[llvm-bugs] [Bug 37823] New: Loop miscompile

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=37823

            Bug ID: 37823
           Summary: Loop miscompile
           Product: clang
           Version: 6.0
          Hardware: Macintosh
                OS: MacOS X
            Status: NEW
          Severity: normal
          Priority: P
         Component: -New Bugs
          Assignee: unassignedclangbugs at nondot.org
          Reporter: ks at claylabs.com
                CC: llvm-bugs at lists.llvm.org

Created attachment 20434
  --> https://bugs.llvm.org/attachment.cgi?id=20434&action=edit
Program to trigger bug in Clang

I've attached a relatively simple program that triggers a miscompile. The
program logic should be straight-forward. I'm fairly certain there is no
undefined behavior.

When compiling with -O2 or -O3, the wrong output is produced. The generated
program also triggers an error with valgrind.

When compiling the attached program with -O0, the correct output is produced,
and there are no errors with valgrind.

I've tested with LLVM 6.0 and also with Xcode 9.4.1. The OS is High Sierra.

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