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

src/code-stubs-hydrogen.cc

Index: src/code-stubs-hydrogen.cc
diff --git a/src/code-stubs-hydrogen.cc b/src/code-stubs-hydrogen.cc
index 852f7b569727b08939d1c952146699d6ff4999f7..9bbab9ab7c69ee67beb514754d117d193ed3c94b 100644
--- a/src/code-stubs-hydrogen.cc
+++ b/src/code-stubs-hydrogen.cc
@@ -92,7 +92,7 @@ class CodeStubGraphBuilderBase : public HGraphBuilder {
     }
 
     ~ArrayContextChecker() {
-      checker_.ElseDeopt();
+      checker_.ElseDeopt("Array constructor called from different context");
       checker_.End();
     }
    private:
@@ -233,7 +233,7 @@ class CodeStubGraphBuilder: public CodeStubGraphBuilderBase {
     IfBuilder builder(this);
     builder.IfNot<HCompareObjectEqAndBranch, HValue*>(undefined, undefined);
     builder.Then();
-    builder.ElseDeopt();
+    builder.ElseDeopt("Forced deopt to runtime");
     return undefined;
   }
 
@@ -387,7 +387,7 @@ HValue* CodeStubGraphBuilder<FastCloneShallowArrayStub>::BuildCodeStub() {
                                                length));
   }
 
-  checker.ElseDeopt();
+  checker.ElseDeopt("Uninitialized boilerplate literals");
   checker.End();
 
   return environment()->Pop();
@@ -434,7 +434,7 @@ HValue* CodeStubGraphBuilder<FastCloneShallowObjectStub>::BuildCodeStub() {
   }
 
   environment()->Push(object);
-  checker.ElseDeopt();
+  checker.ElseDeopt("Uninitialized boilerplate in fast clone");
   checker.End();
 
   return environment()->Pop();
@@ -844,7 +844,7 @@ HValue* CodeStubGraphBuilder<StoreGlobalStub>::BuildCodeInitializedStub() {
     IfBuilder builder(this);
     builder.If<HCompareObjectEqAndBranch>(cell_contents, value);
     builder.Then();
-    builder.ElseDeopt();
+    builder.ElseDeopt("Unexpected cell contents in constant global store");
     builder.End();
   } else {
     // Load the payload of the global parameter cell. A hole indicates that the
@@ -854,7 +854,7 @@ HValue* CodeStubGraphBuilder<StoreGlobalStub>::BuildCodeInitializedStub() {
     HValue* hole_value = Add<HConstant>(hole);
     builder.If<HCompareObjectEqAndBranch>(cell_contents, hole_value);
     builder.Then();
-    builder.Deopt();
+    builder.Deopt("Unexpected cell contents in global store");
     builder.Else();
     Add<HStoreNamedField>(cell, access, value);
     builder.End();
@@ -878,7 +878,8 @@ HValue* CodeStubGraphBuilder<ElementsTransitionAndStoreStub>::BuildCodeStub() {
 
   if (FLAG_trace_elements_transitions) {
     // Tracing elements transitions is the job of the runtime.
-    Add<HDeoptimize>(Deoptimizer::EAGER);
+    Add<HDeoptimize>("Deopt due to --trace-elements-transitions",
+                     Deoptimizer::EAGER);
   } else {
     info()->MarkAsSavesCallerDoubles();