[llvm-bugs] [Bug 43710] New: clang++ miscompiles against MSVC standard library with -O2

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=43710

            Bug ID: 43710
           Summary: clang++ miscompiles against MSVC standard library with
                    -O2
           Product: new-bugs
           Version: 9.0
          Hardware: PC
                OS: other
            Status: NEW
          Severity: normal
          Priority: P
         Component: new bugs
          Assignee: unassignedbugs at nondot.org
          Reporter: karen at codesynthesis.com
                CC: htmldeveloper at gmail.com, llvm-bugs at lists.llvm.org

Created attachment 22688
  --> https://bugs.llvm.org/attachment.cgi?id=22688&action=edit
Test program

Hello,

My system is Windows 10 with clang 9.0.0 (installed with
https://releases.llvm.org/9.0.0/LLVM-9.0.0-win64.exe) and Microsoft Visual
Studio Community 2019, Version 16.3.1.

Under some circumstances a non-empty string passed into a function is seen by
the function code as an empty string.

The attached program reproduces the issue. It is expected to print 'abc' but
prints '' if compiled with -O2 or -O3:

> clang++ -std=c++2a test.cxx
> a.exe
'abc'
>clang++ -std=c++2a -O2 test.cxx
>a.exe
''

Note that the issue also reproduces if compile with clang-cl:

> clang-cl /std:c++17 /EHsc test.cxx
> test.exe
'abc'
> clang-cl /std:c++17 /EHsc /O2 test.cxx
> test.exe
''

Also note that the attached program is not exactly trivial but I was unable to
reduce it further.

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