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cc2/qbe: Change cast() to return Node

Receiving a pointer to the struct were we want to store the
result needs a lot of temporary variables in the code. If
we make that cast() returns a struct instead of a pointer to
struct we can remove a lot of these temporaries and
simplify the code. The code generated is pretty similar
because returning structs usually mean passing an additional
pointer to the function called where it can write the results.

--- a/src/cmd/cc/cc2/target/qbe/cgen.c

+++ b/src/cmd/cc/cc2/target/qbe/cgen.c

@@ -162,21 +162,20 @@

 static Node *rhs(Node *np, Node *new);

 static Node *

-cast(Type *td, Node *ns, Node *nd)

+cast(Type *td, Node *np)

 {

 	Type *ts;

-	Node aux1, aux2;

+	Node *tmp;

 	int op, d_isint, s_isint;

-	ts = &ns->type;

+	ts = &np->type;

 	d_isint = (td->flags & INTF) != 0;

 	s_isint = (ts->flags & INTF) != 0;

 	if (d_isint && s_isint) {

-		if (td->size <= ts->size) {

-			*nd = *ns;

-			return nd;

-		}

+		if (td->size <= ts->size)

+			return np;

+

 		assert(td->size == 4 || td->size == 8);

 		switch (ts->size) {

 		case 1:

@@ -215,7 +214,7 @@

 		case 1:

 		case 2:

 			ts = (ts->flags&SIGNF) ? &int32type : &uint32type;

-			ns = cast(ts, ns, tmpnode(&aux2, ts));

+			np = cast(ts, np);

 		case 4:

 			op = (td->size == 8) ? ASSWTOD : ASSWTOS;

 			break;

@@ -231,8 +230,10 @@

 		op = (td->size == 4) ? ASEXTS : ASTRUNCD;

 	}

-	code(op, tmpnode(nd, td), ns, NULL);

-	return nd;

+	tmp = tmpnode(NULL, td);

+	code(op, tmp, np, NULL);

+

+	return tmp;

 }

 static Node *

@@ -579,7 +580,8 @@

 			l = rhs(l, &aux1);

 		return call(np, l, ret);

 	case OCAST:

-		return cast(tp, rhs(l, &aux1), ret);

+		*ret = *cast(tp, rhs(l, &aux1));

+		return ret;

 	case OASSIG:

 		switch (np->u.subop) {

 		case OINC: