error: OpenMP array shaping operation is not allowed here #34

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1. milfeld commented on Sep 10, 2025

   

   milfeld

   on Sep 10, 2025

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   In godbolt.org I checked nvc++ 25.7 and gcc 15.2 and they appear not to have implemented this feature yet, they didn't understand the shaping syntax.

   nvc++ -fopenmp -mp=gpu *.cpp
   v25.7  NOT compiled
   "<source>", line 27: error: invalid text in pragma
       #pragma omp target update from( (([Nx][Ny+2])a)[0:Nx][1],   (([Nx][Ny+2])a)[0:Nx][ny] )
                                       ^

   I have replicated the problem. The x86-64 clang++ compilers 14->21 complained that it is "not allowed here" for the example, which is modeled after the use case: updating discontiguous elements for a target region. I have created a ticket in the examples-internal repo to discuss this issue.

   $ clang++ -fopenmp -fopenmp-targets=nvptx64-nvidia-cuda snip.cpp
   snip.cpp:29:37: error: OpenMP array shaping operation is not allowed here
      29 |     #pragma omp target update from( (([Nx][Ny+2])a)[0:Nx][1],   (([Nx][Ny+2])a)[0:Nx][Ny] )
         |                                     ^

   I have been using this test code as a quick test:

   #include <stdio.h>
   #include <stdlib.h>
   #include <omp.h>
   #include <string.h>
   
   #define Nx 4
   #define Ny 4
   
   #pragma begin target declare
   void change_ghost(int *a, int nx, int ny){
     for(int i=0; i<nx; i++){
       a[i*(ny+2)+1]  += 2;
       a[i*(ny+2)+ny] += 2;
     }
   }
   #pragma end target declare
   
   int main(){
     int *a;
     a = (int*)malloc(sizeof(int)*Nx*(Ny+2));
     memset(a, 0,     sizeof(int)*Nx*(Ny+2));
     #pragma omp target data map( a[0:Nx*(Ny+2)] )
     {
       #pragma omp target
       change_ghost(a,Nx,Ny);
   
   //  #pragma omp target update from( a[0:Nx*(Ny+2)] )
   //  #pragma omp target update from( ([Nx][Ny+2])a )
       #pragma omp target update from( (([Nx][Ny+2])a)[0:Nx][1],   (([Nx][Ny+2])a)[0:Nx][Ny] )
   //  #pragma omp target update from( (([Nx][Ny+2])a)[0:Nx][1:1], (([Nx][Ny+2])a)[0:Nx][Ny:1] )
       
       printf(" a[1], a[Ny] =  %d %d\n", a[1],a[Ny]);
     }
     printf(" a[1], a[Ny] =  %d %d\n", a[1],a[Ny]);
     return 0;
   }