fix bug in alias analysis

The handling of phi was incorrect
and we would sometimes miss escapes.
We now handle phis at the very end
of the pass to make sure the defs
for their arguments have all been
processed.

alias.c

@@ -132,7 +132,7 @@ store(Ref r, int sz, Fn *fn)
 void
 fillalias(Fn *fn)
 {
-	uint n, m;
+	uint n;
 	int t, sz;
 	int64_t x;
 	Blk *b;
@@ -146,8 +146,6 @@ fillalias(Fn *fn)
 	for (n=0; n<fn->nblk; ++n) {
 		b = fn->rpo[n];
 		for (p=b->phi; p; p=p->link) {
-			for (m=0; m<p->narg; m++)
-				esc(p->arg[m], fn);
 			assert(rtype(p->to) == RTmp);
 			a = &fn->tmp[p->to.val].alias;
 			assert(a->type == ABot);
@@ -217,4 +215,8 @@ fillalias(Fn *fn)
 		if (b->jmp.type != Jretc)
 			esc(b->jmp.arg, fn);
 	}
+	for (b=fn->start; b; b=b->link)
+		for (p=b->phi; p; p=p->link)
+			for (n=0; n<p->narg; n++)
+				esc(p->arg[n], fn);
 }

test/alias1.ssa

@@ -0,0 +1,21 @@
+export function w $main() {
+@start
+	%a =l alloc4 4
+	%b =l alloc4 4
+	storew 4, %a
+	storew 5, %b
+
+@loop
+	# %mem will be %a and %b successively,
+	# but we do not know it when processing
+	# the phi because %b goes through a cpy
+	%mem =l phi @start %a, @loop %bcpy
+
+	%w =w load %mem
+	%eq5 =w ceqw %w, 5
+	%bcpy =l copy %b
+	jnz %eq5, @exit, @loop
+
+@exit
+	ret 0
+}