regexp: speed up onepass prefix check

Remove an unnecessary check that the first operation matches - we know
from compile-time that it is EmptyBeginText which will always match at
position 0.

This enables the call to i.hasPrefix to be done before unpacking the
instruction or the first two runes. If the prefix does not match we
go straight to return, and if it does match we need the runes after the
match, so in either case unpacking the runes was unnecessary.

Author: bboreham

src/regexp/exec.go

@@ -401,45 +401,44 @@ func (re *Regexp) doOnePass(ir io.RuneReader, ib []byte, is string, pos, ncap in
 	}
 
 	m := newOnePassMachine()
+	matched := false
+	i, _ := m.inputs.init(ir, ib, is)
+
+	r, r1 := endOfText, endOfText
+	width, width1 := 0, 0
+	var flag lazyFlag
+	var pc int
+	var inst onePassInst
+
+	// If there is a simple literal prefix, skip over it.
+	if pos == 0 && len(re.prefix) > 0 && i.canCheckPrefix() {
+		// Match requires literal prefix; fast search for it.
+		if !i.hasPrefix(re) {
+			goto Return
+		}
+		pos += len(re.prefix)
+		pc = int(re.prefixEnd)
+	} else {
+		pc = re.onepass.Start
+	}
+
 	if cap(m.matchcap) < ncap {
 		m.matchcap = make([]int, ncap)
 	} else {
 		m.matchcap = m.matchcap[:ncap]
 	}
-
-	matched := false
 	for i := range m.matchcap {
 		m.matchcap[i] = -1
 	}
 
-	i, _ := m.inputs.init(ir, ib, is)
-
-	r, r1 := endOfText, endOfText
-	width, width1 := 0, 0
 	r, width = i.step(pos)
-	if r != endOfText {
-		r1, width1 = i.step(pos + width)
-	}
-	var flag lazyFlag
 	if pos == 0 {
 		flag = newLazyFlag(-1, r)
 	} else {
 		flag = i.context(pos)
 	}
-	pc := re.onepass.Start
-	inst := re.onepass.Inst[pc]
-	// If there is a simple literal prefix, skip over it.
-	if pos == 0 && flag.match(syntax.EmptyOp(inst.Arg)) &&
-		len(re.prefix) > 0 && i.canCheckPrefix() {
-		// Match requires literal prefix; fast search for it.
-		if !i.hasPrefix(re) {
-			goto Return
-		}
-		pos += len(re.prefix)
-		r, width = i.step(pos)
+	if r != endOfText {
 		r1, width1 = i.step(pos + width)
-		flag = i.context(pos)
-		pc = int(re.prefixEnd)
 	}
 	for {
 		inst = re.onepass.Inst[pc]