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From: "drohr at jwdt dot org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/102937] New: Miscompilation with -O3 and aliasing of char*
 and size_t
Date: Mon, 25 Oct 2021 21:02:08 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D102937

            Bug ID: 102937
           Summary: Miscompilation with -O3 and aliasing of char* and
                    size_t
           Product: gcc
           Version: 11.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: drohr at jwdt dot org
  Target Milestone: ---

Created attachment 51663
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=3D51663&action=3Dedit
testcase

The attached testcase miscompiles with GCC 11.2 when using -O3. It compiles
correctly with -O3, and also with GCC 10.3 using -O3. For reference the cod=
e is
also here: https://godbolt.org/z/cjr584sMo

The program prints a ptr twice, which should have been modified in between,=
 but
this is not the case, thus it prints twice a nullptr:
FOO 0x(nil)
BAR 0x(nil)

By either commenting in the printf in line 51 (or also by outcommenting the
call to computePointerWithAlignment in line 50) the miscompilation disappea=
rs,
and the output is
FOO 0x(nil)
test 0x800
BAR 0x0x800=