[Bug middle-end/107115] New: Wrong codegen from TBAA under stores that change effective type?

["bugdal at aerifal dot cx" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107115

            Bug ID: 107115
           Summary: Wrong codegen from TBAA under stores that change
                    effective type?
           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 53648
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=53648&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#comment130510083_42178347,
https://godbolt.org/z/jfv1Ge6v4) and demonstrates what appears to be wrong TBAA
optimization on an object with allocated storage whose effective type changes
under stores. It was first presented as another example of this kind of problem
alongside the example that became
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107107, but it seems likely that
the root cause is distinct.

Reportedly clang/LLVM also transforms this example wrong.

On 64-bit targets, the test program outputs 2/1 with optimization levels that
enable -fstrict-aliasing. The expected output is 2/2. Using
-fno-strict-aliasing fixes it.