encoding/json: Remove extra allocation in scanner.
When the scanner receives a non-whitespace character in stateEndTop,
it creates an error message and caches it to return on the next
transition. nextValue() uses the scanner to sub-scan for a value
inside a larger JSON structure. Since stateEndTop is triggered
*after* the ending byte, whatever character immediately follows the
sub-value gets pulled into the scanner's state machine as well.
Even though it is not used and doesn't cause an error, it does
cause the state machine to allocate an error that will never be used.
The fix is to probe the state machine with whitespace after
scanEndObject or scanEndArray to see if the next character would
result in a scanEnd state transition. If so, we can return right
away without processing the next character and avoid triggering
an allocation.
benchmark old ns/op new ns/op delta
BenchmarkCodeEncoder
17022194 16611336 -2.41%
BenchmarkCodeMarshal
18443250 18090144 -1.91%
BenchmarkCodeDecoder
61502053 61010936 -0.80%
BenchmarkCodeUnmarshal
61410829 60363605 -1.71%
BenchmarkCodeUnmarshalReuse
59124836 58361772 -1.29%
BenchmarkUnmarshalString 602 603 +0.17%
BenchmarkUnmarshalFloat64 535 537 +0.37%
BenchmarkUnmarshalInt64 482 482 +0.00%
BenchmarkIssue10335 1206 799 -33.75%
BenchmarkSkipValue
17605751 18355391 +4.26%
BenchmarkEncoderEncode 612 604 -1.31%
benchmark old MB/s new MB/s speedup
BenchmarkCodeEncoder 114.00 116.82 1.02x
BenchmarkCodeMarshal 105.21 107.27 1.02x
BenchmarkCodeDecoder 31.55 31.81 1.01x
BenchmarkCodeUnmarshal 31.60 32.15 1.02x
BenchmarkSkipValue 111.63 107.07 0.96x
benchmark old allocs new allocs delta
BenchmarkIssue10335 11 4 -63.64%
BenchmarkEncoderEncode 2 2 +0.00%
benchmark old bytes new bytes delta
BenchmarkIssue10335 376 272 -27.66%
BenchmarkEncoderEncode 40 40 +0.00%
Fixes #10335
Change-Id: I3d4f2b67f7a038adfb33ba48bb6b680f528baf18
Reviewed-on: https://go-review.googlesource.com/9074
Reviewed-by: Russ Cox <rsc@golang.org>