Could not move globalized variable to the stack. Variable is potentially captured in call. Mark parameter as __attribute__((noescape)) to override. [OMP113]

This missed remark indicates that a globalized value could not be moved to the stack because it is potentially captured by a call to a function we cannot analyze. In order for a globalized variable to be moved to the stack, copies to its pointer cannot be stored. Otherwise it is considered captured and could potentially be shared between the threads. This can be overridden using a parameter level attribute as suggested in the remark text.

Globalization will occur when a pointer to a thread-local variable escapes the current scope. In most cases it can be determined that the variable cannot be shared if a copy of its pointer is never made. However, this remark indicates a copy of the pointer is present or that sharing is possible because it is used outside the current translation unit.

Examples

If a pointer to a thread-local variable is passed to a function not visible in the current translation unit we need to assume a copy is made of it that can be shared between the threads. This prevents OMP110 from triggering, which will result in a performance penalty when executing on the target device.

extern void use(int *x);

void foo() {
  int x;
  use(&x);
}

int main() {
#pragma omp target parallel
  foo();
}

$ clang++ -fopenmp -fopenmp-targets=nvptx64 -O2 -Rpass-missed=openmp-opt omp113.cpp
missed.cpp:4:7: remark: Could not move globalized variable to the stack. Variable is
potentially captured in call. Mark parameter as `__attribute__((noescape))` to
override. [OMP113]
  int x;
      ^

As the remark suggests, this behaviour can be overridden using the noescape attribute. This tells the compiler that no reference to the object the pointer points to that is derived from the parameter value will survive after the function returns. The user is responsible for verifying that this assertion is correct.

extern void use(__attribute__((noescape)) int *x);

void foo() {
  int x;
  use(&x);
}

int main() {
#pragma omp target parallel
  foo();
}

$ clang++ -fopenmp -fopenmp-targets=nvptx64 -O2 -Rpass=openmp-opt omp113.cpp
missed.cpp:4:7: remark: Moving globalized variable to the stack. [OMP110]
int x;
    ^

Diagnostic Scope

OpenMP target offloading missed remark.