1 files changed, 78 insertions, 0 deletions
diff --git a/flow.c b/flow.c
index f052cdb1..2e20ab75 100644
--- a/flow.c
+++ b/flow.c
@@ -121,6 +121,31 @@ static int bb_depends_on_phi(struct basic_block *target, struct basic_block *src
 	return 0;
 }
 
+///
+// does the BB contains ignorable instructions but a final branch?
+// :note: something could be done for phi-sources but ... we'll see.
+static bool bb_is_forwarder(struct basic_block *bb)
+{
+	struct instruction *insn;
+
+	FOR_EACH_PTR(bb->insns, insn) {
+		if (!insn->bb)
+			continue;
+		switch (insn->opcode) {
+		case OP_NOP:
+		case OP_INLINED_CALL:
+			continue;
+		case OP_CBR:
+		case OP_BR:
+			return true;
+		default:
+			goto out;
+		}
+	} END_FOR_EACH_PTR(insn);
+out:
+	return false;
+}
+
 /*
  * When we reach here, we have:
  *  - a basic block that ends in a conditional branch and
@@ -742,6 +767,22 @@ void vrfy_flow(struct entrypoint *ep)
 	assert(!entry);
 }
 
+static int retarget_parents(struct basic_block *bb, struct basic_block *target)
+{
+	struct basic_block *parent;
+
+	/*
+	 * We can't do FOR_EACH_PTR() here, because the parent list
+	 * may change when we rewrite the parent.
+	 */
+	while ((parent = first_basic_block(bb->parents))) {
+		if (!rewrite_parent_branch(parent, bb, target))
+			return 0;
+	}
+	kill_bb(bb);
+	return REPEAT_CFG_CLEANUP;
+}
+
 static void remove_merging_phisrc(struct basic_block *top, struct instruction *insn)
 {
 	struct instruction *user = get_phinode(insn);
@@ -821,6 +862,43 @@ static int merge_bb(struct basic_block *top, struct basic_block *bot)
 	return REPEAT_CFG_CLEANUP;
 }
 
+///
+// early simplification of the CFG
+// Three things are done here:
+//    # inactive BB are removed
+//    # branches to a 'forwarder' BB are redirected to the forwardee.
+//    # merge single-child/single-parent BBs.
+int simplify_cfg_early(struct entrypoint *ep)
+{
+	struct basic_block *bb;
+	int changed = 0;
+
+	FOR_EACH_PTR_REVERSE(ep->bbs, bb) {
+		struct instruction *insn;
+		struct basic_block *tgt;
+
+		if (!bb->ep) {
+			DELETE_CURRENT_PTR(bb);
+			changed = REPEAT_CFG_CLEANUP;
+			continue;
+		}
+
+		insn = last_instruction(bb->insns);
+		if (!insn)
+			continue;
+		switch (insn->opcode) {
+		case OP_BR:
+			tgt = insn->bb_true;
+			if (bb_is_forwarder(bb))
+				changed |= retarget_parents(bb, tgt);
+			else if (bb_list_size(tgt->parents) == 1)
+				changed |= merge_bb(bb, tgt);
+			break;
+		}
+	} END_FOR_EACH_PTR_REVERSE(bb);
+	return changed;
+}
+
 void pack_basic_blocks(struct entrypoint *ep)
 {
 	struct basic_block *bb;