[interpreter] source positions should not be emitted for dead code.

src/interpreter/source-position-table.cc

Index: src/interpreter/source-position-table.cc
diff --git a/src/interpreter/source-position-table.cc b/src/interpreter/source-position-table.cc
index f408f159db3bef2df859043eab3b41cd7c0da6b6..30507fdd90cbadcf4c5525089225f101e8dc0206 100644
--- a/src/interpreter/source-position-table.cc
+++ b/src/interpreter/source-position-table.cc
@@ -15,20 +15,30 @@ namespace interpreter {
 class IsStatementField : public BitField<bool, 0, 1> {};
 class SourcePositionField : public BitField<int, 1, 30> {};
 
-void SourcePositionTableBuilder::AddStatementPosition(int bytecode_offset,
+void SourcePositionTableBuilder::AddStatementPosition(size_t bytecode_offset,
                                                       int source_position) {
-  AssertMonotonic(bytecode_offset);
+  int offset = static_cast<int>(bytecode_offset);
+  AssertMonotonic(offset);
   uint32_t encoded = IsStatementField::encode(true) |
                      SourcePositionField::encode(source_position);
-  entries_.push_back({bytecode_offset, encoded});
+  entries_.push_back({offset, encoded});
 }
 
-void SourcePositionTableBuilder::AddExpressionPosition(int bytecode_offset,
+void SourcePositionTableBuilder::AddExpressionPosition(size_t bytecode_offset,
                                                        int source_position) {
-  AssertMonotonic(bytecode_offset);
+  int offset = static_cast<int>(bytecode_offset);
+  AssertMonotonic(offset);
   uint32_t encoded = IsStatementField::encode(false) |
                      SourcePositionField::encode(source_position);
-  entries_.push_back({bytecode_offset, encoded});
+  entries_.push_back({offset, encoded});
+}
+
+void SourcePositionTableBuilder::RevertPosition(size_t bytecode_offset) {
+  int offset = static_cast<int>(bytecode_offset);
+  // If we already added a source position table entry, but the bytecode array
+  // builder ended up not outputting a bytecode for the corresponding bytecode
+  // offset, we have to remove that entry.
+  if (entries_.back().bytecode_offset == offset) entries_.pop_back();

[rmcilroy, 2016/02/05 10:05:54]

Should this be a while loop (e.g., could we try to add multiple source positions
for a given bytecode offset before trying to output a bytecode)?

[Yang, 2016/02/05 12:21:31]

No. AssertMonotonic already makes sure that we do not set multiple source
positions for one byte code.

[rmcilroy, 2016/02/05 12:24:31]

Ahh, I was confused since Monotonic usually means a <= b, not a < b. I'm not
sure of a better name though.

 }
 
 Handle<FixedArray> SourcePositionTableBuilder::ToFixedArray() {