[Interpreter] Removes unnecessary jumps and dead code from If and loops.

src/interpreter/bytecode-generator.cc

 // Copyright 2015 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/interpreter/bytecode-generator.h"
 
 #include "src/compiler.h"
 #include "src/interpreter/control-flow-builders.h"
 #include "src/objects.h"
 #include "src/parser.h"
@@ skipping 474 matching lines @@
 
 
 void BytecodeGenerator::VisitEmptyStatement(EmptyStatement* stmt) {
 }
 
 
 void BytecodeGenerator::VisitIfStatement(IfStatement* stmt) {
   // TODO(oth): Spot easy cases where there code would not need to
   // emit the then block or the else block, e.g. condition is
   // obviously true/1/false/0.
 

[mythria, 2015/10/29 17:23:23]

Are there any other cases other than the condition being true/false? I will
update the comment based on your feedback.

[oth, 2015/10/30 09:46:18]

I believe these are the only instances of interest.

   BytecodeLabel else_label, end_label;
-
-  VisitForAccumulatorValue(stmt->condition());
-  builder()->CastAccumulatorToBoolean();
-  builder()->JumpIfFalse(&else_label);
-  Visit(stmt->then_statement());
-  if (stmt->HasElseStatement()) {
-    builder()->Jump(&end_label);
-    builder()->Bind(&else_label);
-    Visit(stmt->else_statement());
+  if (stmt->condition()->ToBooleanIsTrue()) {
+    // Generate only then block.
+    Visit(stmt->then_statement());
+  } else if (stmt->condition()->ToBooleanIsFalse()) {
+    // Generate only else block if it exists.
+    if (stmt->HasElseStatement()) {
+      Visit(stmt->else_statement());
+    }
   } else {
-    builder()->Bind(&else_label);
+    VisitForAccumulatorValue(stmt->condition());
+    builder()->CastAccumulatorToBoolean();
+    builder()->JumpIfFalse(&else_label);
+    Visit(stmt->then_statement());
+    if (stmt->HasElseStatement()) {
+      builder()->Jump(&end_label);
+      builder()->Bind(&else_label);
+      Visit(stmt->else_statement());
+    } else {
+      builder()->Bind(&else_label);
+    }
+    builder()->Bind(&end_label);
   }
-  builder()->Bind(&end_label);
 }
 
 
 void BytecodeGenerator::VisitSloppyBlockFunctionStatement(
     SloppyBlockFunctionStatement* stmt) {
   Visit(stmt->statement());
 }
 
 
 void BytecodeGenerator::VisitContinueStatement(ContinueStatement* stmt) {
@@ skipping 22 matching lines @@
   UNIMPLEMENTED();
 }
 
 
 void BytecodeGenerator::VisitCaseClause(CaseClause* clause) { UNIMPLEMENTED(); }
 
 
 void BytecodeGenerator::VisitDoWhileStatement(DoWhileStatement* stmt) {
   LoopBuilder loop_builder(builder());
   ControlScopeForIteration execution_control(this, stmt, &loop_builder);
+  BytecodeLabel body_label, condition_label, done_label;
 
-  BytecodeLabel body_label, condition_label, done_label;
-  builder()->Bind(&body_label);
-  Visit(stmt->body());
-  builder()->Bind(&condition_label);
-  VisitForAccumulatorValue(stmt->cond());
-  builder()->JumpIfTrue(&body_label);
-  builder()->Bind(&done_label);
-
+  if (stmt->cond()->ToBooleanIsFalse()) {
+    Visit(stmt->body());
+    // Bind condition_label and done_label for
+    // processing continue and break.
+    builder()->Bind(&condition_label);
+    builder()->Bind(&done_label);
+  } else {
+    builder()->Bind(&body_label);
+    Visit(stmt->body());
+    builder()->Bind(&condition_label);
+    if (stmt->cond()->ToBooleanIsTrue()) {
+      builder()->Jump(&body_label);
+    } else {
+      VisitForAccumulatorValue(stmt->cond());
+      builder()->JumpIfTrue(&body_label);
+    }
+    builder()->Bind(&done_label);
+  }
   loop_builder.SetBreakTarget(done_label);
   loop_builder.SetContinueTarget(condition_label);
 }
 
 
 void BytecodeGenerator::VisitWhileStatement(WhileStatement* stmt) {
   LoopBuilder loop_builder(builder());
   ControlScopeForIteration execution_control(this, stmt, &loop_builder);
 
   BytecodeLabel body_label, condition_label, done_label;
-  builder()->Jump(&condition_label);
+  if (stmt->cond()->ToBooleanIsFalse()) {
+    // If the condition is false no need of generating the loop.
+    return;

[mythria, 2015/10/29 17:23:23]

Here, I am not sure if we have to call builder().EnterBlock() and
builder.LeaveBlock(). Since, we are not generating any code, I don't think it is
necessary. We maintain the same state as if we never saw a while statement. 

[oth, 2015/10/30 09:46:19]

Definitely not necessary here.

+  }
+
+  if (!stmt->cond()->ToBooleanIsTrue()) {
+    builder()->Jump(&condition_label);
+  }
   builder()->Bind(&body_label);
   Visit(stmt->body());
   builder()->Bind(&condition_label);
-  VisitForAccumulatorValue(stmt->cond());
-  builder()->JumpIfTrue(&body_label);
+  if (stmt->cond()->ToBooleanIsTrue()) {
+    builder()->Jump(&body_label);
+  } else {
+    VisitForAccumulatorValue(stmt->cond());
+    builder()->JumpIfTrue(&body_label);
+  }
   builder()->Bind(&done_label);
 
   loop_builder.SetBreakTarget(done_label);
   loop_builder.SetContinueTarget(condition_label);
 }
 
 
 void BytecodeGenerator::VisitForStatement(ForStatement* stmt) {
   LoopBuilder loop_builder(builder());
   ControlScopeForIteration execution_control(this, stmt, &loop_builder);
 
   if (stmt->init() != nullptr) {
     Visit(stmt->init());
   }
 
+  if (stmt->cond() && stmt->cond()->ToBooleanIsFalse()) {
+    // If the condition is known to be false, no need of generating

[oth, 2015/10/30 09:46:19]

s/no need of generating/there is no need to generate the/

[mythria, 2015/10/30 15:07:47]

Done.

+    // body, next or condition blocks. Init block should be generated.
+    return;
+  }
+
   BytecodeLabel body_label, condition_label, next_label, done_label;
-  if (stmt->cond() != nullptr) {
+  if (stmt->cond() && !stmt->cond()->ToBooleanIsTrue()) {

[oth, 2015/10/30 09:46:19]

For readability I'd suggest using stmt->cond()->ToBooleanIsFalse() rather than
!stmt->cond()->ToBooleanIsTrue().

[mythria, 2015/10/30 15:07:47]

!stmt->cond()->ToBooleanIsTrue could also mean that the condition is not known
at compile time. I could not think of anyway to avoid
!stmt->cond()->ToBooleanIsTrue() here. Please let me know if you have any
suggestions.

[oth, 2015/10/30 15:31:34]

Sorry, yes, reader error.

     builder()->Jump(&condition_label);
   }
   builder()->Bind(&body_label);
   Visit(stmt->body());
   builder()->Bind(&next_label);
   if (stmt->next() != nullptr) {
     Visit(stmt->next());
   }
-  if (stmt->cond()) {
+  if (stmt->cond() && !stmt->cond()->ToBooleanIsTrue()) {

[oth, 2015/10/30 09:46:18]

Ditto.

[mythria, 2015/10/30 15:07:47]

Here, I changed the condition to avoid using !stmt->cond()->ToBooleanIsTrue().

     builder()->Bind(&condition_label);
     VisitForAccumulatorValue(stmt->cond());
     builder()->JumpIfTrue(&body_label);
   } else {
     builder()->Jump(&body_label);
   }
   builder()->Bind(&done_label);
 
   loop_builder.SetBreakTarget(done_label);
   loop_builder.SetContinueTarget(next_label);
@@ skipping 1334 matching lines @@
 }
 
 
 int BytecodeGenerator::feedback_index(FeedbackVectorSlot slot) const {
   return info()->feedback_vector()->GetIndex(slot);
 }
 
 }  // namespace interpreter
 }  // namespace internal
 }  // namespace v8