tweak of Math.abs in its x64 stub

src/ia32/stub-cache-ia32.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 2461 matching lines @@
   }
 
   // Load the (only) argument into eax.
   __ mov(eax, Operand(esp, 1 * kPointerSize));
 
   // Check if the argument is a smi.
   Label not_smi;
   STATIC_ASSERT(kSmiTag == 0);
   __ JumpIfNotSmi(eax, &not_smi);
 
+  // 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.
   __ mov(ebx, eax);
   __ sar(ebx, kBitsPerInt - 1);
 
   // Do bitwise not or do nothing depending on ebx.
   __ xor_(eax, ebx);
 
   // Add 1 or do nothing depending on ebx.
   __ sub(eax, ebx);
@@ skipping 1274 matching lines @@
     TailCallBuiltin(masm, Builtins::kKeyedStoreIC_Slow);
   }
 }
 
 
 #undef __
 
 } }  // namespace v8::internal
 
 #endif  // V8_TARGET_ARCH_IA32