Change the x64 optimizing compiler to stop using the fixed context register rsi.

src/x64/macro-assembler-x64.cc

 // Copyright 2011 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 243 matching lines @@
     and_(scratch, Immediate(static_cast<int32_t>(HEAP->NewSpaceMask())));
     j(cc, branch, near_jump);
   }
 }
 
 
 void MacroAssembler::RecordWrite(Register object,
                                  int offset,
                                  Register value,
                                  Register index) {
-  // The compiled code assumes that record write doesn't change the
-  // context register, so we check that none of the clobbered
-  // registers are rsi.
-  ASSERT(!object.is(rsi) && !value.is(rsi) && !index.is(rsi));
-
   // First, check if a write barrier is even needed. The tests below
   // catch stores of smis and stores into the young generation.
   Label done;
   JumpIfSmi(value, &done);
 
   RecordWriteNonSmi(object, offset, value, index);
   bind(&done);
 
   // Clobber all input registers when running with the debug-code flag
   // turned on to provoke errors. This clobbering repeats the
   // clobbering done inside RecordWriteNonSmi but it's necessary to
   // avoid having the fast case for smis leave the registers
   // unchanged.
   if (emit_debug_code()) {
     movq(object, BitCast<int64_t>(kZapValue), RelocInfo::NONE);
     movq(value, BitCast<int64_t>(kZapValue), RelocInfo::NONE);
     movq(index, BitCast<int64_t>(kZapValue), RelocInfo::NONE);
   }
 }
 
 
 void MacroAssembler::RecordWrite(Register object,
                                  Register address,
                                  Register value) {
-  // The compiled code assumes that record write doesn't change the
-  // context register, so we check that none of the clobbered
-  // registers are rsi.
-  ASSERT(!object.is(rsi) && !value.is(rsi) && !address.is(rsi));
-
   // First, check if a write barrier is even needed. The tests below
   // catch stores of smis and stores into the young generation.
   Label done;
   JumpIfSmi(value, &done);
 
   InNewSpace(object, value, equal, &done);
 
   RecordWriteHelper(object, address, value);
 
   bind(&done);
@@ skipping 3580 matching lines @@
   CPU::FlushICache(address_, size_);
 
   // Check that the code was patched as expected.
   ASSERT(masm_.pc_ == address_ + size_);
   ASSERT(masm_.reloc_info_writer.pos() == address_ + size_ + Assembler::kGap);
 }
 
 } }  // namespace v8::internal
 
 #endif  // V8_TARGET_ARCH_X64