cmd/compile: minor rulegen simplifications

The commuteDepth variable is no longer necessary; remove it.

Else branches after a log.Fatal call are unnecessary.

Also make the unbalanced return an integer, so we can differentiate
positive from negative cases. We only want to continue a rule with the
following lines if this balance is positive, for example.

While at it, make the balance loop stop when it goes negative, to not
let ")(" seem balanced.

Change-Id: I8aa313343ca5a2f07f638b62a0398fdf108fc9eb
Reviewed-on: https://go-review.googlesource.com/c/go/+/228822
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>

src/cmd/compile/internal/ssa/gen/rulegen.go

@@ -137,10 +137,12 @@ func genRulesSuffix(arch arch, suff string) {
 			ruleLineno = lineno
 		}
 		if strings.HasSuffix(rule, "->") || strings.HasSuffix(rule, "=>") {
-			continue
+			continue // continue on the next line
 		}
-		if unbalanced(rule) {
-			continue
+		if n := balance(rule); n > 0 {
+			continue // open parentheses remain, continue on the next line
+		} else if n < 0 {
+			break // continuing the line can't help, and it will only make errors worse
 		}
 
 		loc := fmt.Sprintf("%s%s.rules:%d", arch.name, suff, ruleLineno)
@@ -162,7 +164,7 @@ func genRulesSuffix(arch arch, suff string) {
 	if err := scanner.Err(); err != nil {
 		log.Fatalf("scanner failed: %v\n", err)
 	}
-	if unbalanced(rule) {
+	if balance(rule) != 0 {
 		log.Fatalf("%s.rules:%d: unbalanced rule: %v\n", arch.name, lineno, rule)
 	}
 
@@ -607,9 +609,8 @@ func fprint(w io.Writer, n Node) {
 					}
 					if prev, ok := seenRewrite[k]; ok {
 						log.Fatalf("duplicate rule %s, previously seen at %s\n", rr.Loc, prev)
-					} else {
-						seenRewrite[k] = rr.Loc
 					}
+					seenRewrite[k] = rr.Loc
 				}
 			}
 			fmt.Fprintf(w, "}\n")
@@ -878,7 +879,7 @@ func genBlockRewrite(rule Rule, arch arch, data blockData) *RuleRewrite {
 				if rr.Check == "" {
 					rr.Check = check
 				} else {
-					rr.Check = rr.Check + " && " + check
+					rr.Check += " && " + check
 				}
 			}
 			if p == "" {
@@ -1118,10 +1119,8 @@ func genMatch0(rr *RuleRewrite, arch arch, match, v string, cnt map[string]int,
 			rr.add(declf(vname, "%s.Args[%d]", v, i))
 		}
 	}
-	var commuteDepth int
 	if commutative {
-		commuteDepth = rr.CommuteDepth
-		rr.add(StartCommuteLoop{commuteDepth, v})
+		rr.add(StartCommuteLoop{rr.CommuteDepth, v})
 		rr.CommuteDepth++
 	}
 	for i, arg := range args {
@@ -1507,17 +1506,23 @@ func typeName(typ string) string {
 	}
 }
 
-// unbalanced reports whether there are a different number of ( and ) in the string.
-func unbalanced(s string) bool {
+// balance returns the number of unclosed '(' characters in s.
+// If a ')' appears without a corresponding '(', balance returns -1.
+func balance(s string) int {
 	balance := 0
 	for _, c := range s {
-		if c == '(' {
+		switch c {
+		case '(':
 			balance++
-		} else if c == ')' {
+		case ')':
 			balance--
+			if balance < 0 {
+				// don't allow ")(" to return 0
+				return -1
+			}
 		}
 	}
-	return balance != 0
+	return balance
 }
 
 // findAllOpcode is a function to find the opcode portion of s-expressions.