llvm-project

afbcf952 - [OpenMP 6.0 ]Codegen for Reduction over private variables with reduction clause (#134709)

[OpenMP 6.0 ]Codegen for Reduction over private variables with reduction clause (#134709) Codegen support for reduction over private variable with reduction clause. Section 7.6.10 in in OpenMP 6.0 spec. - An internal shared copy is initialized with an initializer value. - The shared copy is updated by combining its value with the values from the private copies created by the clause. - Once an encountering thread verifies that all updates are complete, its original list item is updated by merging its value with that of the shared copy and then broadcast to all threads. Sample Test Case from OpenMP 6.0 Example ``` #include <assert.h> #include <omp.h> #define N 10 void do_red(int n, int *v, int &sum_v) { sum_v = 0; // sum_v is private #pragma omp for reduction(original(private),+: sum_v) for (int i = 0; i < n; i++) { sum_v += v[i]; } } int main(void) { int v[N]; for (int i = 0; i < N; i++) v[i] = i; #pragma omp parallel num_threads(4) { int s_v; // s_v is private do_red(N, v, s_v); assert(s_v == 45); } return 0; } ``` Expected Codegen: ``` // A shared global/static variable is introduced for the reduction result. // This variable is initialized (e.g., using memset or a UDR initializer) // e.g., .omp.reduction.internal_private_var // Barrier before any thread performs combination call void @__kmpc_barrier(...) // Initialization block (executed by thread 0) // e.g., call void @llvm.memset.p0.i64(...) or call @udr_initializer(...) call void @__kmpc_critical(...) // Inside critical section: // Load the current value from the shared variable // Load the thread-local private variable's value // Perform the reduction operation // Store the result back to the shared variable call void @__kmpc_end_critical(...) // Barrier after all threads complete their combinations call void @__kmpc_barrier(...) // Broadcast phase: // Load the final result from the shared variable) // Store the final result to the original private variable in each thread // Final barrier after broadcast call void @__kmpc_barrier(...) ``` --------- Co-authored-by: Chandra Ghale <ghale@pe31.hpc.amslabs.hpecorp.net>