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, 8 insertions, 4 deletions

diff --git a/copy.c b/copy.c
index 1f9e1f3..77a8f06 100644
--- a/copy.c
+++ b/copy.c
@@ -125,7 +125,7 @@ subst(Ref *pr, Ref *cpy)
 	*pr = copyof(*pr, cpy);
 }
 
-/* requires use and rpo, breaks use */
+/* requires use and dom, breaks use */
 void
 copy(Fn *fn)
 {
@@ -155,12 +155,16 @@ copy(Fn *fn)
 			for (a=0; a<p->narg; a++)
 				if (p->blk[a]->id < n) {
 					r1 = copyof(p->arg[a], cpy);
-					if (req(r, R) || req(r1, r))
+					if (req(r, R) || req(r, UNDEF))
+						r = r1;
+					if (req(r1, r) || req(r1, UNDEF))
 						eq++;
-					r = r1;
 				}
 			assert(!req(r, R));
-			if (eq == p->narg)
+			if (rtype(r) == RTmp
+			&& !dom(fn->rpo[fn->tmp[r.val].bid], b))
+				cpy[p->to.val] = p->to;
+			else if (eq == p->narg)
 				cpy[p->to.val] = r;
 			else {
 				cpy[p->to.val] = p->to;