gemm_ performs one of the matrix-matrix operations

C := alpha*op( A )*op( B ) + beta*C,

where op( X ) is one of

op( X ) = X or op( X ) = X**T or op( X ) = X**H,

alpha and beta are scalars, and A, B and C are matrices, with op( A ) an m ⨉ k matrix, op( B ) a k ⨉ n matrix and C an m ⨉ n matrix.