a1826b4d26
A proposed fix for the issue #95611, [OpenMP][SIMD] ordered has no effect in a loop SIMD region as of LLVM 18.1.0 Changes: - Implement new lowering behavior: Conservatively serialize "omp simd" loops that have `omp simd ordered` directive to prevent incorrect vectorization (which results in incorrect execution behavior of the miscompiled program). Implementation outline: - We start with the optimistic default initial value of `LoopStack.setParallel(/Enable=/true);` in `CodeGenFunction::EmitOMPSimdInit(const OMPLoopDirective &D)`. - We only disable the loop parallel memory access assumption with `if (HasOrderedDirective) LoopStack.setParallel(/Enable=/false);` using the `HasOrderedDirective` (which tests for the presence of an `OMPOrderedDirective`). - This results in no longer incorrectly vectorizing the loop when the `omp simd ordered` directive is present. Motivation: We'd like to prevent incorrect vectorization of the loops marked with the `#pragma omp ordered simd` directive which has previously resulted in miscompiled code. At the same time, we'd like the usage outside of the `#pragma omp ordered simd` context to remain unaffected: Note that in the test "clang/test/OpenMP/ordered_codegen.cpp" we only "lose" the `!llvm.access.group` metadata in `foo_simd` alone. This is conservative, in that it's possible some of the loops would be possible to vectorize, but we prefer to avoid miscompilation of the loops that are currently illegal to vectorize. A concrete example follows: ```cpp // "test.c" #include <float.h> #include <math.h> #include <omp.h> #include <stdio.h> #include <stdlib.h> #include <time.h> int compare_float(float x1, float x2, float scalar) { const float diff = fabsf(x1 - x2); x1 = fabsf(x1); x2 = fabsf(x2); const float l = (x2 > x1) ? x2 : x1; if (diff <= l * scalar * FLT_EPSILON) return 1; else return 0; } #define ARRAY_SIZE 256 __attribute__((noinline)) void initialization_loop( float X[ARRAY_SIZE][ARRAY_SIZE], float Y[ARRAY_SIZE][ARRAY_SIZE]) { const float max = 1000.0; srand(time(NULL)); for (int r = 0; r < ARRAY_SIZE; r++) { for (int c = 0; c < ARRAY_SIZE; c++) { X[r][c] = ((float)rand() / (float)(RAND_MAX)) * max; Y[r][c] = X[r][c]; } } } __attribute__((noinline)) void omp_simd_loop(float X[ARRAY_SIZE][ARRAY_SIZE]) { for (int r = 1; r < ARRAY_SIZE; ++r) { for (int c = 1; c < ARRAY_SIZE; ++c) { #pragma omp simd for (int k = 2; k < ARRAY_SIZE; ++k) { #pragma omp ordered simd X[r][k] = X[r][k - 2] + sinf((float)(r / c)); } } } } __attribute__((noinline)) int comparison_loop(float X[ARRAY_SIZE][ARRAY_SIZE], float Y[ARRAY_SIZE][ARRAY_SIZE]) { int totalErrors_simd = 0; const float scalar = 1.0; for (int r = 1; r < ARRAY_SIZE; ++r) { for (int c = 1; c < ARRAY_SIZE; ++c) { for (int k = 2; k < ARRAY_SIZE; ++k) { Y[r][k] = Y[r][k - 2] + sinf((float)(r / c)); } } // check row for simd update for (int k = 0; k < ARRAY_SIZE; ++k) { if (!compare_float(X[r][k], Y[r][k], scalar)) { ++totalErrors_simd; } } } return totalErrors_simd; } int main(void) { float X[ARRAY_SIZE][ARRAY_SIZE]; float Y[ARRAY_SIZE][ARRAY_SIZE]; initialization_loop(X, Y); omp_simd_loop(X); const int totalErrors_simd = comparison_loop(X, Y); if (totalErrors_simd) { fprintf(stdout, "totalErrors_simd: %d \n", totalErrors_simd); fprintf(stdout, "%s : %d - FAIL: error in ordered simd computation.\n", __FILE__, __LINE__); } else { fprintf(stdout, "Success!\n"); } return totalErrors_simd; } ``` Before: ``` $ clang -fopenmp-simd -O3 -ffast-math -lm test.c -o test && ./test totalErrors_simd: 15408 test.c : 76 - FAIL: error in ordered simd computation. ``` clang 19.1.0: https://godbolt.org/z/6EvhxqEhe After: ``` $ clang -fopenmp-simd -O3 -ffast-math test.c -o test && ./test Success! ``` Co-authored-by: Matt P. Dziubinski <matt-p.dziubinski@hpe.com>
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