runtime: optimize growslice

This is tiny optimization for growslice, which is probably too small to
measure easily.

Move the for loop to avoid multiple checks inside the loop.
Also, use >> 2 instead of /4, which generates fewer instructions.

Change-Id: I9ab09bdccb56f98ab22073f23d9e102c252238c7
Reviewed-on: https://go-review.googlesource.com/c/go/+/493795
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Egon Elbre <egonelbre@gmail.com>
Auto-Submit: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Keith Randall <khr@google.com>

src/runtime/slice.go

@@ -186,14 +186,21 @@ func growslice(oldPtr unsafe.Pointer, newLen, oldCap, num int, et *_type) slice
 		if oldCap < threshold {
 			newcap = doublecap
 		} else {
-			// Check 0 < newcap to detect overflow
-			// and prevent an infinite loop.
-			for 0 < newcap && newcap < newLen {
+			for {
 				// Transition from growing 2x for small slices
 				// to growing 1.25x for large slices. This formula
 				// gives a smooth-ish transition between the two.
-				newcap += (newcap + 3*threshold) / 4
+				newcap += (newcap + 3*threshold) >> 2
+
+				// We need to check `newcap >= newLen` and whether `newcap` overflowed.
+				// newLen is guaranteed to be larger than zero, hence
+				// when newcap overflows then `uint(newcap) > uint(newLen)`.
+				// This allows to check for both with the same comparison.
+				if uint(newcap) >= uint(newLen) {
+					break
+				}
 			}
+
 			// Set newcap to the requested cap when
 			// the newcap calculation overflowed.
 			if newcap <= 0 {