Use fucomi instruction to compare floats and put results directly into EFLAGS...

src/x64/codegen-x64.cc

 // Copyright 2009 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 6324 matching lines @@
   __ movl(rdx, FieldOperand(rax, String::kLengthOffset));
   __ shr(rdx, Immediate(String::kShortLengthShift));
   __ j(zero, &false_result);
   __ jmp(&true_result);
 
   __ bind(&not_string);
   // HeapNumber => false iff +0, -0, or NaN.
   // These three cases set C3 when compared to zero in the FPU.
   __ CompareRoot(rdx, Heap::kHeapNumberMapRootIndex);
   __ j(not_equal, &true_result);
-  // TODO(x64): Don't use fp stack, use MMX registers?
   __ fldz();  // Load zero onto fp stack
   // Load heap-number double value onto fp stack
   __ fld_d(FieldOperand(rax, HeapNumber::kValueOffset));
-  __ fucompp();  // Compare and pop both values.
-  __ movq(kScratchRegister, rax);
-  __ fnstsw_ax();  // Store fp status word in ax, no checking for exceptions.
-  __ testl(rax, Immediate(0x4000));  // Test FP condition flag C3, bit 16.
-  __ movq(rax, kScratchRegister);
-  __ j(not_zero, &false_result);
+  __ FCmp();
+  __ j(zero, &false_result);
   // Fall through to |true_result|.
 
   // Return 1/0 for true/false in rax.
   __ bind(&true_result);
   __ movq(rax, Immediate(1));
   __ ret(1 * kPointerSize);
   __ bind(&false_result);
   __ xor_(rax, rax);
   __ ret(1 * kPointerSize);
 }
@@ skipping 1377 matching lines @@
         CpuFeatures::Scope scope(CpuFeatures::SSE3);
         __ fisttp_s(Operand(rsp, 0 * kPointerSize));
         __ fisttp_s(Operand(rsp, 1 * kPointerSize));
         __ fnstsw_ax();
         __ testl(rax, Immediate(1));
         __ j(not_zero, &operand_conversion_failure);
       } else {
         // Check if right operand is int32.
         __ fist_s(Operand(rsp, 0 * kPointerSize));
         __ fild_s(Operand(rsp, 0 * kPointerSize));
-        __ fucompp();
-        __ fnstsw_ax();
-        if (CpuFeatures::IsSupported(CpuFeatures::SAHF)) {
-          __ sahf();
-          __ j(not_zero, &operand_conversion_failure);
-          __ j(parity_even, &operand_conversion_failure);
-        } else {
-          __ and_(rax, Immediate(0x4400));
-          __ cmpl(rax, Immediate(0x4000));
-          __ j(not_zero, &operand_conversion_failure);
-        }
+        __ FCmp();
+        __ j(not_zero, &operand_conversion_failure);
+        __ j(parity_even, &operand_conversion_failure);
+
         // Check if left operand is int32.
         __ fist_s(Operand(rsp, 1 * kPointerSize));
         __ fild_s(Operand(rsp, 1 * kPointerSize));
-        __ fucompp();
-        __ fnstsw_ax();
-        if (CpuFeatures::IsSupported(CpuFeatures::SAHF)) {
-          __ sahf();
-          __ j(not_zero, &operand_conversion_failure);
-          __ j(parity_even, &operand_conversion_failure);
-        } else {
-          __ and_(rax, Immediate(0x4400));
-          __ cmpl(rax, Immediate(0x4000));
-          __ j(not_zero, &operand_conversion_failure);
-        }
+        __ FCmp();
+        __ j(not_zero, &operand_conversion_failure);
+        __ j(parity_even, &operand_conversion_failure);
       }
 
       // Get int32 operands and perform bitop.
       __ pop(rcx);
       __ pop(rax);
       switch (op_) {
         case Token::BIT_OR:  __ or_(rax, rcx); break;
         case Token::BIT_AND: __ and_(rax, rcx); break;
         case Token::BIT_XOR: __ xor_(rax, rcx); break;
         case Token::SAR: __ sarl(rax); break;
@@ skipping 117 matching lines @@
 int CompareStub::MinorKey() {
   // Encode the two parameters in a unique 16 bit value.
   ASSERT(static_cast<unsigned>(cc_) < (1 << 15));
   return (static_cast<unsigned>(cc_) << 1) | (strict_ ? 1 : 0);
 }
 
 
 #undef __
 
 } }  // namespace v8::internal