gc: O(1) string comparison when lengths differ

R=ken2
CC=golang-dev
https://golang.org/cl/2331045

src/cmd/5g/cgen.c

@@ -820,6 +820,9 @@ bgen(Node *n, int true, Prog *to)
 	if(n == N)
 		n = nodbool(1);
 
+	if(n->ninit != nil)
+		genlist(n->ninit);
+
 	nl = n->left;
 	nr = n->right;
 

src/cmd/6g/cgen.c

@@ -699,6 +699,9 @@ bgen(Node *n, int true, Prog *to)
 	if(n == N)
 		n = nodbool(1);
 
+	if(n->ninit != nil)
+		genlist(n->ninit);
+
 	nl = n->left;
 	nr = n->right;
 

src/cmd/8g/cgen.c

@@ -754,6 +754,9 @@ bgen(Node *n, int true, Prog *to)
 	if(n == N)
 		n = nodbool(1);
 
+	if(n->ninit != nil)
+		genlist(n->ninit);
+
 	nl = n->left;
 	nr = n->right;
 

src/cmd/gc/gen.c

@@ -248,9 +248,6 @@ gen(Node *n)
 
 		gen(n->nincr);				// contin:	incr
 		patch(p1, pc);				// test:
-		if(n->ntest != N)
-			if(n->ntest->ninit != nil)
-				genlist(n->ntest->ninit);
 		bgen(n->ntest, 0, breakpc);		//		if(!test) goto break
 		genlist(n->nbody);				//		body
 		gjmp(continpc);
@@ -263,9 +260,6 @@ gen(Node *n)
 		p1 = gjmp(P);			//		goto test
 		p2 = gjmp(P);			// p2:		goto else
 		patch(p1, pc);				// test:
-		if(n->ntest != N)
-			if(n->ntest->ninit != nil)
-				genlist(n->ntest->ninit);
 		bgen(n->ntest, 0, p2);			//		if(!test) goto p2
 		genlist(n->nbody);				//		then
 		p3 = gjmp(P);			//		goto done

src/cmd/gc/go.h

@@ -1109,6 +1109,7 @@ Type*	ptrto(Type *t);
 void*	remal(void *p, int32 on, int32 n);
 Sym*	restrictlookup(char *name, Pkg *pkg);
 Node*	safeexpr(Node *n, NodeList **init);
+Node*	cheapexpr(Node *n, NodeList **init);
 int32	setlineno(Node *n);
 void	setmaxarg(Type *t);
 Type*	shallow(Type *t);

src/cmd/gc/subr.c

@@ -2595,7 +2595,7 @@ brrev(int a)
 }
 
 /*
- * return side effect-free appending side effects to init.
+ * return side effect-free n, appending side effects to init.
  * result is assignable if n is.
  */
 Node*
@@ -2652,6 +2652,24 @@ safeexpr(Node *n, NodeList **init)
 	// make a copy; must not be used as an lvalue
 	if(islvalue(n))
 		fatal("missing lvalue case in safeexpr: %N", n);
+	return cheapexpr(n, init);
+}
+
+/*
+ * return side-effect free and cheap n, appending side effects to init.
+ * result may not be assignable.
+ */
+Node*
+cheapexpr(Node *n, NodeList **init)
+{
+	Node *a, *l;
+
+	switch(n->op) {
+	case ONAME:
+	case OLITERAL:
+		return n;
+	}
+
 	l = nod(OXXX, N, N);
 	tempname(l, n->type);
 	a = nod(OAS, l, n);

src/cmd/gc/walk.c

@@ -654,8 +654,6 @@ walkexpr(Node **np, NodeList **init)
 	case OAND:
 	case OOR:
 	case OXOR:
-	case OANDAND:
-	case OOROR:
 	case OSUB:
 	case OMUL:
 	case OEQ:
@@ -670,6 +668,17 @@ walkexpr(Node **np, NodeList **init)
 		walkexpr(&n->right, init);
 		goto ret;
 	
+	case OANDAND:
+	case OOROR:
+		walkexpr(&n->left, init);
+		// cannot put side effects from n->right on init,
+		// because they cannot run before n->left is checked.
+		// save elsewhere and store on the eventual n->right.
+		ll = nil;
+		walkexpr(&n->right, &ll);
+		n->right->ninit = concat(n->right->ninit, ll);
+		goto ret;
+
 	case OPRINT:
 	case OPRINTN:
 		walkexprlist(n->list, init);
@@ -1196,11 +1205,27 @@ walkexpr(Node **np, NodeList **init)
 			goto ret;
 		}
 
+		// prepare for rewrite below
+		if(n->etype == OEQ || n->etype == ONE) {
+			n->left = cheapexpr(n->left, init);
+			n->right = cheapexpr(n->right, init);
+		}
+
 		// sys_cmpstring(s1, s2) :: 0
 		r = mkcall("cmpstring", types[TINT], init,
 			conv(n->left, types[TSTRING]),
 			conv(n->right, types[TSTRING]));
 		r = nod(n->etype, r, nodintconst(0));
+
+		// quick check of len before full compare for == or !=
+		if(n->etype == OEQ || n->etype == ONE) {
+			if(n->etype == OEQ)
+				r = nod(OANDAND, nod(OEQ, nod(OLEN, n->left, N), nod(OLEN, n->right, N)), r);
+			else
+				r = nod(OOROR, nod(ONE, nod(OLEN, n->left, N), nod(OLEN, n->right, N)), r);
+			typecheck(&r, Erv);
+			walkexpr(&r, nil);
+		}
 		typecheck(&r, Erv);
 		n = r;
 		goto ret;