[Bug middle-end/107107] New: Wrong codegen from TBAA when stores to distinct same-mode types are collapsed?

[bugdal at aerifal dot cx]

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.