tweak of Math.abs in its x64 stub

src/x64/stub-cache-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 2228 matching lines @@
                                 &miss);
     GenerateLoadFunctionFromCell(cell, function, &miss);
   }
   // Load the (only) argument into rax.
   __ movq(rax, Operand(rsp, 1 * kPointerSize));
 
   // Check if the argument is a smi.
   Label not_smi;
   STATIC_ASSERT(kSmiTag == 0);
   __ JumpIfNotSmi(rax, &not_smi);
-  __ SmiToInteger32(rax, rax);
 
+  // Branchless abs implementation, refer to below:
+  // http://graphics.stanford.edu/~seander/bithacks.html#IntegerAbs
   // Set ebx to 1...1 (== -1) if the argument is negative, or to 0...0
   // otherwise.
-  __ movl(rbx, rax);
-  __ sarl(rbx, Immediate(kBitsPerInt - 1));
+  __ movq(rbx, rax);
+  __ sar(rbx, Immediate(kBitsPerPointer - 1));
 
   // Do bitwise not or do nothing depending on ebx.
-  __ xorl(rax, rbx);
+  __ xor_(rax, rbx);
 
   // Add 1 or do nothing depending on ebx.
-  __ subl(rax, rbx);
+  __ subq(rax, rbx);
 
   // If the result is still negative, go to the slow case.
   // This only happens for the most negative smi.
   Label slow;
   __ j(negative, &slow);
 
-  // Smi case done.
-  __ Integer32ToSmi(rax, rax);
   __ ret(2 * kPointerSize);
 
   // Check if the argument is a heap number and load its value.
   __ bind(&not_smi);
   __ CheckMap(rax, factory()->heap_number_map(), &slow, DONT_DO_SMI_CHECK);
   __ movq(rbx, FieldOperand(rax, HeapNumber::kValueOffset));
 
   // Check the sign of the argument. If the argument is positive,
   // just return it.
   Label negative_sign;
@@ skipping 1235 matching lines @@
     TailCallBuiltin(masm, Builtins::kKeyedStoreIC_Slow);
   }
 }
 
 
 #undef __
 
 } }  // namespace v8::internal
 
 #endif  // V8_TARGET_ARCH_X64