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From: "bugdal at aerifal dot cx" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug middle-end/107115] New: Wrong codegen from TBAA under stores
 that change effective type?
Date: Sat, 01 Oct 2022 20:00:39 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D107115

            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=3D53648&action=3Dedit
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-s=
torage-and-sockaddr-in-break-strict-aliasing/42178347?noredirect=3D1#commen=
t130510083_42178347,
https://godbolt.org/z/jfv1Ge6v4) and demonstrates what appears to be wrong =
TBAA
optimization on an object with allocated storage whose effective type chang=
es
under stores. It was first presented as another example of this kind of pro=
blem
alongside the example that became
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D107107, 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 th=
at
enable -fstrict-aliasing. The expected output is 2/2. Using
-fno-strict-aliasing fixes it.=