cc: set unspecified elements to zero in local variable initializers

the compiler used to skip zero initialization when initializer
list was given not covering unspecified elements. now we zero
all non explicitely initialized elements. for example:

typedef struct F F;
struct F
{
	int a;
	int b;
	int c;
};

void
main(void)
{
	char a[16] = { 1, 2, 3 };	/* a[3..15] initialized to zero */
	F f = { .b = 1 };			/* f.a, f.c initialized to zero */
}

1 files changed, 59 insertions, 41 deletions

diff --git a/sys/src/cmd/cc/dcl.c b/sys/src/cmd/cc/dcl.c
index 2984feec8..f6c567669 100644
--- a/sys/src/cmd/cc/dcl.c
+++ b/sys/src/cmd/cc/dcl.c
@@ -1542,6 +1542,48 @@ symadjust(Sym *s, Node *n, long del)
 	}
 }
 
+static int
+covered1(long o, Node *n)
+{
+	if(n->op == OLIST)
+		return covered1(o, n->left) || covered1(o, n->right);
+	if(n->op == OASI)
+		return covered1(o, n->left);
+	if(n->op != ONAME)
+		return 0;
+	return o >= n->xoffset && o < n->xoffset+n->type->width;
+}
+
+static int
+covered(long o, long w, Node *n)
+{
+	while(w > 0 && covered1(o, n)) {
+		o++;
+		w--;
+	}
+	return w == 0;
+}
+
+static Node*
+initz(Node *q, Type *t, long o, Node *n)
+{
+	Node *p;
+
+	if(covered(o, t->width, n))
+		return n;
+
+	p = new(ONAME, Z, Z);
+	*p = *q;
+	p->type = t;
+	p->xoffset = o;
+
+	q = new(OCONST, Z, Z);
+	q->vconst = 0;
+	q->type = t;
+
+	return new(OLIST, new(OASI, p, q), n);
+}
+
 Node*
 contig(Sym *s, Node *n, long v)
 {
@@ -1573,12 +1615,6 @@ contig(Sym *s, Node *n, long v)
 	}
 	if(n->op == OAS)
 		diag(Z, "oops in contig");
-/*ZZZ this appears incorrect
-need to check if the list completely covers the data.
-if not, bail
- */
-	if(n->op == OLIST)
-		goto no;
 	if(n->op == OASI)
 		if(n->left->type)
 		if(n->left->type->width == w)
@@ -1589,47 +1625,29 @@ if not, bail
 
 	v = s->offset;
 
-	/* unaligned front */ 
-	while(w > 0 && (v % ewidth[TIND]) != 0){
-		p = new(ONAME, Z, Z);
-		*p = *q;
-
+	/* unaligned front */
+	while(w > 0 && (v % ewidth[TIND]) != 0) {
+		zt = types[TCHAR];
 		if(w >= ewidth[TLONG] && (v % ewidth[TLONG]) == 0)
-			p->type = types[TLONG];
-		else
-			p->type = types[TCHAR];
-
-		p->xoffset = v;
-		v += p->type->width;
-		w -= p->type->width;
-
-		m = new(OCONST, Z, Z);
-		m->vconst = 0;
-		m->type = p->type;
+			zt = types[TLONG];
+		n = initz(q, zt, v, n);
+		v += zt->width;
+		w -= zt->width;
+	}
 
-		r = new(OAS, p, m);
-		n = new(OLIST, r, n);
+	/* skip initialized words */
+	while(w >= ewidth[TIND] && covered(v, ewidth[TIND], n)) {
+		v += ewidth[TIND];
+		w -= ewidth[TIND];
 	}
 
 	/* unaligned (or small) back */
-	while(w > 0 && ((w % ewidth[TIND]) != 0 || w <= 16)){
-		p = new(ONAME, Z, Z);
-		*p = *q;
-
+	while(w > 0 && ((w % ewidth[TIND]) != 0 || w <= 16)) {
+		zt = types[TCHAR];
 		if(w >= ewidth[TLONG] && (w % ewidth[TLONG]) == 0)
-			p->type = types[TLONG];
-		else
-			p->type = types[TCHAR];
-
-		w -= p->type->width;
-		p->xoffset = v + w;
-
-		m = new(OCONST, Z, Z);
-		m->vconst = 0;
-		m->type = p->type;
-
-		r = new(OAS, p, m);
-		n = new(OLIST, r, n);
+			zt = types[TLONG];
+		w -= zt->width;
+		n = initz(q, zt, v + w, n);
 	}
 
 	if(w == 0)