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

src/hydrogen.cc

 // Copyright 2013 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 4301 matching lines @@
   ASSERT(current_block() != NULL);
   HBasicBlock* loop_entry = BuildLoopEntry(stmt);
 
   BreakAndContinueInfo break_info(stmt);
   CHECK_BAILOUT(VisitLoopBody(stmt, loop_entry, &break_info));
   HBasicBlock* body_exit =
       JoinContinue(stmt, current_block(), break_info.continue_block());
   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()) {
+    if (stmt->cond()->ToBooleanIsFalse()) {
+      Goto(loop_successor);
+      body_exit = NULL;
+    } else {
+      // 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());
-    } else {
-      body_exit = NULL;
     }
-    if (loop_successor->HasPredecessor()) {
-      loop_successor->SetJoinId(stmt->ExitId());
-    } else {
-      loop_successor = NULL;
-    }
+    ASSERT(loop_successor->HasPredecessor());
+    loop_successor->SetJoinId(stmt->ExitId());
   }
   HBasicBlock* loop_exit = CreateLoop(stmt,
                                       loop_entry,
                                       body_exit,
                                       loop_successor,
                                       break_info.break_block());
   set_current_block(loop_exit);
 }
 
 
 void HOptimizedGraphBuilder::VisitWhileStatement(WhileStatement* stmt) {
   ASSERT(!HasStackOverflow());
   ASSERT(current_block() != NULL);
   ASSERT(current_block()->HasPredecessor());
   ASSERT(current_block() != NULL);
   HBasicBlock* loop_entry = BuildLoopEntry(stmt);
 
   // If the condition is constant true, do not generate a branch.
   HBasicBlock* loop_successor = NULL;
   if (!stmt->cond()->ToBooleanIsTrue()) {
     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);
   if (current_block() != NULL) {
     CHECK_BAILOUT(VisitLoopBody(stmt, loop_entry, &break_info));
   }
   HBasicBlock* body_exit =
       JoinContinue(stmt, current_block(), break_info.continue_block());
   HBasicBlock* loop_exit = CreateLoop(stmt,
                                       loop_entry,
@@ skipping 12 matching lines @@
     CHECK_ALIVE(Visit(stmt->init()));
   }
   ASSERT(current_block() != NULL);
   HBasicBlock* loop_entry = BuildLoopEntry(stmt);
 
   HBasicBlock* loop_successor = NULL;
   if (stmt->cond() != NULL) {
     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);
   if (current_block() != NULL) {
     CHECK_BAILOUT(VisitLoopBody(stmt, loop_entry, &break_info));
   }
   HBasicBlock* body_exit =
       JoinContinue(stmt, current_block(), break_info.continue_block());
 
   if (stmt->next() != NULL && body_exit != NULL) {
@@ skipping 6524 matching lines @@
   if (ShouldProduceTraceOutput()) {
     isolate()->GetHTracer()->TraceHydrogen(name(), graph_);
   }
 
 #ifdef DEBUG
   graph_->Verify(false);  // No full verify.
 #endif
 }
 
 } }  // namespace v8::internal