[Interpreter] Ensure that implicit return undefined is generated.

src/interpreter/bytecode-generator.cc

Index: src/interpreter/bytecode-generator.cc
diff --git a/src/interpreter/bytecode-generator.cc b/src/interpreter/bytecode-generator.cc
index 487b86543794b9effc862db130ea37a2e0e36ca7..0f24995c92a9b150b38f002a73ea5d70ff61a360 100644
--- a/src/interpreter/bytecode-generator.cc
+++ b/src/interpreter/bytecode-generator.cc
@@ -45,6 +45,13 @@ Handle<BytecodeArray> BytecodeGenerator::MakeBytecode(CompilationInfo* info) {
   // Visit statements in the function body.
   VisitStatements(info->literal()->body());
 
+  // If the last bytecode wasn't a return, then return 'undefined' to avoid
+  // falling off the end.
+  if (!builder_.HasExplicitReturn()) {

[Michael Starzinger, 2015/09/08 08:25:10]

This looks dangerous. Depending on how control flow is modeled there could very
well be a case where the last instruction in the byte-code stream is a return
statement, but it is not guaranteed post-dominate all executions paths. Consider
the following ...

function f(a) {
  if (a) { return 23; }
}

[rmcilroy, 2015/09/08 09:41:59]

Good point, we need to take this into account, but we would need to take this
into account once we start adding control flow in any case since the if block
will need to jump to an actual bytecode, and the bytecode array builder will
need to back-patch the jump instruction with the target of the jump.

The last thing the bytecode-generator would do in the graph above would be to
try and back-patch the if jump to target the next bytecode which is to be
emitted. Since that bytecode doesn't exist yet we could easily spot in the
bytecode-array-builder that there is a jump instruction which is patched to a
non-existent bytecode (e.g., just having a field tracking the greatest bytecode
indice which was patched into a jump instruction and check if that is
greater-than/equal than bytecodes_.size()), and add that to the
HasExplicitReturn conditions? WDYT? 

+    builder_.LoadUndefined();
+    builder_.Return();
+  }
+
   set_scope(nullptr);
   set_info(nullptr);
   return builder_.ToBytecodeArray();