diff --git a/src/encoding/json/stream.go b/src/encoding/json/stream.go
index 7d5137fbc7..e29127499b 100644
--- a/src/encoding/json/stream.go
+++ b/src/encoding/json/stream.go
@@ -92,20 +92,23 @@ 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:
@@ -113,7 +116,6 @@ Input:
 				return 0, dec.scan.err
 			}
 		}
-		scanp = len(dec.buf)
 
 		// Did the last read have an error?
 		// Delayed until now to allow buffer scan.