encoding/json: remove a bounds check in readValue

readValue is a hot function, clocking in at ~13% flat CPU use in
CodeDecoder. In particular, looping over the bytes is slow. That's
partially because the code contains a bounds check at the start of the
loop.

The source of the problem is that scanp is a signed integer, and comes
from a field, so the compiler doesn't know that it's non-negative. Help
it with a simple and comparatively cheap hint.

While at it, use scanp as the index variable directly, removing the need
for a duplicate index variable which is later added back into scanp.

name           old time/op    new time/op    delta
CodeDecoder-8    11.3ms ± 1%    11.2ms ± 1%  -0.98%  (p=0.000 n=9+9)

name           old speed      new speed      delta
CodeDecoder-8   172MB/s ± 1%   174MB/s ± 1%  +0.99%  (p=0.000 n=9+9)

Updates #28923.

Change-Id: I138f83babdf316fc97697cc18f595c3403c1ddb7
Reviewed-on: https://go-review.googlesource.com/c/go/+/170939
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

stream.go

@@ -92,28 +92,30 @@ func (dec *Decoder) readValue() (int, error) {
 	scanp := dec.scanp
 	var err error
 Input:
-	for {
+	// help the compiler see that scanp is never negative, so it can remove
+	// some bounds checks below.
+	for scanp >= 0 {
+
 		// Look in the buffer for a new value.
-		for i, c := range dec.buf[scanp:] {
+		for ; scanp < len(dec.buf); scanp++ {
+			c := dec.buf[scanp]
 			dec.scan.bytes++
 			switch dec.scan.step(&dec.scan, c) {
 			case scanEnd:
-				scanp += i
 				break Input
 			case scanEndObject, scanEndArray:
 				// scanEnd is delayed one byte.
 				// We might block trying to get that byte from src,
 				// so instead invent a space byte.
 				if stateEndValue(&dec.scan, ' ') == scanEnd {
-					scanp += i + 1
+					scanp++
 					break Input
 				}
 			case scanError:
 				dec.err = dec.scan.err
 				return 0, dec.scan.err
 			}
 		}
-		scanp = len(dec.buf)
 
 		// Did the last read have an error?
 		// Delayed until now to allow buffer scan.