[compiler] Drop the CompareNilIC.

src/full-codegen/x64/full-codegen-x64.cc

 // Copyright 2012 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.
 
 #if V8_TARGET_ARCH_X64
 
 #include "src/ast/scopes.h"
 #include "src/code-factory.h"
 #include "src/code-stubs.h"
 #include "src/codegen.h"
@@ skipping 3901 matching lines @@
 
   VisitForAccumulatorValue(sub_expr);
   PrepareForBailoutBeforeSplit(expr, true, if_true, if_false);
   if (expr->op() == Token::EQ_STRICT) {
     Heap::RootListIndex nil_value = nil == kNullValue ?
         Heap::kNullValueRootIndex :
         Heap::kUndefinedValueRootIndex;
     __ CompareRoot(rax, nil_value);
     Split(equal, if_true, if_false, fall_through);
   } else {
-    Handle<Code> ic = CompareNilICStub::GetUninitialized(isolate(), nil);
-    CallIC(ic, expr->CompareOperationFeedbackId());
-    __ CompareRoot(rax, Heap::kTrueValueRootIndex);
-    Split(equal, if_true, if_false, fall_through);
+    __ JumpIfSmi(rax, if_false);
+    __ movp(rax, FieldOperand(rax, HeapObject::kMapOffset));
+    __ testb(FieldOperand(rax, Map::kBitFieldOffset),
+             Immediate(1 << Map::kIsUndetectable));

[mvstanton, 2016/02/25 23:08:57]

Adam and I were speculating that this allows a Map (internal object) to fall out
into a later ToBooleanStub input through rax, per today's Canary. Could it be
so?

[adamk, 2016/02/25 23:38:38]

If there's something wrong with this patch, I don't think this is it. I can't
figure a way that rax leaks into anything else past this block (certainly
haven't been able to generate a test case where we do this compare and then try
to use the ToBoolean stub).

+    Split(not_zero, if_true, if_false, fall_through);
   }
   context()->Plug(if_true, if_false);
 }
 
 
 void FullCodeGenerator::VisitThisFunction(ThisFunction* expr) {
   __ movp(rax, Operand(rbp, JavaScriptFrameConstants::kFunctionOffset));
   context()->Plug(rax);
 }
 
@@ skipping 183 matching lines @@
   DCHECK_EQ(isolate->builtins()->OsrAfterStackCheck()->entry(),
             Assembler::target_address_at(call_target_address,
                                          unoptimized_code));
   return OSR_AFTER_STACK_CHECK;
 }
 
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_TARGET_ARCH_X64