Use zig-zag encoding in the source position table.

test/unittests/interpreter/source-position-table-unittest.cc

 // Copyright 2016 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/v8.h"
 
 #include "src/interpreter/source-position-table.h"
 #include "test/unittests/test-utils.h"
 
 namespace v8 {
@@ skipping 38 matching lines @@
   SourcePositionTableBuilder builder(isolate(), zone());
   for (int i = 0; i < arraysize(offsets); i++) {
     builder.AddPosition(offsets[i], offsets[i], false);
   }
   CHECK(!builder.ToSourcePositionTable().is_null());
 }
 
 TEST_F(SourcePositionTableTest, EncodeAscending) {
   SourcePositionTableBuilder builder(isolate(), zone());
 
-  int accumulator = 0;
+  int code_offset = 0;
+  int source_position = 0;
   for (int i = 0; i < arraysize(offsets); i++) {
-    accumulator += offsets[i];
+    code_offset += offsets[i];
+    source_position += offsets[i];
     if (i % 2) {
-      builder.AddPosition(accumulator, accumulator, true);
+      builder.AddPosition(code_offset, source_position, true);
     } else {
-      builder.AddPosition(accumulator, accumulator, false);
+      builder.AddPosition(code_offset, source_position, false);
     }
   }
 
-  // Also test negative offsets:
+  // Also test negative offsets for source positions:
   for (int i = 0; i < arraysize(offsets); i++) {
-    accumulator -= offsets[i];
+    code_offset += offsets[i];
+    source_position -= offsets[i];
     if (i % 2) {
-      builder.AddPosition(accumulator, accumulator, true);
+      builder.AddPosition(code_offset, source_position, true);
     } else {
-      builder.AddPosition(accumulator, accumulator, false);
+      builder.AddPosition(code_offset, source_position, false);
     }
   }
 
   CHECK(!builder.ToSourcePositionTable().is_null());
 }
 
 }  // namespace interpreter
 }  // namespace internal
 }  // namespace v8