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, 7 insertions, 2 deletions

diff --git a/parse.c b/parse.c
index 68488a2..aed9427 100644
--- a/parse.c
+++ b/parse.c
@@ -867,7 +867,7 @@ parsefn(Lnk *lnk)
 	curi = insb;
 	curf = alloc(sizeof *curf);
 	curf->ntmp = 0;
-	curf->ncon = 1; /* first constant must be 0 */
+	curf->ncon = 2;
 	curf->tmp = vnew(curf->ntmp, sizeof curf->tmp[0], PFn);
 	curf->con = vnew(curf->ncon, sizeof curf->con[0], PFn);
 	for (i=0; i<Tmp0; ++i)
@@ -876,6 +876,8 @@ parsefn(Lnk *lnk)
 		else
 			newtmp(0, Kl, curf);
 	curf->con[0].type = CBits;
+	curf->con[0].bits.i = 0xdeaddead;  /* UNDEF */
+	curf->con[1].type = CBits;
 	curf->lnk = *lnk;
 	blink = &curf->start;
 	curf->retty = Kx;
@@ -1230,7 +1232,10 @@ printref(Ref r, Fn *fn, FILE *f)
 			fprintf(f, "%%%s", fn->tmp[r.val].name);
 		break;
 	case RCon:
-		printcon(&fn->con[r.val], f);
+		if (req(r, UNDEF))
+			fprintf(f, "UNDEF");
+		else
+			printcon(&fn->con[r.val], f);
 		break;
 	case RSlot:
 		fprintf(f, "S%d", rsval(r));