cmd/compile, go/parser: simpler binary expression parsing

The existing nested loops are too tricky for me to grok and don't seem
necessary.

Change-Id: I75c65c8470b799d6f4cfb05bb1b4796c5d7d32e7
Reviewed-on: https://go-review.googlesource.com/19927
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

src/cmd/compile/internal/gc/parser.go

@@ -1142,17 +1142,14 @@ func (p *parser) bexpr(prec int) *Node {
 	// don't trace bexpr - only leads to overly nested trace output
 
 	x := p.uexpr()
+	for {
 		t := prectab[p.tok]
-	for tprec := t.prec; tprec >= prec; tprec-- {
-		for tprec == prec {
-			p.next()
-			y := p.bexpr(t.prec + 1)
-			x = Nod(t.op, x, y)
-			t = prectab[p.tok]
-			tprec = t.prec
-		}
-	}
+		if t.prec < prec {
 			return x
+		}
+		p.next()
+		x = Nod(t.op, x, p.bexpr(t.prec+1))
+	}
 }
 
 func (p *parser) expr() *Node {

src/go/parser/parser.go

@@ -1597,23 +1597,19 @@ func (p *parser) parseBinaryExpr(lhs bool, prec1 int) ast.Expr {
 	}
 
 	x := p.parseUnaryExpr(lhs)
-	for _, prec := p.tokPrec(); prec >= prec1; prec-- {
 	for {
 		op, oprec := p.tokPrec()
-			if oprec != prec {
-				break
+		if oprec < prec1 {
+			return x
 		}
 		pos := p.expect(op)
 		if lhs {
 			p.resolve(x)
 			lhs = false
 		}
-			y := p.parseBinaryExpr(false, prec+1)
+		y := p.parseBinaryExpr(false, oprec+1)
 		x = &ast.BinaryExpr{X: p.checkExpr(x), OpPos: pos, Op: op, Y: p.checkExpr(y)}
 	}
-	}
-
-	return x
 }
 
 // If lhs is set and the result is an identifier, it is not resolved.