[Interpreter] Preserve source positions in peephole optimizer.

src/interpreter/bytecode-peephole-optimizer.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-peephole-optimizer.h"
 
 #include "src/interpreter/constant-array-builder.h"
 #include "src/objects-inl.h"
 #include "src/objects.h"
 
@@ skipping 76 matching lines @@
 }
 
 bool BytecodePeepholeOptimizer::LastBytecodePutsNameInAccumulator() const {
   DCHECK(LastIsValid());
   return (last_.bytecode() == Bytecode::kTypeOf ||
           last_.bytecode() == Bytecode::kToName ||
           (last_.bytecode() == Bytecode::kLdaConstant &&
            GetConstantForIndexOperand(&last_, 0)->IsName()));
 }
 
-void BytecodePeepholeOptimizer::UpdateCurrentBytecode(BytecodeNode* current) {
+void BytecodePeepholeOptimizer::UpdateLastAndCurrentBytecodes(
+    BytecodeNode* current) {
   if (Bytecodes::IsJumpIfToBoolean(current->bytecode()) &&
       Bytecodes::WritesBooleanToAccumulator(last_.bytecode())) {
     // Conditional jumps with boolean conditions are emitted in
     // ToBoolean form by the bytecode array builder,
     // i.e. JumpIfToBooleanTrue rather JumpIfTrue. The ToBoolean element
     // can be removed if the previous bytecode put a boolean value in
     // the accumulator.
     Bytecode jump = Bytecodes::GetJumpWithoutToBoolean(current->bytecode());
     current->set_bytecode(jump, current->operand(0), current->operand_scale());
   } else if (current->bytecode() == Bytecode::kToBooleanLogicalNot &&
              Bytecodes::WritesBooleanToAccumulator(last_.bytecode())) {
     // Logical-nots are emitted in ToBoolean form by the bytecode array
     // builder, The ToBoolean element can be removed if the previous bytecode
     // put a boolean value in the accumulator.
     current->set_bytecode(Bytecode::kLogicalNot);
   }
+
+  if (current->source_info().is_statement() &&
+      last_.source_info().is_expression() &&
+      Bytecodes::IsWithoutExternalSideEffects(last_.bytecode())) {
+    // The last bytecode has been marked as expression. It has no
+    // external effects so can't throw and the current bytecode is a
+    // source position. Remove the expression position on the last
+    // bytecode to open up potential peephole optimizations and to
+    // save the memory and perf cost of storing the unneeded
+    // expression position.
+    last_.source_info().set_invalid();
+  }
 }
 
 bool BytecodePeepholeOptimizer::CanElideCurrent(
     const BytecodeNode* const current) const {
   if (Bytecodes::IsLdarOrStar(last_.bytecode()) &&
       Bytecodes::IsLdarOrStar(current->bytecode()) &&
       current->operand(0) == last_.operand(0)) {
     // Ldar and Star make the accumulator and register hold equivalent
     // values. Only the first bytecode is needed if there's a sequence
     // of back-to-back Ldar and Star bytecodes with the same operand.
     return true;
   } else if (current->bytecode() == Bytecode::kToName &&
              LastBytecodePutsNameInAccumulator()) {
     // If the previous bytecode ensured a name was in the accumulator,
     // the type coercion ToName() can be elided.
     return true;
   } else {
     // Additional candidates for eliding current:
     // (i) ToNumber if the last puts a number in the accumulator.
     return false;
   }
 }
 
+bool BytecodePeepholeOptimizer::CanElideLastBasedOnSourcePosition(
+    const BytecodeNode* const current) const {
+  //
+  // The rules for allowing the elision of the last bytecode based
+  // on source position are:
+  //
+  //                     C U R R E N T
+  //              +--------+--------+--------+
+  //              |  None  |  Expr  |  Stmt  |
+  //  L  +--------+--------+--------+--------+
+  //     |  None  |  YES   |  YES   |  YES   |
+  //  A  +--------+--------+--------+--------+
+  //     |  Expr  |  YES   | MAYBE  |  MAYBE |
+  //  S  +--------+--------+--------+--------+
+  //     |  Stmt  |  YES   |   NO   |   NO   |
+  //  T  +--------+--------+--------+--------+
+  //
+  // The goal is not lose any statement positions and not lose useful
+  // expression positions. Whenever the last bytecode is elided it's
+  // source position information is applied to the current node
+  // updating it if necessary.
+  //
+  // The last bytecode can be elided for the MAYBE cases if the last
+  // bytecode is known not to throw. If it throws, the system would
+  // not have correct stack trace information. The appropriate check
+  // for this would be Bytecodes::IsWithoutExternalSideEffects(), but
+  // this is not performed here to keep the check simple.

[rmcilroy, 2016/05/23 10:59:26]

nit - /s/but this is not.../which is checked in UpdateLastAndCurrentBytecodes to
keep the check here simple.

[oth, 2016/05/23 11:22:42]

Done.

+  //
+  return (!last_.source_info().is_valid() ||
+          !current->source_info().is_valid());
+}
+
 bool BytecodePeepholeOptimizer::CanElideLast(
     const BytecodeNode* const current) const {
   if (!last_is_discardable_) {
     return false;
   }
 
   if (last_.bytecode() == Bytecode::kNop) {
-    // Nop are placeholders for holding source position information
-    // and can be elided.
+    // Nop are placeholders for holding source position information.
     return true;
   } else if (Bytecodes::IsAccumulatorLoadWithoutEffects(current->bytecode()) &&
              Bytecodes::IsAccumulatorLoadWithoutEffects(last_.bytecode())) {
     // The accumulator is invisible to the debugger. If there is a sequence of
     // consecutive accumulator loads (that don't have side effects) then only
     // the final load is potentially visible.
     return true;
   } else {
     return false;
   }
 }
 
 BytecodeNode* BytecodePeepholeOptimizer::Optimize(BytecodeNode* current) {
-  UpdateCurrentBytecode(current);
-
+  UpdateLastAndCurrentBytecodes(current);
   if (CanElideCurrent(current)) {
     if (current->source_info().is_valid()) {
       current->set_bytecode(Bytecode::kNop);
     } else {
       current = nullptr;
     }
-  } else if (CanElideLast(current)) {
-    if (last_.source_info().is_valid()) {
-      current->source_info().Update(last_.source_info());
-    }
+  } else if (CanElideLast(current) &&
+             CanElideLastBasedOnSourcePosition(current)) {
+    current->source_info().Update(last_.source_info());
     InvalidateLast();
   }
   return current;
 }
 
 }  // namespace interpreter
 }  // namespace internal
 }  // namespace v8