[llvm-bugs] [Bug 36228] New: -O2 optimization bug

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=36228

            Bug ID: 36228
           Summary: -O2 optimization bug
           Product: clang
           Version: 4.0
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: C++
          Assignee: unassignedclangbugs at nondot.org
          Reporter: xu_zhong_xing at 163.com
                CC: dgregor at apple.com, llvm-bugs at lists.llvm.org

Created attachment 19809
  --> https://bugs.llvm.org/attachment.cgi?id=19809&action=edit
test case

The attached code behaves differently under -O0 and -O2.

clang version 4.0.1-6 (tags/RELEASE_401/final)
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/bin
Found candidate GCC installation: /usr/bin/../lib/gcc/x86_64-linux-gnu/7
Found candidate GCC installation: /usr/bin/../lib/gcc/x86_64-linux-gnu/7.2.0
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/7
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/7.2.0
Selected GCC installation: /usr/bin/../lib/gcc/x86_64-linux-gnu/7.2.0
Candidate multilib: .;@m64
Selected multilib: .;@m64


$ clang++ -O0 main.cpp
$ ./a.out 
BEGIN
END

$ clang++ -O2 main.cpp
$ ./a.out 
BEGIN
ERROR: s.a = 255

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