[NOT FOR COMMIT] Native Client builds of V8 on ia32 and x64.

src/arm/simulator-arm.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 1629 matching lines @@
 // uses the ObjectPair which is essentially two 32-bit values stuffed into a
 // 64-bit value. With the code below we assume that all runtime calls return
 // 64 bits of result. If they don't, the r1 result register contains a bogus
 // value, which is fine because it is caller-saved.
 typedef int64_t (*SimulatorRuntimeCall)(int32_t arg0,
                                         int32_t arg1,
                                         int32_t arg2,
                                         int32_t arg3,
                                         int32_t arg4,
                                         int32_t arg5);
+#if defined(V8_ARM_ON_X86_64)
+// This prototype will handle calls with up to two double args
+// and up to four integer args in any order. Note that arguments
+// on x86_64 are passed in registers. With this prototype the
+// six cited argument registers are loaded. Note that the callee
+// will simply ignore any arguments it doesn't actually need.
+// This prototype will fail for structures passed by value.
+typedef double (*SimulatorRuntimeFPCall)(double darg0,
+                                         double darg1,
+                                         int32_t arg0,
+                                         int32_t arg1,
+                                         int32_t arg2,
+                                         int32_t arg3);
+#else
 typedef double (*SimulatorRuntimeFPCall)(int32_t arg0,
                                          int32_t arg1,
                                          int32_t arg2,
                                          int32_t arg3);
+#endif
 
 // This signature supports direct call in to API function native callback
 // (refer to InvocationCallback in v8.h).
 typedef v8::Handle<v8::Value> (*SimulatorRuntimeDirectApiCall)(int32_t arg0);
 
 // This signature supports direct call to accessor getter callback.
 typedef v8::Handle<v8::Value> (*SimulatorRuntimeDirectGetterCall)(int32_t arg0,
                                                                   int32_t arg1);
 
+
+#if defined(V8_ARM_ON_X86_64)
+static double ArgAsDouble(uint32_t lo, uint32_t hi) {
+  union { uint64_t i64; double d; } darg;
+  darg.i64 = hi;
+  darg.i64 = (darg.i64 << 32) | lo;
+  return darg.d;
+}
+#endif
+
 // Software interrupt instructions are used by the simulator to call into the
 // C-based V8 runtime.
 void Simulator::SoftwareInterrupt(Instruction* instr) {
   int svc = instr->SvcValue();
   switch (svc) {
     case kCallRtRedirected: {
       // Check if stack is aligned. Error if not aligned is reported below to
       // include information on the function called.
       bool stack_aligned =
           (get_register(sp)
@@ skipping 59 matching lines @@
             GetFpArgs(&dval0);
             PrintF("Call to host function at %p with arg %f",
                 FUNCTION_ADDR(target), dval0);
             break;
           case ExternalReference::BUILTIN_FP_INT_CALL:
             GetFpArgs(&dval0, &ival);
             PrintF("Call to host function at %p with args %f, %d",
                 FUNCTION_ADDR(target), dval0, ival);
             break;
           default:
             UNREACHABLE();

[ulan, 2013/04/04 16:38:17]

Yes, I think so because of this line.
BUILTIN_FP_INT_CALL from the others in simulator-arm.cc
If I understood the question correctly, you can check redirection->type() like
it is done in this switch statement.

             break;
           }
           if (!stack_aligned) {
             PrintF(" with unaligned stack %08x\n", get_register(sp));
           }
           PrintF("\n");
         }
         CHECK(stack_aligned);
         if (redirection->type() != ExternalReference::BUILTIN_COMPARE_CALL) {
           SimulatorRuntimeFPCall target =
               reinterpret_cast<SimulatorRuntimeFPCall>(external);
+#if defined(V8_ARM_ON_X86_64)
+          double result = target(ArgAsDouble(arg0, arg1),
+                                 ArgAsDouble(arg2, arg3),
+                                 arg0, arg1, arg2, arg3);
+#else
           double result = target(arg0, arg1, arg2, arg3);
+#endif
           SetFpResult(result);
         } else {
           SimulatorRuntimeCall target =
               reinterpret_cast<SimulatorRuntimeCall>(external);
           int64_t result = target(arg0, arg1, arg2, arg3, arg4, arg5);
           int32_t lo_res = static_cast<int32_t>(result);
           int32_t hi_res = static_cast<int32_t>(result >> 32);
           if (::v8::internal::FLAG_trace_sim) {
             PrintF("Returned %08x\n", lo_res);
           }
@@ skipping 1701 matching lines @@
   uintptr_t address = *stack_slot;
   set_register(sp, current_sp + sizeof(uintptr_t));
   return address;
 }
 
 } }  // namespace v8::internal
 
 #endif  // USE_SIMULATOR
 
 #endif  // V8_TARGET_ARCH_ARM