Add faster, but unsafe version of LoadInternalField.

src/fast-accessor-assembler.cc

Index: src/fast-accessor-assembler.cc
diff --git a/src/fast-accessor-assembler.cc b/src/fast-accessor-assembler.cc
index a261eb01a63a5edfbd3a311b963cac30b1bc464d..82b4918b438023d21a8180804849400c2b778c3b 100644
--- a/src/fast-accessor-assembler.cc
+++ b/src/fast-accessor-assembler.cc
@@ -91,6 +91,15 @@ FastAccessorAssembler::ValueId FastAccessorAssembler::LoadInternalField(
   return FromRaw(result.value());
 }
 
+FastAccessorAssembler::ValueId FastAccessorAssembler::LoadInternalFieldUnsafe(

[vogelheim, 2016/07/27 09:50:56]

I briefly discussed handling the LoadInternalField[Unsafe] variants. How about
we change it like this:

1, we'll have both versions, LoadInternalFieldChecked and
LoadInternalFieldUnsafe.

2, There's a FLAG_debug_code, that we use elsewhere to instruct the compiler to
emit additional runtime checks (and crash if they fail). If this is set, we'll
still check the assumptions in the *Unsafe case. That way, users of *Unsafe
still have some reasonable means to guard themselves against errors.

(FLAG_debug_code is hugely helpful for testing, since it allows us to run our
entire test suite (+ all the fuzzers) and check some of the implicit
assumptions.)

So: LoadInternalFieldChecked does all the checks, and returns Undefined, as it
currently does. LoadInternalFieldUnchecked does - if FLAG_debug_code is true -
the same checks, but CRASH()es when they fail. (This is the signal that fuzzers
+ test infrastructure need.) And if !FLAG_debug_code, it would not do any
checks.

You'll probably want to factor out the actual generating code into suitable
methods.

[Alfonso, 2016/07/28 14:39:13]

The C++ counterpart returns NULL (instead of undefined) if the argument is not a
JSObject.
Refactoring into smaller composable parts.

+    ValueId value, int field_no) {
+  CHECK_EQ(kBuilding, state_);
+  Node* result = assembler_->LoadObjectField(
+      FromId(value), JSObject::kHeaderSize + kPointerSize * field_no,
+      MachineType::Pointer());
+  return FromRaw(result);
+}
+
 FastAccessorAssembler::ValueId FastAccessorAssembler::LoadValue(ValueId value,
                                                                 int offset) {
   CHECK_EQ(kBuilding, state_);