Copy ICData descriptors when starting background compilation, so that they do not change while comp…

runtime/vm/compiler.cc

Index: runtime/vm/compiler.cc
diff --git a/runtime/vm/compiler.cc b/runtime/vm/compiler.cc
index cf452bd71ea6bd45078844ff0092e3d388ad007f..8861712e2aaa4fa099852855f6470f1bb1d64602 100644
--- a/runtime/vm/compiler.cc
+++ b/runtime/vm/compiler.cc
@@ -427,7 +427,15 @@ static bool CompileParsedFunctionHelper(CompilationPipeline* pipeline,
           // builder uses it to attach it to nodes.
           ASSERT(function.deoptimization_counter() <
                  FLAG_deoptimization_counter_threshold);
-          function.RestoreICDataMap(ic_data_array);
+          if ((osr_id == Compiler::kNoOSRDeoptId) &&
+              FLAG_background_compilation && optimized) {
+            ASSERT(!Thread::Current()->IsMutatorThread());
+            // This 'freezes' ICData so that it does not change while compiling.
+            function.CopyICDataMap(ic_data_array);
+          } else {
+            ASSERT(Thread::Current()->IsMutatorThread());
+            function.RestoreICDataMap(ic_data_array);
+          }
           if (FLAG_print_ic_data_map) {
             for (intptr_t i = 0; i < ic_data_array->length(); i++) {
               if ((*ic_data_array)[i] != NULL) {
@@ -1537,9 +1545,8 @@ void BackgroundCompiler::Run() {
         ASSERT(error.IsNull());
         temp_function = RemoveFunctionOrNull();
         ASSERT(temp_function.raw() == function.raw());
-        // Reset to 0 so that it can be recompiled if needed.
-        function.set_usage_counter(0);
         function = LastFunctionOrNull();
+        ASSERT(!code.IsNull());
         AddCode(code);
       }
     }
@@ -1575,8 +1582,12 @@ void BackgroundCompiler::InstallGeneratedCode() {
     code = queue.PopBackCode();
     if (code.IsDisabled()) continue;
     owner = code.owner();
-    ASSERT(owner.IsFunction());
-    Function::Cast(owner).InstallOptimizedCode(code, false /* not OSR */);
+    const Function& function = Function::Cast(owner);
+    function.InstallOptimizedCode(code, false /* not OSR */);
+    if (function.usage_counter() < 0) {
+      // Reset to 0 so that it can be recompiled if needed.
+      function.set_usage_counter(0);
+    }
   }
 }