[llvm-bugs] [Bug 45635] New: InstCombine propagates values across omp barrier, after D75010

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=45635

            Bug ID: 45635
           Summary: InstCombine propagates values across omp barrier,
                    after D75010
           Product: OpenMP
           Version: unspecified
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: Clang Compiler Support
          Assignee: unassignedclangbugs at nondot.org
          Reporter: chang-sun.lin.jr at intel.com
                CC: llvm-bugs at lists.llvm.org

Created attachment 23394
  --> https://bugs.llvm.org/attachment.cgi?id=23394&action=edit
test case

The attached test started giving different results @ O2, after commit
D75010/rG72b51d6f93b5. It has a code sequence like this:

  #pragma omp critical
  shared_var++; 
  #pragma omp barrier
  printf("expecting 2: %d\n", shared_var);

D75010 marks the kmpc_barrier function with inaccessiblemem_or_argmemonly,
which allows InstCombine to propagate the local thread's shared_var value
across the barrier, to the printf. The different threads print different values
of shared_var. The standard seems to say that barrier implies a flush of the
data visible to the thread, ensuring that all threads see the same view of
memory after the barrier.

Compiled with:
clang -fopenmp -O2 barrier.c
Expecting this output:

expecting 2: 2
expecting 2: 2

Got this output:

expecting 2: 2
expecting 2: 1

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