[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 148 matching lines @@
   UpdateCurrentBytecode(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());
+      if (current->source_info().is_valid() &&
+          current->source_info() != last_.source_info()) {

[rmcilroy, 2016/05/20 11:08:03]

I think this would be clearer as:

// The last bytecode can't be elided without dropping a source
// position.
if (!current->source_info().is_valid() || current->source_info() ==
last_.source_info()) {
  // Transfer source position from bytecode that's being dropped
  // to its successor.
  current->source_info().Update(last_.source_info());
  InvalidateLast();
}

[oth, 2016/05/20 14:56:43]

Revised with a simpler path that produces the same results. The suggestion is
subtly different from what it'd replace.

+        // The last bytecode can't be elided without dropping a source
+        // position.
+        return current;
+      } else {
+        // Transfer source position from bytecode that's being dropped
+        // to it's successor.
+        current->source_info().Update(last_.source_info());
+      }
     }
     InvalidateLast();
   }
   return current;
 }
 
 }  // namespace interpreter
 }  // namespace internal
 }  // namespace v8