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

src/arm/lithium-codegen-arm.cc

Index: src/arm/lithium-codegen-arm.cc
diff --git a/src/arm/lithium-codegen-arm.cc b/src/arm/lithium-codegen-arm.cc
index d22bcb59100ddf6abd751288c422dbca189ff285..9d2c9bfe55ee5f713e16d3d4687fa67b5f5b4c11 100644
--- a/src/arm/lithium-codegen-arm.cc
+++ b/src/arm/lithium-codegen-arm.cc
@@ -5680,6 +5680,8 @@ void LCodeGen::DoDeoptimize(LDeoptimize* instr) {
   if (info()->IsStub() && type == Deoptimizer::EAGER) {
     type = Deoptimizer::LAZY;
   }
+
+  Comment(";;; deoptimize: %s", instr->hydrogen()->reason());
   DeoptimizeIf(al, instr->environment(), type);
 }