Deoptimization is easier to diagnose when there is a text reason.

src/x64/lithium-codegen-x64.cc

 // Copyright 2013 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 5429 matching lines @@
 
 void LCodeGen::DoDeoptimize(LDeoptimize* instr) {
   Deoptimizer::BailoutType type = instr->hydrogen()->type();
   // TODO(danno): Stubs expect all deopts to be lazy for historical reasons (the
   // needed return address), even though the implementation of LAZY and EAGER is
   // now identical. When LAZY is eventually completely folded into EAGER, remove
   // the special case below.
   if (info()->IsStub() && type == Deoptimizer::EAGER) {
     type = Deoptimizer::LAZY;
   }
+
+  Comment(";;; deoptimize: %s", instr->hydrogen()->reason());
   DeoptimizeIf(no_condition, instr->environment(), type);
 }
 
 
 void LCodeGen::DoDummyUse(LDummyUse* instr) {
   // Nothing to see here, move on!
 }
 
 
 void LCodeGen::DoDeferredStackCheck(LStackCheck* instr) {
@@ skipping 157 matching lines @@
                                FixedArray::kHeaderSize - kPointerSize));
   __ bind(&done);
 }
 
 
 #undef __
 
 } }  // namespace v8::internal
 
 #endif  // V8_TARGET_ARCH_X64