[llvm-bugs] [Bug 27005] New: Dead store elimination incorrectly removes store to function pointer in struct

[via llvm-bugs]

https://llvm.org/bugs/show_bug.cgi?id=27005

            Bug ID: 27005
           Summary: Dead store elimination incorrectly removes store to
                    function pointer in struct
           Product: libraries
           Version: 3.8
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: Backend: X86
          Assignee: unassignedbugs at nondot.org
          Reporter: maniandram01+bugzilla at gmail.com
                CC: llvm-bugs at lists.llvm.org
    Classification: Unclassified

Created attachment 16057
  --> https://llvm.org/bugs/attachment.cgi?id=16057&action=edit
Code that causes the miscompilation

In the attached code incorrect_dse.ll, the dead store elimination optimization
incorrectly removes in create() a store instruction to a function pointer in
the structure {void()*}, even though that pointer is load'ed later in main().
This may be due to the bitcast to the structure {} later. This removal of the
store instruction does not occur if the function is replaced with an i32. Note
that one can add a dummy i32 to both {} and {void()*} structures, but the big
still occurs (it has nothing to do with {} being empty).

The end result of this miscompilation is a segmentation fault. However, if one
uses -O2 and -O3 levels, LLVM recognizes that the function being load'ed is
undefined and hence optimizes away main() to `ret undef`.

You can reproduce the problem with: opt bugpoint-tooptimize.bc -dse

I am using the Clang 3.8 binaries for Ubuntu 15.10 downloaded from llvm.org.

bugpoint_out.txt is from running:

~/clang+llvm-3.8.0-x86_64-linux-gnu-ubuntu-15.10/bin$ ./bugpoint -llc-safe -dse
~/incorrect_dse.ll > bugpoint_out.txt

I have also attached the bitcode files bugpoint-tonotoptimize.bc and
bugpoint-tooptimize.bc generated by bugpoint.

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