OpenMathLib / OpenMathLib/OpenBLAS
AVX2: dgemm on HASWELL leads to invalid results (low accuracy)
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- C
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Description
After upgrading Scilab OpenBLAS build, we detected a poor precision result impacting dgemm when using AVX2 kernels.
With OpenBLAS 0.3.27:
$ OPENBLAS_CORETYPE=NEHALEM scilab/test_dgemm
[1, 0.666667] * [-6 0 ; 9 0]
0 0
$ OPENBLAS_CORETYPE=HASWELL scilab/test_dgemm
[1, 0.666667] * [-6 0 ; 9 0]
-3.33067E-16 0
With refBLAS 3.12.0 :
$ scilab/test_dgemm
[1, 0.666667] * [-6 0 ; 9 0]
0 0
test code
#include <math.h>
#include <stdio.h>
extern void dgemm_(char *transa, char *transb, int *m, int *n, int *k, double *alpha, double *a, int *lda, double *b, int *ldb, double *beta, double *c, int *ldc);
int main()
{
int ONE = 1;
int TWO = 2;
double dZERO = 0;
double dONE = 1;
double A_tab[] = {1., 2. / 3.};
double* A = A_tab;
double B_tab[] = {-6., 9., 0., 0.};
double* B = B_tab;
double C_tab[] = {NAN, NAN};
double* C = C_tab;
dgemm_("n", "n", &ONE, &TWO, &TWO, &dONE, A, &ONE, B, &TWO, &dZERO, C, &ONE);
printf("[%lG, %lG] * [%lG %lG ; %lG %lG]\n", A[0], A[1], B[0], B[2], B[1], B[3]);
printf("\t%lG\t%lG\n", C[0], C[1]);
return 0;
}
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First steps
- Read the whole issue, then the project's contributing guide.
- Comment on the issue to say you are picking it up — it saves two people doing the same work.
- Fork the repository and make your change on a branch.
- Open a pull request that references the issue number.
Research direction
Start by compiling and running the provided C reproducer through the dgemm_ entry point with OPENBLAS_CORETYPE set to HASWELL, then compare it with NEHALEM and refBLAS. Trace the AVX2 dgemm path implicated by the HASWELL result; done means the reproducer returns the expected zero result without reducing correctness elsewhere.
Written by the indexing model from the issue text.
Assessment
- Tech stack
- c
- Domain
- performance
- Issue type
- Bug
- Difficulty
- 4/5
- Estimated time
- 3-5 days
- Activity status
- Stale
- Clarity
- Mostly clear
- Newbie friendliness
- 42/100