[ignition] desugar GetIterator() via bytecode rather than via AST

test/cctest/interpreter/bytecode_expectations/ForOf.golden

Index: test/cctest/interpreter/bytecode_expectations/ForOf.golden
diff --git a/test/cctest/interpreter/bytecode_expectations/ForOf.golden b/test/cctest/interpreter/bytecode_expectations/ForOf.golden
index 21abc69caed20ccf596e1bb78cedfac68ea97125..22a9703b85d3dfa45f246a1f88eb524fc4566268 100644
--- a/test/cctest/interpreter/bytecode_expectations/ForOf.golden
+++ b/test/cctest/interpreter/bytecode_expectations/ForOf.golden
@@ -11,7 +11,7 @@ snippet: "
 "
 frame size: 15
 parameter count: 1
-bytecode array length: 268
+bytecode array length: 274
 bytecodes: [
   /*   30 E> */ B(StackCheck),
                 B(LdaZero),
@@ -19,11 +19,12 @@ bytecodes: [
                 B(Mov), R(context), R(11),
                 B(Mov), R(context), R(12),
   /*   48 S> */ B(CreateArrayLiteral), U8(0), U8(0), U8(9),
-                B(Star), R(14),
-                B(LdaConstant), U8(1),
-  /*   48 E> */ B(LdaKeyedProperty), R(14), U8(4),
                 B(Star), R(13),
-  /*   48 E> */ B(CallProperty), R(13), R(14), U8(1), U8(2),
+                B(LdaNamedProperty), R(13), U8(1), U8(2),
+                B(Star), R(14),
+                B(CallProperty), R(14), R(13), U8(1), U8(4),
+                B(JumpIfJSReceiver), U8(7),
+                B(CallRuntime), U16(Runtime::kThrowSymbolIteratorInvalid), R(0), U8(0),

[caitp, 2016/12/06 05:49:11]

well, the other downside is how much bigger the bytecode is because of the
JumpIfJSReceiver + runtime call (implementing step 3 of
https://tc39.github.io/ecma262/#sec-getiterator) --- but the iterator.next()
call may make this somewhat redundant. Might actually shrink the bytecode a bit
if that's gotten rid of. In upstream v8, this exception is only thrown before
the call to .next() for `yield*`, and it's possible to get the same semantics
here.

[rmcilroy, 2016/12/06 15:02:28]

I'm slightly confused by this - it looks like the parser previously should have
added nodes for doing this check / throw, but I don't see the bytecode added in
the old expectations. Am I missing something? Shouldn't this be strictly less
bytecode than before since we now have the special bytecode rather than having
to do the runtime call?

[caitp, 2016/12/06 15:14:58]

These nodes were only added for yield* desugaring, and not for for-of loops, so
you see them in this case:

```
function* g() { yield* { [Symbol.iterator]() { return 1; } } }
g().next(); // Throws "TypeError: Result of the Symbol.iterator method is not an
object"
```

They were never added for the for-of loop, and we could probably get away with
never adding it, since .next() is always called immediately after GetIterator()
in the current spec. It would be nice to have it, but since it grows the
bytecode so much, it may not be worth it.

So, there are a couple of options:

1) always get rid of the receiver check/runtime call, and rely on the TypeError
thrown by Call. This should shrink the generated bytecode a bit
2) get rid of the receiver check for for-of loops only (this would match the
current behaviour in trunk)
3) always have the check, and be spec compliant at the cost of some bigger
bytecode
4) add a new opcode for throwing if this check fails, and do the runtime call
directly from interpreter.cc and reduce it appropriately in TF

just a question of how to balance strict compliance with the spec and the size
of bytecode

[caitp, 2016/12/06 16:07:07]

Also note, not including the typecheck is an observable spec violation (which is
incompatible with SpiderMonkey), but this is admittedly unlikely to matter in
real code. (https://jsfiddle.net/gj3uaw1t/)

                 B(Star), R(2),
   /*   45 S> */ B(LdaNamedProperty), R(2), U8(2), U8(8),
                 B(Star), R(14),
@@ -142,9 +143,9 @@ constant pool: [
   FIXED_ARRAY_TYPE,
 ]
 handlers: [
-  [7, 120, 126],
-  [10, 84, 86],
-  [195, 205, 207],
+  [7, 126, 132],
+  [10, 90, 92],
+  [201, 211, 213],
 ]
 
 ---
@@ -154,7 +155,7 @@ snippet: "
 "
 frame size: 16
 parameter count: 1
-bytecode array length: 279
+bytecode array length: 288
 bytecodes: [
   /*   30 E> */ B(StackCheck),
   /*   42 S> */ B(LdaConstant), U8(0),
@@ -163,10 +164,12 @@ bytecodes: [
                 B(Star), R(5),
                 B(Mov), R(context), R(12),
                 B(Mov), R(context), R(13),
-  /*   68 S> */ B(LdaConstant), U8(1),
-  /*   68 E> */ B(LdaKeyedProperty), R(0), U8(4),
-                B(Star), R(14),
-  /*   68 E> */ B(CallProperty), R(14), R(0), U8(1), U8(2),
+  /*   68 S> */ B(LdaNamedProperty), R(0), U8(1), U8(2),
+                B(Star), R(15),
+                B(CallProperty), R(15), R(0), U8(1), U8(4),
+                B(Mov), R(0), R(14),
+                B(JumpIfJSReceiver), U8(7),
+                B(CallRuntime), U16(Runtime::kThrowSymbolIteratorInvalid), R(0), U8(0),
                 B(Star), R(3),
   /*   65 S> */ B(LdaNamedProperty), R(3), U8(2), U8(8),
                 B(Star), R(15),
@@ -291,9 +294,9 @@ constant pool: [
   FIXED_ARRAY_TYPE,
 ]
 handlers: [
-  [11, 120, 126],
-  [14, 84, 86],
-  [196, 206, 208],
+  [11, 129, 135],
+  [14, 93, 95],
+  [205, 215, 217],
 ]
 
 ---
@@ -305,7 +308,7 @@ snippet: "
 "
 frame size: 15
 parameter count: 1
-bytecode array length: 286
+bytecode array length: 292
 bytecodes: [
   /*   30 E> */ B(StackCheck),
                 B(LdaZero),
@@ -313,11 +316,12 @@ bytecodes: [
                 B(Mov), R(context), R(11),
                 B(Mov), R(context), R(12),
   /*   48 S> */ B(CreateArrayLiteral), U8(0), U8(0), U8(9),
-                B(Star), R(14),
-                B(LdaConstant), U8(1),
-  /*   48 E> */ B(LdaKeyedProperty), R(14), U8(4),
                 B(Star), R(13),
-  /*   48 E> */ B(CallProperty), R(13), R(14), U8(1), U8(2),
+                B(LdaNamedProperty), R(13), U8(1), U8(2),
+                B(Star), R(14),
+                B(CallProperty), R(14), R(13), U8(1), U8(4),
+                B(JumpIfJSReceiver), U8(7),
+                B(CallRuntime), U16(Runtime::kThrowSymbolIteratorInvalid), R(0), U8(0),
                 B(Star), R(2),
   /*   45 S> */ B(LdaNamedProperty), R(2), U8(2), U8(8),
                 B(Star), R(14),
@@ -444,9 +448,9 @@ constant pool: [
   FIXED_ARRAY_TYPE,
 ]
 handlers: [
-  [7, 138, 144],
-  [10, 102, 104],
-  [213, 223, 225],
+  [7, 144, 150],
+  [10, 108, 110],
+  [219, 229, 231],
 ]
 
 ---
@@ -456,7 +460,7 @@ snippet: "
 "
 frame size: 14
 parameter count: 1
-bytecode array length: 293
+bytecode array length: 299
 bytecodes: [
   /*   30 E> */ B(StackCheck),
   /*   42 S> */ B(CreateObjectLiteral), U8(0), U8(0), U8(1), R(8),
@@ -466,11 +470,12 @@ bytecodes: [
                 B(Mov), R(context), R(10),
                 B(Mov), R(context), R(11),
   /*   77 S> */ B(CreateArrayLiteral), U8(1), U8(1), U8(9),
-                B(Star), R(13),
-                B(LdaConstant), U8(2),
-  /*   77 E> */ B(LdaKeyedProperty), R(13), U8(4),
                 B(Star), R(12),
-  /*   77 E> */ B(CallProperty), R(12), R(13), U8(1), U8(2),
+                B(LdaNamedProperty), R(12), U8(2), U8(2),
+                B(Star), R(13),
+                B(CallProperty), R(13), R(12), U8(1), U8(4),
+                B(JumpIfJSReceiver), U8(7),
+                B(CallRuntime), U16(Runtime::kThrowSymbolIteratorInvalid), R(0), U8(0),
                 B(Star), R(1),
   /*   74 S> */ B(LdaNamedProperty), R(1), U8(3), U8(8),
                 B(Star), R(13),
@@ -599,8 +604,8 @@ constant pool: [
   FIXED_ARRAY_TYPE,
 ]
 handlers: [
-  [15, 134, 140],
-  [18, 98, 100],
-  [210, 220, 222],
+  [15, 140, 146],
+  [18, 104, 106],
+  [216, 226, 228],
 ]