new UNDEF Ref

Crashing loads of uninitialized memory
proved to be a problem when implementing
unions using qbe.  This patch introduces
a new UNDEF Ref to represent data that is
known to be uninitialized.  Optimization
passes can make use of it to eliminate
some code.  In the last compilation stages,
UNDEF is treated as the constant 0xdeaddead.

1 files changed, 15 insertions, 10 deletions

diff --git a/mem.c b/mem.c
index 5d59b96..1db632f 100644
--- a/mem.c
+++ b/mem.c
@@ -201,7 +201,7 @@ coalesce(Fn *fn)
 	Ref *arg;
 	bits x;
 	int64_t off0, off1;
-	int n, m, sz, nsl, nbl, ip, *stk;
+	int n, m, ip, sz, nsl, nbl, *stk;
 	uint total, freed, fused;
 
 	/* minimize the stack usage
@@ -317,26 +317,31 @@ coalesce(Fn *fn)
 	while (n--) {
 		t = &fn->tmp[stk[n]];
 		assert(t->ndef == 1 && t->def);
-		*t->def = (Ins){.op = Onop};
+		i = t->def;
+		if (isload(i->op)) {
+			i->op = Ocopy;
+			i->arg[0] = UNDEF;
+			continue;
+		}
+		*i = (Ins){.op = Onop};
 		for (u=t->use; u<&t->use[t->nuse]; u++) {
 			if (u->type == UJmp) {
 				b = fn->rpo[u->bid];
-				b->jmp.arg = CON_Z;
+				assert(isret(b->jmp.type));
+				b->jmp.type = Jret0;
+				b->jmp.arg = R;
 				continue;
 			}
 			assert(u->type == UIns);
 			i = u->u.ins;
-			/* make loads crash */
-			if (isload(i->op))
-				i->arg[0] = CON_Z;
-			else if (i->op == Oargc)
-				i->arg[1] = CON_Z;
-			else if (!req(i->to, R)) {
+			if (!req(i->to, R)) {
 				assert(rtype(i->to) == RTmp);
 				vgrow(&stk, ++n);
 				stk[n-1] = i->to.val;
+			} else if (isarg(i->op)) {
+				assert(i->op == Oargc);
+				i->arg[1] = CON_Z;  /* crash */
 			} else {
-				assert(!isarg(i->op));
 				if (i->op == Oblit0)
 					*(i+1) = (Ins){.op = Onop};
 				*i = (Ins){.op = Onop};