action=edit original test case by supercat The attached test case is from user supercat on Stack Overflow (original source: https://stackoverflow.com/questions/42178179/will-casting-around-sockaddr-storage-and-sockaddr-in-break-strict-aliasing/42178347?noredirect=1#comment130509588_42178347, https://godbolt.org/z/83v4ssrn4) and demonstrates wrong TBAA apparently assuming an object of type long long was not modified after the code path modifying it was collapsed with a different code path performing the modification via an lvalue of type long. On 64-bit targets, the test program outputs 1/2 with optimization levels that enable -fstrict-aliasing. The expected output is 2/2. Using -fno-strict-aliasing fixes it. I have not checked this myself, but according to others who have looked at the test case, the regression came between GCC 4.7 and 4.8.