implement a simple alias analysis

1 files changed, 154 insertions, 0 deletions

diff --git a/alias.c b/alias.c
new file mode 100644
index 0000000..3d1d00c
--- /dev/null
+++ b/alias.c
@@ -0,0 +1,154 @@
+#include "all.h"
+
+static void
+getalias(Alias *a, Ref r, Fn *fn)
+{
+	Con *c;
+
+	switch (rtype(r)) {
+	default:
+		die("unreachable");
+	case RTmp:
+		*a = fn->tmp[r.val].alias;
+		assert(a->type != ABot);
+		break;
+	case RCon:
+		c = &fn->con[r.val];
+		if (c->type == CAddr) {
+			a->type = ASym;
+			strcpy(a->label, c->label);
+		} else
+			a->type = ACon;
+		a->offset = c->bits.i;
+		break;
+	}
+}
+
+int
+alias(Ref p, int sp, Ref q, int sq, int *delta, Fn *fn)
+{
+	Alias ap, aq;
+	int ovlap;
+
+	getalias(&ap, p, fn);
+	getalias(&aq, q, fn);
+	*delta = ap.offset - aq.offset;
+	ovlap = ap.offset < aq.offset + sq && aq.offset < ap.offset + sp;
+
+	if (ap.type & aq.type & 1) {
+		/* if both are offsets of the same
+		 * stack slot, they alias iif they
+		 * overlap */
+		if (req(ap.base, aq.base) && ovlap)
+			return MustAlias;
+		return NoAlias;
+	}
+
+	if (ap.type == ASym && aq.type == ASym) {
+		/* they conservatively alias if the
+		 * symbols are different, or they
+		 * alias for sure if they overlap */
+		if (strcmp(ap.label, aq.label) != 0)
+			return MayAlias;
+		if (ovlap)
+			return MustAlias;
+		return NoAlias;
+	}
+
+	if ((ap.type == ACon && aq.type == ACon)
+	|| (ap.type == aq.type && req(ap.base, aq.base))) {
+		assert(ap.type == ACon || ap.type == AUnk);
+		/* if they have the same base, we
+		 * can rely on the offsets only */
+		if (ovlap)
+			return MustAlias;
+		return NoAlias;
+	}
+
+	/* if one of the two is unknown
+	 * there may be aliasing unless
+	 * the other is provably local */
+	if (ap.type == AUnk && aq.type != ALoc)
+		return MayAlias;
+	if (aq.type == AUnk && ap.type != ALoc)
+		return MayAlias;
+
+	return NoAlias;
+}
+
+int
+escapes(Ref r, Fn *fn)
+{
+	if (rtype(r) != RTmp)
+		return 1;
+	return fn->tmp[r.val].alias.type != ALoc;
+}
+
+static void
+esc(Ref r, Fn *fn)
+{
+	Alias *a;
+
+	assert(rtype(r) <= RType);
+	if (rtype(r) == RTmp) {
+		a = &fn->tmp[r.val].alias;
+		assert(a->type != ABot);
+		if (a->type == ALoc)
+			a->type = AEsc;
+	}
+}
+
+void
+fillalias(Fn *fn)
+{
+	int n;
+	Blk *b;
+	Phi *p;
+	Ins *i;
+	Alias *a, a0, a1;
+
+	for (n=0; n<fn->nblk; ++n) {
+		b = fn->rpo[n];
+		for (p=b->phi; p; p=p->link) {
+			assert(rtype(p->to) == RTmp);
+			a = &fn->tmp[p->to.val].alias;
+			assert(a->type == ABot);
+			a->type = AUnk;
+			a->base = p->to;
+			a->offset = 0;
+		}
+		for (i=b->ins; i<&b->ins[b->nins]; ++i) {
+			a = 0;
+			if (!req(i->to, R)) {
+				assert(rtype(i->to) == RTmp);
+				a = &fn->tmp[i->to.val].alias;
+				assert(a->type == ABot);
+				if (Oalloc <= i->op && i->op <= Oalloc1)
+					a->type = ALoc;
+				else
+					a->type = AUnk;
+				a->base = i->to;
+				a->offset = 0;
+			}
+			if (i->op == Oadd) {
+				getalias(&a0, i->arg[0], fn);
+				getalias(&a1, i->arg[1], fn);
+				if (a0.type == ACon) {
+					*a = a1;
+					a->offset += a0.offset;
+				}
+				else if (a1.type == ACon) {
+					*a = a0;
+					a->offset += a1.offset;
+				}
+			}
+			if (req(i->to, R) || a->type == AUnk) {
+				if (!isload(i->op))
+					esc(i->arg[0], fn);
+				if (!isstore(i->op))
+					esc(i->arg[1], fn);
+			}
+		}
+		esc(b->jmp.arg, fn);
+	}
+}