Skip back edge creation for "do ... while(false)" loops

src/hydrogen.cc

Index: src/hydrogen.cc
diff --git a/src/hydrogen.cc b/src/hydrogen.cc
index 3c4ff159eb22b9b92595a5843f83c492c6c2d589..e8cbabca5d56fba707fe7342afc125956fa6b4a4 100644
--- a/src/hydrogen.cc
+++ b/src/hydrogen.cc
@@ -4319,21 +4319,20 @@ void HOptimizedGraphBuilder::VisitDoWhileStatement(DoWhileStatement* stmt) {
   HBasicBlock* loop_successor = NULL;
   if (body_exit != NULL && !stmt->cond()->ToBooleanIsTrue()) {
     set_current_block(body_exit);
-    // The block for a true condition, the actual predecessor block of the
-    // back edge.
-    body_exit = graph()->CreateBasicBlock();
     loop_successor = graph()->CreateBasicBlock();
-    CHECK_BAILOUT(VisitForControl(stmt->cond(), body_exit, loop_successor));
-    if (body_exit->HasPredecessor()) {
-      body_exit->SetJoinId(stmt->BackEdgeId());
-    } else {
+    if (stmt->cond()->ToBooleanIsFalse()) {
+      Goto(loop_successor);
       body_exit = NULL;
-    }
-    if (loop_successor->HasPredecessor()) {
-      loop_successor->SetJoinId(stmt->ExitId());
     } else {
-      loop_successor = NULL;
+      // The block for a true condition, the actual predecessor block of the
+      // back edge.
+      body_exit = graph()->CreateBasicBlock();
+      CHECK_BAILOUT(VisitForControl(stmt->cond(), body_exit, loop_successor));
+      ASSERT(body_exit->HasPredecessor());

[titzer, 2014/01/09 14:19:53]

I don't think this is strictly correct, e.g. if the loop condition leads to a
block being generated that ends in a deoptimize...then there may not be any
edges out of the loop.

[Jakob Kummerow, 2014/01/09 15:01:44]

I disagree. We are very careful to ensure that all parts of the graph are
connected properly; I can't imagine any reason why this would change in the
future, making this code useful again.
But as per offline discussion, I've reverted this part of the change.

+      body_exit->SetJoinId(stmt->BackEdgeId());
     }
+    ASSERT(loop_successor->HasPredecessor());
+    loop_successor->SetJoinId(stmt->ExitId());
   }
   HBasicBlock* loop_exit = CreateLoop(stmt,
                                       loop_entry,
@@ -4357,15 +4356,11 @@ void HOptimizedGraphBuilder::VisitWhileStatement(WhileStatement* stmt) {
     HBasicBlock* body_entry = graph()->CreateBasicBlock();
     loop_successor = graph()->CreateBasicBlock();
     CHECK_BAILOUT(VisitForControl(stmt->cond(), body_entry, loop_successor));
-    if (body_entry->HasPredecessor()) {
-      body_entry->SetJoinId(stmt->BodyId());
-      set_current_block(body_entry);
-    }
-    if (loop_successor->HasPredecessor()) {
-      loop_successor->SetJoinId(stmt->ExitId());
-    } else {
-      loop_successor = NULL;
-    }
+    ASSERT(body_entry->HasPredecessor());

[titzer, 2014/01/09 14:19:53]

Same here.

+    body_entry->SetJoinId(stmt->BodyId());
+    set_current_block(body_entry);
+    ASSERT(loop_successor->HasPredecessor());
+    loop_successor->SetJoinId(stmt->ExitId());
   }
 
   BreakAndContinueInfo break_info(stmt);
@@ -4398,15 +4393,11 @@ void HOptimizedGraphBuilder::VisitForStatement(ForStatement* stmt) {
     HBasicBlock* body_entry = graph()->CreateBasicBlock();
     loop_successor = graph()->CreateBasicBlock();
     CHECK_BAILOUT(VisitForControl(stmt->cond(), body_entry, loop_successor));
-    if (body_entry->HasPredecessor()) {
-      body_entry->SetJoinId(stmt->BodyId());
-      set_current_block(body_entry);
-    }
-    if (loop_successor->HasPredecessor()) {
-      loop_successor->SetJoinId(stmt->ExitId());
-    } else {
-      loop_successor = NULL;
-    }
+    ASSERT(body_entry->HasPredecessor());
+    body_entry->SetJoinId(stmt->BodyId());
+    set_current_block(body_entry);
+    ASSERT(loop_successor->HasPredecessor());
+    loop_successor->SetJoinId(stmt->ExitId());
   }
 
   BreakAndContinueInfo break_info(stmt);