regexp: optimize for provably too short inputs

For many patterns we can compute the minimum input length at compile time.
If the input is shorter, we can return early and get a huge speedup.

As pointed out by Damian Gryski, Perl's regex engine contains a number of
these kinds of fail-fast optimizations:
https://perldoc.perl.org/perlreguts.html#Peep-hole-Optimisation-and-Analysis

Benchmarks: (including new ones for compile time)

name               old time/op    new time/op    delta
Compile/Onepass-8    4.39µs ± 1%    4.40µs ± 0%  +0.34%  (p=0.029 n=9+8)
Compile/Medium-8     9.80µs ± 0%    9.91µs ± 0%  +1.17%  (p=0.000 n=10+10)
Compile/Hard-8       72.7µs ± 0%    73.5µs ± 0%  +1.10%  (p=0.000 n=9+10)

name                       old time/op    new time/op      delta
Match/Easy0/16-8             52.6ns ± 5%       4.9ns ± 0%     -90.68%  (p=0.000 n=10+9)
Match/Easy0/32-8             64.1ns ±10%      61.4ns ± 1%        ~     (p=0.188 n=10+9)
Match/Easy0/1K-8              280ns ± 1%       277ns ± 2%      -0.97%  (p=0.004 n=10+10)
Match/Easy0/32K-8            4.61µs ± 1%      4.55µs ± 1%      -1.49%  (p=0.000 n=9+10)
Match/Easy0/1M-8              229µs ± 0%       226µs ± 1%      -1.29%  (p=0.000 n=8+10)
Match/Easy0/32M-8            7.50ms ± 1%      7.47ms ± 1%        ~     (p=0.165 n=10+10)
Match/Easy0i/16-8             533ns ± 1%         5ns ± 2%     -99.07%  (p=0.000 n=10+10)
Match/Easy0i/32-8             950ns ± 0%       950ns ± 1%        ~     (p=0.920 n=10+9)
Match/Easy0i/1K-8            27.5µs ± 1%      27.5µs ± 0%        ~     (p=0.739 n=10+10)
Match/Easy0i/32K-8           1.13ms ± 0%      1.13ms ± 1%        ~     (p=0.079 n=9+10)
Match/Easy0i/1M-8            36.7ms ± 2%      36.1ms ± 0%      -1.64%  (p=0.000 n=10+9)
Match/Easy0i/32M-8            1.17s ± 0%       1.16s ± 1%      -0.80%  (p=0.004 n=8+9)
Match/Easy1/16-8             55.5ns ± 6%       4.9ns ± 1%     -91.19%  (p=0.000 n=10+9)
Match/Easy1/32-8             58.3ns ± 8%      56.6ns ± 1%        ~     (p=0.449 n=10+8)
Match/Easy1/1K-8              750ns ± 0%       748ns ± 1%        ~     (p=0.072 n=8+10)
Match/Easy1/32K-8            31.8µs ± 0%      31.6µs ± 1%      -0.50%  (p=0.035 n=10+9)
Match/Easy1/1M-8             1.10ms ± 1%      1.09ms ± 0%      -0.95%  (p=0.000 n=10+9)
Match/Easy1/32M-8            35.5ms ± 0%      35.2ms ± 1%      -1.05%  (p=0.000 n=9+10)
Match/Medium/16-8             442ns ± 2%         5ns ± 1%     -98.89%  (p=0.000 n=10+10)
Match/Medium/32-8             875ns ± 0%       878ns ± 1%        ~     (p=0.071 n=9+10)
Match/Medium/1K-8            26.1µs ± 0%      25.9µs ± 0%      -0.64%  (p=0.000 n=10+10)
Match/Medium/32K-8           1.09ms ± 1%      1.08ms ± 0%      -0.84%  (p=0.000 n=10+9)
Match/Medium/1M-8            34.9ms ± 0%      34.6ms ± 1%      -0.98%  (p=0.000 n=9+10)
Match/Medium/32M-8            1.12s ± 1%       1.11s ± 1%      -0.98%  (p=0.000 n=10+9)
Match/Hard/16-8               721ns ± 1%         5ns ± 0%     -99.32%  (p=0.000 n=10+9)
Match/Hard/32-8              1.32µs ± 1%      1.31µs ± 0%      -0.71%  (p=0.000 n=9+9)
Match/Hard/1K-8              39.8µs ± 1%      39.7µs ± 1%        ~     (p=0.165 n=10+10)
Match/Hard/32K-8             1.57ms ± 0%      1.56ms ± 0%      -0.70%  (p=0.000 n=10+9)
Match/Hard/1M-8              50.4ms ± 1%      50.1ms ± 1%      -0.57%  (p=0.007 n=10+10)
Match/Hard/32M-8              1.62s ± 1%       1.60s ± 0%      -0.98%  (p=0.000 n=10+10)
Match/Hard1/16-8             3.88µs ± 1%      3.86µs ± 0%        ~     (p=0.118 n=10+10)
Match/Hard1/32-8             7.44µs ± 1%      7.46µs ± 1%        ~     (p=0.109 n=10+10)
Match/Hard1/1K-8              232µs ± 1%       229µs ± 1%      -1.31%  (p=0.000 n=10+9)
Match/Hard1/32K-8            7.41ms ± 2%      7.41ms ± 0%        ~     (p=0.237 n=10+8)
Match/Hard1/1M-8              238ms ± 1%       238ms ± 0%        ~     (p=0.481 n=10+10)
Match/Hard1/32M-8             7.69s ± 1%       7.61s ± 0%      -1.00%  (p=0.000 n=10+10)

Fixes #31329

Change-Id: I04640e8c59178ec8b3106e13ace9b109b6bdbc25
Reviewed-on: https://go-review.googlesource.com/c/go/+/171023
Reviewed-by: Rob Pike <r@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>
Run-TryBot: Rob Pike <r@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

5 files changed, 90 insertions, 10 deletions

diff --git a/src/regexp/all_test.go b/src/regexp/all_test.go
index 623f82df72..626a69142f 100644
--- a/src/regexp/all_test.go
+++ b/src/regexp/all_test.go
@@ -860,6 +860,25 @@ func BenchmarkQuoteMetaNone(b *testing.B) {
 	}
 }
 
+var compileBenchData = []struct{ name, re string }{
+	{"Onepass", `^a.[l-nA-Cg-j]?e$`},
+	{"Medium", `^((a|b|[d-z0-9])*(日){4,5}.)+$`},
+	{"Hard", strings.Repeat(`((abc)*|`, 50) + strings.Repeat(`)`, 50)},
+}
+
+func BenchmarkCompile(b *testing.B) {
+	for _, data := range compileBenchData {
+		b.Run(data.name, func(b *testing.B) {
+			b.ReportAllocs()
+			for i := 0; i < b.N; i++ {
+				if _, err := Compile(data.re); err != nil {
+					b.Fatal(err)
+				}
+			}
+		})
+	}
+}
+
 func TestDeepEqual(t *testing.T) {
 	re1 := MustCompile("a.*b.*c.*d")
 	re2 := MustCompile("a.*b.*c.*d")
@@ -882,3 +901,31 @@ func TestDeepEqual(t *testing.T) {
 		t.Errorf("DeepEqual(re1, re2) = false, want true")
 	}
 }
+
+var minInputLenTests = []struct {
+	Regexp string
+	min    int
+}{
+	{``, 0},
+	{`a`, 1},
+	{`aa`, 2},
+	{`(aa)a`, 3},
+	{`(?:aa)a`, 3},
+	{`a?a`, 1},
+	{`(aaa)|(aa)`, 2},
+	{`(aa)+a`, 3},
+	{`(aa)*a`, 1},
+	{`(aa){3,5}`, 6},
+	{`[a-z]`, 1},
+	{`日`, 3},
+}
+
+func TestMinInputLen(t *testing.T) {
+	for _, tt := range minInputLenTests {
+		re, _ := syntax.Parse(tt.Regexp, syntax.Perl)
+		m := minInputLen(re)
+		if m != tt.min {
+			t.Errorf("regexp %#q has minInputLen %d, should be %d", tt.Regexp, m, tt.min)
+		}
+	}
+}
diff --git a/src/regexp/exec.go b/src/regexp/exec.go
index efe764e2dc..4411e4c3e6 100644
--- a/src/regexp/exec.go
+++ b/src/regexp/exec.go
@@ -524,6 +524,10 @@ func (re *Regexp) doExecute(r io.RuneReader, b []byte, s string, pos int, ncap i
 		dstCap = arrayNoInts[:0:0]
 	}
 
+	if r == nil && len(b)+len(s) < re.minInputLen {
+		return nil
+	}
+
 	if re.onepass != nil {
 		return re.doOnePass(r, b, s, pos, ncap, dstCap)
 	}
diff --git a/src/regexp/exec_test.go b/src/regexp/exec_test.go
index 1489219328..1e8795525d 100644
--- a/src/regexp/exec_test.go
+++ b/src/regexp/exec_test.go
@@ -717,6 +717,7 @@ var benchSizes = []struct {
 	name string
 	n    int
 }{
+	{"16", 16},
 	{"32", 32},
 	{"1K", 1 << 10},
 	{"32K", 32 << 10},
diff --git a/src/regexp/onepass_test.go b/src/regexp/onepass_test.go
index a0f2e39048..32264d5f1e 100644
--- a/src/regexp/onepass_test.go
+++ b/src/regexp/onepass_test.go
@@ -223,13 +223,3 @@ func TestRunOnePass(t *testing.T) {
 		}
 	}
 }
-
-func BenchmarkCompileOnepass(b *testing.B) {
-	b.ReportAllocs()
-	const re = `^a.[l-nA-Cg-j]?e$`
-	for i := 0; i < b.N; i++ {
-		if _, err := Compile(re); err != nil {
-			b.Fatal(err)
-		}
-	}
-}
diff --git a/src/regexp/regexp.go b/src/regexp/regexp.go
index 54cbd3777b..19ca6f2223 100644
--- a/src/regexp/regexp.go
+++ b/src/regexp/regexp.go
@@ -94,6 +94,7 @@ type Regexp struct {
 	matchcap       int            // size of recorded match lengths
 	prefixComplete bool           // prefix is the entire regexp
 	cond           syntax.EmptyOp // empty-width conditions required at start of match
+	minInputLen    int            // minimum length of the input in bytes
 
 	// This field can be modified by the Longest method,
 	// but it is otherwise read-only.
@@ -191,6 +192,7 @@ func compile(expr string, mode syntax.Flags, longest bool) (*Regexp, error) {
 		cond:        prog.StartCond(),
 		longest:     longest,
 		matchcap:    matchcap,
+		minInputLen: minInputLen(re),
 	}
 	if regexp.onepass == nil {
 		regexp.prefix, regexp.prefixComplete = prog.Prefix()
@@ -264,6 +266,42 @@ func (re *Regexp) put(m *machine) {
 	matchPool[re.mpool].Put(m)
 }
 
+// minInputLen walks the regexp to find the minimum length of any matchable input
+func minInputLen(re *syntax.Regexp) int {
+	switch re.Op {
+	default:
+		return 0
+	case syntax.OpAnyChar, syntax.OpAnyCharNotNL, syntax.OpCharClass:
+		return 1
+	case syntax.OpLiteral:
+		l := 0
+		for _, r := range re.Rune {
+			l += utf8.RuneLen(r)
+		}
+		return l
+	case syntax.OpCapture, syntax.OpPlus:
+		return minInputLen(re.Sub[0])
+	case syntax.OpRepeat:
+		return re.Min * minInputLen(re.Sub[0])
+	case syntax.OpConcat:
+		l := 0
+		for _, sub := range re.Sub {
+			l += minInputLen(sub)
+		}
+		return l
+	case syntax.OpAlternate:
+		l := minInputLen(re.Sub[0])
+		var lnext int
+		for _, sub := range re.Sub[1:] {
+			lnext = minInputLen(sub)
+			if lnext < l {
+				l = lnext
+			}
+		}
+		return l
+	}
+}
+
 // MustCompile is like Compile but panics if the expression cannot be parsed.
 // It simplifies safe initialization of global variables holding compiled regular
 // expressions.