cmd/gc: do not generate code for var _ = ... unless necessary

Fixes #2443.

R=ken2
CC=golang-dev
https://golang.org/cl/6997048

src/cmd/gc/go.h

@@ -1375,6 +1375,7 @@ void	walkexprlistsafe(NodeList *l, NodeList **init);
 void	walkstmt(Node **np);
 void	walkstmtlist(NodeList *l);
 Node*	conv(Node*, Type*);
+int	candiscard(Node*);
 
 /*
  *	arch-specific ggen.c/gsubr.c/gobj.c/pgen.c

src/cmd/gc/init.c

@@ -55,6 +55,10 @@ anyinit(NodeList *n)
 		case ODCLTYPE:
 		case OEMPTY:
 			break;
+		case OAS:
+			if(isblank(l->n->left) && candiscard(l->n->right))
+				break;
+			// fall through
 		default:
 			return 1;
 		}

src/cmd/gc/sinit.c

@@ -108,6 +108,13 @@ init1(Node *n, NodeList **out)
 		case OAS:
 			if(n->defn->left != n)
 				goto bad;
+			if(isblank(n->defn->left) && candiscard(n->defn->right)) {
+				n->defn->op = OEMPTY;
+				n->defn->left = N;
+				n->defn->right = N;
+				break;
+			}
+
 		/*
 			n->defn->dodata = 1;
 			init1(n->defn->right, out);

src/cmd/gc/walk.c

@@ -183,8 +183,8 @@ walkstmt(Node **np)
 		dump("nottop", n);
 		break;
 
-	case OASOP:
 	case OAS:
+	case OASOP:
 	case OAS2:
 	case OAS2DOTTYPE:
 	case OAS2RECV:
@@ -3226,3 +3226,112 @@ usefield(Node *n)
 	curfn->paramfld = l;
 }
 
+static int
+candiscardlist(NodeList *l)
+{
+	for(; l; l=l->next)
+		if(!candiscard(l->n))
+			return 0;
+	return 1;
+}
+
+int
+candiscard(Node *n)
+{
+	if(n == N)
+		return 1;
+	
+	switch(n->op) {
+	default:
+		return 0;
+
+	case ONAME:
+	case ONONAME:
+	case OTYPE:
+	case OPACK:
+	case OLITERAL:
+	case OADD:
+	case OSUB:
+	case OOR:
+	case OXOR:
+	case OADDSTR:
+	case OADDR:
+	case OANDAND:
+	case OARRAYBYTESTR:
+	case OARRAYRUNESTR:
+	case OSTRARRAYBYTE:
+	case OSTRARRAYRUNE:
+	case OCAP:
+	case OCMPIFACE:
+	case OCMPSTR:
+	case OCOMPLIT:
+	case OMAPLIT:
+	case OSTRUCTLIT:
+	case OARRAYLIT:
+	case OPTRLIT:
+	case OCONV:
+	case OCONVIFACE:
+	case OCONVNOP:
+	case ODOT:
+	case OEQ:
+	case ONE:
+	case OLT:
+	case OLE:
+	case OGT:
+	case OGE:
+	case OKEY:
+	case OLEN:
+	case OMUL:
+	case OLSH:
+	case ORSH:
+	case OAND:
+	case OANDNOT:
+	case ONEW:
+	case ONOT:
+	case OCOM:
+	case OPLUS:
+	case OMINUS:
+	case OOROR:
+	case OPAREN:
+	case ORUNESTR:
+	case OREAL:
+	case OIMAG:
+	case OCOMPLEX:
+		// Discardable as long as the subpieces are.
+		break;
+
+	case ODIV:
+	case OMOD:
+		// Discardable as long as we know it's not division by zero.
+		if(isconst(n->right, CTINT) && mpcmpfixc(n->right->val.u.xval, 0) != 0)
+			break;
+		if(isconst(n->right, CTFLT) && mpcmpfltc(n->right->val.u.fval, 0) != 0)
+			break;
+		return 0;
+
+	case OMAKECHAN:
+	case OMAKEMAP:
+		// Discardable as long as we know it won't fail because of a bad size.
+		if(isconst(n->left, CTINT) && mpcmpfixc(n->left->val.u.xval, 0) == 0)
+			break;
+		return 0;
+	
+	case OMAKESLICE:
+		// Difficult to tell what sizes are okay.
+		return 0;		
+	}
+	
+	if(!candiscard(n->left) ||
+	   !candiscard(n->right) ||
+	   !candiscard(n->ntest) ||
+	   !candiscard(n->nincr) ||
+	   !candiscardlist(n->ninit) ||
+	   !candiscardlist(n->nbody) ||
+	   !candiscardlist(n->nelse) ||
+	   !candiscardlist(n->list) ||
+	   !candiscardlist(n->rlist)) {
+		return 0;
+	}
+	
+	return 1;
+}

test/golden.out

@@ -16,13 +16,9 @@
 == fixedbugs/
 
 =========== fixedbugs/bug429.go
-throw: all goroutines are asleep - deadlock!
+fatal error: all goroutines are asleep - deadlock!
 
 == bugs/
 
 =========== bugs/bug395.go
 bug395 is broken
-
-=========== bugs/bug434.go
-bugs/bug434.dir/two.go:10: one.t.int undefined (cannot refer to unexported field or method one.int)
-BUG:bug434

test/sinit.go

@@ -259,3 +259,13 @@ var copy_pt0a = pt0a
 var copy_pt0b = pt0b
 var copy_pt1 = pt1
 var copy_pt1a = pt1a
+
+var _ interface{} = 1
+
+type T1 int
+
+func (t *T1) M() {}
+
+type Mer interface { M() }
+
+var _ Mer = (*T1)(nil)