There's no point in computing whether we're at the
beginning of the line if the NFA isn't going to ask.
Wait to compute that until asked.

Whatever minor slowdowns were introduced by
the conversion to pools that were not repaid by
other optimizations are taken care of by this one.

name                             old time/op    new time/op    delta
Find-12                             252ns ± 0%     260ns ± 0%   +3.34%  (p=0.000 n=10+8)
FindAllNoMatches-12                 136ns ± 4%     134ns ± 4%   -0.96%  (p=0.033 n=10+10)
FindString-12                       246ns ± 0%     250ns ± 0%   +1.46%  (p=0.000 n=8+10)
FindSubmatch-12                     332ns ± 1%     332ns ± 0%     ~     (p=0.101 n=9+10)
FindStringSubmatch-12               321ns ± 1%     322ns ± 1%     ~     (p=0.717 n=9+10)
Literal-12                         91.6ns ± 0%    92.3ns ± 0%   +0.74%  (p=0.000 n=9+9)
NotLiteral-12                      1.47µs ± 0%    1.47µs ± 0%   +0.38%  (p=0.000 n=9+8)
MatchClass-12                      2.15µs ± 0%    2.15µs ± 0%   +0.39%  (p=0.000 n=10+10)
MatchClass_InRange-12              2.09µs ± 0%    2.11µs ± 0%   +0.75%  (p=0.000 n=9+9)
ReplaceAll-12                      1.40µs ± 0%    1.40µs ± 0%     ~     (p=0.525 n=10+10)
AnchoredLiteralShortNonMatch-12    83.5ns ± 0%    81.6ns ± 0%   -2.28%  (p=0.000 n=9+10)
AnchoredLiteralLongNonMatch-12      101ns ± 0%      97ns ± 1%   -3.54%  (p=0.000 n=10+10)
AnchoredShortMatch-12               131ns ± 0%     128ns ± 0%   -2.29%  (p=0.000 n=10+9)
AnchoredLongMatch-12                268ns ± 1%     252ns ± 1%   -6.04%  (p=0.000 n=10+10)
OnePassShortA-12                    614ns ± 0%     587ns ± 1%   -4.33%  (p=0.000 n=6+10)
NotOnePassShortA-12                 552ns ± 0%     547ns ± 1%   -0.89%  (p=0.000 n=10+10)
OnePassShortB-12                    494ns ± 0%     455ns ± 0%   -7.96%  (p=0.000 n=9+9)
NotOnePassShortB-12                 411ns ± 0%     406ns ± 0%   -1.30%  (p=0.000 n=9+9)
OnePassLongPrefix-12                109ns ± 0%     108ns ± 1%     ~     (p=0.064 n=8+9)
OnePassLongNotPrefix-12             403ns ± 0%     349ns ± 0%  -13.30%  (p=0.000 n=9+8)
MatchParallelShared-12             38.9ns ± 1%    37.9ns ± 1%   -2.65%  (p=0.000 n=10+8)
MatchParallelCopied-12             39.2ns ± 1%    38.3ns ± 2%   -2.20%  (p=0.001 n=10+10)
QuoteMetaAll-12                    94.5ns ± 0%    94.7ns ± 0%   +0.18%  (p=0.043 n=10+9)
QuoteMetaNone-12                   52.7ns ± 0%    52.7ns ± 0%     ~     (all equal)
Match/Easy0/32-12                  72.2ns ± 0%    71.9ns ± 0%   -0.38%  (p=0.009 n=8+10)
Match/Easy0/1K-12                   296ns ± 1%     297ns ± 0%   +0.51%  (p=0.001 n=10+9)
Match/Easy0/32K-12                 4.57µs ± 3%    4.61µs ± 2%     ~     (p=0.280 n=10+10)
Match/Easy0/1M-12                   234µs ± 0%     234µs ± 0%     ~     (p=0.986 n=10+10)
Match/Easy0/32M-12                 7.96ms ± 0%    7.98ms ± 0%   +0.22%  (p=0.010 n=10+9)
Match/Easy0i/32-12                 1.09µs ± 0%    1.10µs ± 0%   +0.23%  (p=0.000 n=8+9)
Match/Easy0i/1K-12                 31.7µs ± 0%    31.7µs ± 0%   +0.09%  (p=0.003 n=9+8)
Match/Easy0i/32K-12                1.61ms ± 0%    1.27ms ± 1%  -21.03%  (p=0.000 n=8+10)
Match/Easy0i/1M-12                 51.4ms ± 0%    40.4ms ± 0%  -21.29%  (p=0.000 n=8+8)
Match/Easy0i/32M-12                 1.65s ± 0%     1.30s ± 1%  -21.22%  (p=0.000 n=9+9)
Match/Easy1/32-12                  67.6ns ± 1%    67.2ns ± 0%     ~     (p=0.085 n=10+9)
Match/Easy1/1K-12                   873ns ± 2%     880ns ± 0%   +0.78%  (p=0.006 n=9+7)
Match/Easy1/32K-12                 39.7µs ± 1%    34.3µs ± 3%  -13.53%  (p=0.000 n=10+10)
Match/Easy1/1M-12                  1.41ms ± 1%    1.19ms ± 3%  -15.48%  (p=0.000 n=10+10)
Match/Easy1/32M-12                 44.9ms ± 1%    38.0ms ± 2%  -15.21%  (p=0.000 n=10+10)
Match/Medium/32-12                 1.04µs ± 0%    1.03µs ± 0%   -0.57%  (p=0.000 n=9+9)
Match/Medium/1K-12                 31.2µs ± 0%    31.4µs ± 1%   +0.61%  (p=0.000 n=8+10)
Match/Medium/32K-12                1.45ms ± 1%    1.20ms ± 0%  -17.70%  (p=0.000 n=10+8)
Match/Medium/1M-12                 46.4ms ± 0%    38.4ms ± 2%  -17.32%  (p=0.000 n=6+9)
Match/Medium/32M-12                 1.49s ± 1%     1.24s ± 1%  -16.81%  (p=0.000 n=10+10)
Match/Hard/32-12                   1.47µs ± 0%    1.47µs ± 0%   -0.31%  (p=0.000 n=9+10)
Match/Hard/1K-12                   44.5µs ± 1%    44.4µs ± 0%     ~     (p=0.075 n=10+10)
Match/Hard/32K-12                  2.09ms ± 0%    1.78ms ± 7%  -14.88%  (p=0.000 n=8+10)
Match/Hard/1M-12                   67.8ms ± 5%    56.9ms ± 7%  -16.05%  (p=0.000 n=10+10)
Match/Hard/32M-12                   2.17s ± 5%     1.84s ± 6%  -15.21%  (p=0.000 n=10+10)
Match/Hard1/32-12                  7.89µs ± 0%    7.94µs ± 0%   +0.61%  (p=0.000 n=9+9)
Match/Hard1/1K-12                   246µs ± 0%     245µs ± 0%   -0.30%  (p=0.010 n=9+10)
Match/Hard1/32K-12                 8.93ms ± 0%    8.17ms ± 0%   -8.44%  (p=0.000 n=9+8)
Match/Hard1/1M-12                   286ms ± 0%     269ms ± 9%   -5.66%  (p=0.028 n=9+10)
Match/Hard1/32M-12                  9.16s ± 0%     8.61s ± 8%   -5.98%  (p=0.028 n=9+10)
Match_onepass_regex/32-12           825ns ± 0%     712ns ± 0%  -13.75%  (p=0.000 n=8+8)
Match_onepass_regex/1K-12          28.7µs ± 1%    19.8µs ± 0%  -30.99%  (p=0.000 n=9+8)
Match_onepass_regex/32K-12          950µs ± 1%     628µs ± 0%  -33.83%  (p=0.000 n=9+8)
Match_onepass_regex/1M-12          30.4ms ± 0%    20.1ms ± 0%  -33.74%  (p=0.000 n=9+8)
Match_onepass_regex/32M-12          974ms ± 1%     646ms ± 0%  -33.73%  (p=0.000 n=9+8)
CompileOnepass-12                  4.60µs ± 0%    4.59µs ± 0%     ~     (p=0.063 n=8+9)
[Geo mean]                         23.1µs         21.3µs        -7.44%

https://perf.golang.org/search?q=upload:20181004.4

Change-Id: I47cdd09f6dcde1d7c317080e9b4df42c7d0a8d24
Reviewed-on: https://go-review.googlesource.com/c/139782
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>