gc: fix another blank bug

R=ken2
CC=golang-dev
https://golang.org/cl/5478051

src/cmd/5g/gsubr.c

@@ -515,6 +515,12 @@ nodarg(Type *t, int fp)
 	n->orig = t->nname;
 
 fp:
+	// Rewrite argument named _ to __,
+	// or else the assignment to _ will be
+	// discarded during code generation.
+	if(isblank(n))
+		n->sym = lookup("__");
+
 	switch(fp) {
 	default:
 		fatal("nodarg %T %d", t, fp);

src/cmd/6g/gsubr.c

@@ -481,6 +481,7 @@ nodarg(Type *t, int fp)
 	n = nod(ONAME, N, N);
 	n->type = t->type;
 	n->sym = t->sym;
+	
 	if(t->width == BADWIDTH)
 		fatal("nodarg: offset not computed for %T", t);
 	n->xoffset = t->width;
@@ -488,6 +489,12 @@ nodarg(Type *t, int fp)
 	n->orig = t->nname;
 
 fp:
+	// Rewrite argument named _ to __,
+	// or else the assignment to _ will be
+	// discarded during code generation.
+	if(isblank(n))
+		n->sym = lookup("__");
+
 	switch(fp) {
 	case 0:		// output arg
 		n->op = OINDREG;

src/cmd/8g/gsubr.c

@@ -968,6 +968,12 @@ nodarg(Type *t, int fp)
 		break;
 	}
 	
+	// Rewrite argument named _ to __,
+	// or else the assignment to _ will be
+	// discarded during code generation.
+	if(isblank(n))
+		n->sym = lookup("__");
+
 	switch(fp) {
 	default:
 		fatal("nodarg %T %d", t, fp);

test/blank.go

@@ -118,12 +118,29 @@ func (TI) M(x int, y int) {
 	}
 }
 
+var fp = func(_ int, y int) {}
+
+func init() {
+	fp = fp1
+}
+
+func fp1(x, y int) {
+	if x != y {
+		println("invalid fp1 call:", x, y)
+		panic("bad fp1")
+	}
+}
+
+
 func m() {
 	var i I
 	
 	i = TI{}
 	i.M(1, 1)
 	i.M(2, 2)
+	
+	fp(1, 1)
+	fp(2, 2)
 }
 
 // useless but legal