Eliminate redundant smi checks

src/x64/lithium-codegen-x64.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 2371 matching lines @@
   // Load the result.
   __ movq(result,
           BuildFastArrayOperand(instr->elements(),
                                 instr->key(),
                                 FAST_ELEMENTS,
                                 FixedArray::kHeaderSize - kHeapObjectTag,
                                 instr->additional_index()));
 
   // Check for the hole value.
   if (instr->hydrogen()->RequiresHoleCheck()) {
-    __ CompareRoot(result, Heap::kTheHoleValueRootIndex);
-    DeoptimizeIf(equal, instr->environment());
+    if (IsFastSmiElementsKind(instr->hydrogen()->elements_kind())) {
+      Condition smi = __ CheckSmi(result);
+      DeoptimizeIf(NegateCondition(smi), instr->environment());
+    } else {
+      __ CompareRoot(result, Heap::kTheHoleValueRootIndex);
+      DeoptimizeIf(equal, instr->environment());
+    }
   }
 }
 
 
 void LCodeGen::DoLoadKeyedFastDoubleElement(
     LLoadKeyedFastDoubleElement* instr) {
   XMMRegister result(ToDoubleRegister(instr->result()));
 
   if (instr->hydrogen()->IsDehoisted() && !instr->key()->IsConstantOperand()) {
     // Sign extend key because it could be a 32 bit negative value
@@ skipping 1379 matching lines @@
   __ Integer32ToSmi(input, input);
 }
 
 
 void LCodeGen::DoSmiUntag(LSmiUntag* instr) {
   ASSERT(instr->InputAt(0)->Equals(instr->result()));
   Register input = ToRegister(instr->InputAt(0));
   if (instr->needs_check()) {
     Condition is_smi = __ CheckSmi(input);
     DeoptimizeIf(NegateCondition(is_smi), instr->environment());
+  } else {
+#if DEBUG
+    Label ok;
+    Condition is_smi = __ CheckSmi(input);
+    __ j(is_smi, &ok, Label::kNear);
+    __ int3();
+    __ bind(&ok);
+#endif
   }
   __ SmiToInteger32(input, input);
 }
 
 
 void LCodeGen::EmitNumberUntagD(Register input_reg,
                                 XMMRegister result_reg,
                                 bool deoptimize_on_undefined,
                                 bool deoptimize_on_minus_zero,
                                 LEnvironment* env) {
@@ skipping 1093 matching lines @@
                                FixedArray::kHeaderSize - kPointerSize));
   __ bind(&done);
 }
 
 
 #undef __
 
 } }  // namespace v8::internal
 
 #endif  // V8_TARGET_ARCH_X64