Use register allocator for context on x64.

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 47 matching lines @@
   DoubleToIStub stub(source_reg, destination_reg, offset, true);
   byte* start = stub.GetCode(isolate)->instruction_start();
 
   __ push(rbx);
   __ push(rcx);
   __ push(rdx);
   __ push(rsi);
   __ push(rdi);
 
   if (!source_reg.is(rsp)) {
-    __ lea(source_reg, MemOperand(rsp, -8 * kPointerSize - offset));
+    // 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 =
+        (Register::NumAllocatableRegisters() - 1) * kPointerSize + kDoubleSize;
+    __ lea(source_reg, MemOperand(rsp, -double_argument_slot - offset));
   }
 
-  int param_offset = 7 * kPointerSize;
   // Save registers make sure they don't get clobbered.
   int reg_num = 0;
   for (;reg_num < Register::NumAllocatableRegisters(); ++reg_num) {
-    Register reg = Register::from_code(reg_num);
+    Register reg = Register::FromAllocationIndex(reg_num);
     if (!reg.is(rsp) && !reg.is(rbp) && !reg.is(destination_reg)) {
       __ push(reg);
-      param_offset += kPointerSize;
     }
   }
 
-  // Re-push the double argument
+  // Put the double argument into the designated double argument slot.
   __ subq(rsp, Immediate(kDoubleSize));
   __ movsd(MemOperand(rsp, 0), xmm0);
 
   // Call through to the actual stub
   __ Call(start, RelocInfo::EXTERNAL_REFERENCE);
 
   __ addq(rsp, Immediate(kDoubleSize));
 
   // Make sure no registers have been unexpectedly clobbered
   for (--reg_num; reg_num >= 0; --reg_num) {
-    Register reg = Register::from_code(reg_num);
+    Register reg = Register::FromAllocationIndex(reg_num);
     if (!reg.is(rsp) && !reg.is(rbp) && !reg.is(destination_reg)) {
       __ cmpq(reg, MemOperand(rsp, 0));
       __ Assert(equal, kRegisterWasClobbered);
       __ addq(rsp, Immediate(kPointerSize));
     }
   }
 
   __ movq(rax, destination_reg);
 
   __ pop(rdi);
@@ skipping 36 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]));
     }
   }
 }