From ab2f83c387fcc00efbdcc0a5c249af26c84ff425 Mon Sep 17 00:00:00 2001 From: =?utf8?q?Daniel=20Mart=C3=AD?= Date: Fri, 5 Apr 2019 23:06:49 +0200 Subject: [PATCH] encoding/json: remove a bounds check in readValue MIME-Version: 1.0 Content-Type: text/plain; charset=utf8 Content-Transfer-Encoding: 8bit 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 TryBot-Result: Gobot Gobot Reviewed-by: Brad Fitzpatrick --- src/encoding/json/stream.go | 12 +++++++----- 1 file changed, 7 insertions(+), 5 deletions(-) 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. -- 2.48.1