Improve and simplify removal of unreachable code

src/hydrogen-mark-unreachable.cc

Index: src/hydrogen-mark-unreachable.cc
diff --git a/src/hydrogen-sce.cc b/src/hydrogen-mark-unreachable.cc
similarity index 54%
copy from src/hydrogen-sce.cc
copy to src/hydrogen-mark-unreachable.cc
index a6995f647afc00437783f057110c7654a28265c3..d7c5ed2b180281628e79d92f3522c71a4e9518b9 100644
--- a/src/hydrogen-sce.cc
+++ b/src/hydrogen-mark-unreachable.cc
@@ -25,38 +25,53 @@
 // (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 // OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
-#include "hydrogen-sce.h"
-#include "v8.h"
+#include "hydrogen-mark-unreachable.h"
 
 namespace v8 {
 namespace internal {
 
-void HStackCheckEliminationPhase::Run() {
-  // For each loop block walk the dominator tree from the backwards branch to
-  // the loop header. If a call instruction is encountered the backwards branch
-  // is dominated by a call and the stack check in the backwards branch can be
-  // removed.
-  for (int i = 0; i < graph()->blocks()->length(); i++) {
-    HBasicBlock* block = graph()->blocks()->at(i);
-    if (block->IsLoopHeader()) {
-      HBasicBlock* back_edge = block->loop_information()->GetLastBackEdge();
-      HBasicBlock* dominator = back_edge;
-      while (true) {
-        for (HInstructionIterator it(dominator); !it.Done(); it.Advance()) {
-          if (it.Current()->IsCall()) {
-            block->loop_information()->stack_check()->Eliminate();
+
+void HMarkUnreachableBlocksPhase::MarkUnreachableBlocks() {
+  // If there is unreachable code in the graph, propagate the unreachable marks
+  // using a fixed-point iteration.
+  bool changed = true;
+  const ZoneList<HBasicBlock*>* blocks = graph()->blocks();
+  while (changed) {
+    changed = false;
+    for (int i = 0; i < blocks->length(); i++) {
+      HBasicBlock* block = blocks->at(i);
+      if (!block->IsReachable()) continue;
+      bool is_reachable = blocks->at(0) == block;
+      for (HPredecessorIterator it(block); !it.Done(); it.Advance()) {
+        HBasicBlock* predecessor = it.Current();
+        // A block is reachable if one of its predecessors is reachable,
+        // doesn't deoptimize and either is known to transfer control to the
+        // block or has a control flow instruction for which the next block
+        // cannot be determined.
+        if (predecessor->IsReachable() && !predecessor->IsDeoptimizing()) {
+          HBasicBlock* pred_succ;
+          bool known_pred_succ =
+              predecessor->end()->KnownSuccessorBlock(&pred_succ);
+          if (!known_pred_succ || pred_succ == block) {
+            is_reachable = true;
             break;
           }
         }
-
-        // Done when the loop header is processed.
-        if (dominator == block) break;
-
-        // Move up the dominator tree.
-        dominator = dominator->dominator();
+        if (block->is_osr_entry()) {
+          is_reachable = true;
+        }
+      }
+      if (!is_reachable) {
+        block->MarkUnreachable();
+        changed = true;
       }
     }
   }
 }
 
+
+void HMarkUnreachableBlocksPhase::Run() {
+  MarkUnreachableBlocks();
+}
+
 } }  // namespace v8::internal