regexp/syntax: don't waste time checking for one pass algorithm

The code recurs very deeply in cases like (?:x{1,1000}){1,1000}
Since if much time is spent checking whether one pass is possible, it's not
worth doing at all, a simple fix is proposed: Stop if the check takes too long.
To do this, we simply avoid machines with >1000 instructions.

Benchmarks show a percent or less change either way, effectively zero.

Fixes #7608.

LGTM=rsc
R=rsc
CC=golang-codereviews
https://golang.org/cl/92290043

src/pkg/regexp/all_test.go

@@ -473,6 +473,11 @@ func TestSplit(t *testing.T) {
 	}
 }
 
+// This ran out of stack before issue 7608 was fixed.
+func TestOnePassCutoff(t *testing.T) {
+	MustCompile(`^(?:x{1,1000}){1,1000}$`)
+}
+
 func BenchmarkLiteral(b *testing.B) {
 	x := strings.Repeat("x", 50) + "y"
 	b.StopTimer()
@@ -588,6 +593,7 @@ func BenchmarkOnePassShortA(b *testing.B) {
 		re.Match(x)
 	}
 }
+
 func BenchmarkNotOnePassShortA(b *testing.B) {
 	b.StopTimer()
 	x := []byte("abcddddddeeeededd")
@@ -597,6 +603,7 @@ func BenchmarkNotOnePassShortA(b *testing.B) {
 		re.Match(x)
 	}
 }
+
 func BenchmarkOnePassShortB(b *testing.B) {
 	b.StopTimer()
 	x := []byte("abcddddddeeeededd")
@@ -606,6 +613,7 @@ func BenchmarkOnePassShortB(b *testing.B) {
 		re.Match(x)
 	}
 }
+
 func BenchmarkNotOnePassShortB(b *testing.B) {
 	b.StopTimer()
 	x := []byte("abcddddddeeeededd")
@@ -615,6 +623,7 @@ func BenchmarkNotOnePassShortB(b *testing.B) {
 		re.Match(x)
 	}
 }
+
 func BenchmarkOnePassLongPrefix(b *testing.B) {
 	b.StopTimer()
 	x := []byte("abcdefghijklmnopqrstuvwxyz")
@@ -624,6 +633,7 @@ func BenchmarkOnePassLongPrefix(b *testing.B) {
 		re.Match(x)
 	}
 }
+
 func BenchmarkOnePassLongNotPrefix(b *testing.B) {
 	b.StopTimer()
 	x := []byte("abcdefghijklmnopqrstuvwxyz")

src/pkg/regexp/syntax/prog.go

@@ -600,6 +600,11 @@ func (p runeSlice) Sort() {
 // onepass Prog, the Prog syntax.NotOnePass is returned. makeOnePass is recursive
 // to the size of the Prog
 func (p *Prog) makeOnePass() *Prog {
+	// If the machine is very long, it's not worth the time to check if we can use one pass.
+	if len(p.Inst) >= 1000 {
+		return NotOnePass
+	}
+
 	var (
 		instQueue    = newQueue(len(p.Inst))
 		visitQueue   = newQueue(len(p.Inst))