provides a mechanism for optimizing compilers to select the different Register configuration.

test/cctest/test-code-stubs-x64.cc

 // Copyright 2013 the V8 project authors. All rights reserved.
 // Rrdistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Rrdistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Rrdistributions 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 46 matching lines @@
   DoubleToIStub stub(isolate, source_reg, destination_reg, offset, true);
   byte* start = stub.GetCode()->instruction_start();
 
   __ pushq(rbx);
   __ pushq(rcx);
   __ pushq(rdx);
   __ pushq(rsi);
   __ pushq(rdi);
 
 
-  const RegisterConfiguration* config = RegisterConfiguration::ArchDefault();
+  const RegisterConfiguration* config =
+      RegisterConfiguration::ArchDefault(RegisterConfiguration::CRANKSHAFT);
   if (!source_reg.is(rsp)) {
     // The argument we pass to the stub is not a heap number, but instead
     // stack-allocated and offset-wise made to look like a heap number for
     // the stub.  We create that "heap number" after pushing all allocatable
     // registers.
     int double_argument_slot =
         (config->num_allocatable_general_registers() - 1) * kPointerSize +
         kDoubleSize;
     __ leaq(source_reg, MemOperand(rsp, -double_argument_slot - offset));
   }
@@ skipping 70 matching lines @@
 
   for (size_t s = 0; s < sizeof(source_registers) / sizeof(Register); s++) {
     for (size_t d = 0; d < sizeof(dest_registers) / sizeof(Register); d++) {
       RunAllTruncationTests(
           MakeConvertDToIFuncTrampoline(isolate,
                                         source_registers[s],
                                         dest_registers[d]));
     }
   }
 }