native: small cleanup in Dgeqrf

Author: vladimir-ch

native/dgeqrf.go

@@ -43,20 +43,21 @@ func (impl Implementation) Dgeqrf(m, n int, a []float64, lda int, tau, work []fl
 	if k == 0 {
 		return
 	}
-	nbmin := 2 // Minimal number of blocks
+	nbmin := 2 // Minimal block size.
 	var nx int // Use unblocked (unless changed in the next for loop)
 	iws := n
 	ldwork := nb
-	// Only consider blocked if the suggested number of blocks is > 1 and the
-	// number of columns is sufficiently large.
-	if nb > 1 && k > nb {
-		// nx is the crossover point. Above this value the blocked routine should be used.
+	// Only consider blocked if the suggested block size is > 1 and the
+	// number of rows or columns is sufficiently large.
+	if 1 < nb && nb < k {
+		// nx is the block size at which the code switches from blocked
+		// to unblocked.
 		nx = max(0, impl.Ilaenv(3, "DGEQRF", " ", m, n, -1, -1))
 		if k > nx {
 			iws = ldwork * n
 			if lwork < iws {
-				// Not enough workspace to use the optimal number of blocks. Instead,
-				// get the maximum allowable number of blocks.
+				// Not enough workspace to use the optimal block
+				// size. Get the minimum block size instead.
 				nb = lwork / n
 				nbmin = max(2, impl.Ilaenv(2, "DGEQRF", " ", m, n, -1, -1))
 			}
@@ -67,7 +68,7 @@ func (impl Implementation) Dgeqrf(m, n int, a []float64, lda int, tau, work []fl
 	}
 	// Compute QR using a blocked algorithm.
 	var i int
-	if nb >= nbmin && nb < k && nx < k {
+	if nbmin <= nb && nb < k && nx < k {
 		for i = 0; i < k-nx; i += nb {
 			ib := min(k-i, nb)
 			// Compute the QR factorization of the current block.