[llvm-bugs] [Bug 41572] New: Miscompile of collapse(2) with non-rectangular loop nest

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=41572

            Bug ID: 41572
           Summary: Miscompile of collapse(2) with non-rectangular loop
                    nest
           Product: clang
           Version: trunk
          Hardware: PC
                OS: Windows NT
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: OpenMP
          Assignee: unassignedclangbugs at nondot.org
          Reporter: llvm at meinersbur.de
                CC: a.bataev at hotmail.com, hfinkel at anl.gov,
                    jdoerfert at anl.gov, llvm-bugs at lists.llvm.org

The following program miscompiles on current trunk (r358973):

$ cat collapse.c
#include <stdbool.h>
#include <stdio.h>
#include <stdlib.h>

int main() {
        int n = 2;
        bool executed = false;
#pragma omp parallel for collapse(2)
        for (int i = 0; i < n; i += 1)
                for (int j = i; j < n; j += 1)
                        executed = true;
        printf("executed? %d\n", executed);
        return EXIT_SUCCESS;
}

$ ~/build/llvm/release/bin/clang collapse.c -fopenmp -O3 -o collapse  &&
./collapse
executed? 0

Emitting the output reveals that outlined function has been optimized away:
$ ~/build/llvm/release/bin/clang collapse.c -fopenmp -O3 -emit-llvm -S -o -
[...]
define internal void @.omp_outlined.(i32* noalias nocapture %.global_tid., i32*
noalias nocapture %.bound_tid., i32* nocapture dereferenceable(4) %n, i8*
nocapture dereferenceable(1) %executed) #2 {
entry:
  ret void
}
[...]


The following do return the correct result:

$ ~/build/llvm/release/bin/clang collapse.c -o collapse -O3 && ./collapse
executed? 1

$ ~/build/llvm/release/bin/clang collapse.c -fopenmp -o collapse -O0 &&
./collapse
executed? 1

[...]
#pragma omp parallel for collapse(2)
        for (int i = 0; i < n; i += 1)
                for (int j = 0; j < n; j += 1) // <= rectangular loop nest
[...]
$ ~/build/llvm/release/bin/clang collapse-rect.c -fopenmp -o collapse-rect -O3
&& ./collapse-rect
executed? 1

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