[Bug c++/102937] New: Miscompilation with -O3 and aliasing of char* and size_t

["drohr at jwdt dot org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102937

            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=51663&action=edit
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 code 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 disappears,
and the output is
FOO 0x(nil)
test 0x800
BAR 0x0x800