Sort: reduced stack depth to lg(n) in quickSort

Doing the tail recursion elimination explicitly
seems safer than leaving it to the compiler;
the code is still clean and easy to understand.

R=r, r2, gri
CC=golang-dev
https://golang.org/cl/3373041

src/pkg/sort/sort.go

@@ -122,11 +122,19 @@ func doPivot(data Interface, lo, hi int) (midlo, midhi int) {
 }
 
 func quickSort(data Interface, a, b int) {
-	if b-a > 7 {
+	for b-a > 7 {
 		mlo, mhi := doPivot(data, a, b)
-		quickSort(data, a, mlo)
-		quickSort(data, mhi, b)
-	} else if b-a > 1 {
+		// Avoiding recursion on the larger subproblem guarantees
+		// a stack depth of at most lg(b-a).
+		if mlo-a < b-mhi {
+			quickSort(data, a, mlo)
+			a = mhi // i.e., quickSort(data, mhi, b)
+		} else {
+			quickSort(data, mhi, b)
+			b = mlo // i.e., quickSort(data, a, mlo)
+		}
+	}
+	if b-a > 1 {
 		insertionSort(data, a, b)
 	}
 }