regexp: speed up onepass prefix check

src/regexp/exec.go

@@ -401,46 +401,45 @@ 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)
-		r1, width1 = i.step(pos + width)
-		flag = i.context(pos)
-		pc = int(re.prefixEnd)
-	}
 	for {
 		inst = re.onepass.Inst[pc]
 		pc = int(inst.Out)