Value has potential side effects preventing SPMD-mode execution. Add [[omp::assume("ompx_spmd_amenable")]] to the called function to override. [OMP121]

This analysis remarks indicates that a potential side-effect that cannot be guarded prevents the target region from executing in SPMD-mode. SPMD-mode requires that each thread is active inside the region. Any instruction that cannot be either recomputed by each thread independently or guarded and executed by a single thread prevents the region from executing in SPMD-mode.

This remark will attempt to print out the instructions preventing the region from being executed in SPMD-mode. Calls to functions outside the current translation unit will prevent this transformation from occurring as well, but can be overridden using an assumption stating that it contains no calls that prevent SPMD execution.

Examples

Calls to functions outside the current translation unit may contain instructions or operations that cannot be executed in SPMD-mode.

extern int work();

void use(int x);

void foo() {
#pragma omp target teams
  {
    int x = work();
#pragma omp parallel
      use(x);

  }
}

$ clang++ -fopenmp -fopenmp-targets=nvptx64 -O2 -Rpass-analysis=openmp-opt omp121.cpp
omp121.cpp:8:13: remark: Value has potential side effects preventing SPMD-mode
execution.  Add `[[omp::assume("ompx_spmd_amenable")]]` to the called function
to override. [OMP121]
int x = work();
         ^

As the remark suggests, the problem is caused by the unknown call to the external function work. This can be overridden by asserting that it does not contain any code that prevents SPMD-mode execution.

[[omp::assume("ompx_spmd_amenable")]] extern int work();

void use(int x);

void foo() {
#pragma omp target teams
  {
    int x = work();
#pragma omp parallel
      use(x);

  }
}

$ clang++ -fopenmp -fopenmp-targets=nvptx64 -O2 -Rpass=openmp-opt omp121.cpp
omp121.cpp:6:1: remark: Transformed generic-mode kernel to SPMD-mode. [OMP120]
#pragma omp target teams
^

Diagnostic Scope

OpenMP target offloading analysis remark.