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ref: dc20a94f244ab7bbaec47e5106362957a53a570b
parent: f6fb4afa7527852036972c7c5795ce46e9bbdbfb
author: cinap_lenrek <cinap_lenrek@felloff.net>
date: Tue May 26 15:25:06 EDT 2015 

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 */
}


--- a/sys/src/cmd/cc/dcl.c
+++ b/sys/src/cmd/cc/dcl.c
@@ -1542,6 +1542,48 @@
 	}
 }
 
+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 @@
 	}
 	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 @@
 
 	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];
+			zt = types[TLONG];
+		n = initz(q, zt, v, n);
+		v += zt->width;
+		w -= zt->width;
+	}
 
-		p->xoffset = v;
-		v += p->type->width;
-		w -= p->type->width;
-
-		m = new(OCONST, Z, Z);
-		m->vconst = 0;
-		m->type = p->type;
-
-		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)
-- 
⑨