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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/107107] New: Wrong codegen from TBAA when stores to
 distinct same-mode types are collapsed?
Date: Fri, 30 Sep 2022 21:34:15 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D107107

            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=3D53646&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=
t130509588_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 th=
at
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.=