Is there any Dgemm like matrix call that scales the a and b matrix? (A, B, C are scalars).
IE c = C * c + (A*op(a)) * (B*op(b));
Bonus points is therer any reason this isn't supported or is it just uncommon in Linear-Algebra and therefor generally unwarranted?
The standard BLAS interface does not provide a direct mean to scale both A and B matrices independently (in this jargon, they usually devote uppercase letters for matrices/vectors and lowercase for scalars). See the MKL cblas documentation for cblas_?gemm (it will be the same across BLAS implementations).
What you can do though is to generate the scaled version of B with an intermediate call, setting a and b to zero:
C = c.*C + (0.*A) * (0.*B)
However, if you decompose the matrix multiplication into single operations (example in row-major order):
for( i = 0; i < N; i++ ) {
for( j = 0; j < M; j++ ) {
for( ii = 0; ii < K; ii++ ) {
C[i][j] = C[i][j] + a*A[i][ii]*B[ii][j];
}
}
}
You will see that the scaling affects both matrices' elements. Since scalar product is commutative and associative, using an a argument equal to the product of both your scalars has the same effect.