Faster code for megamorphic calls fo get:hashCode for Smi and OneByteString

runtime/vm/flow_graph_compiler_arm.cc

Index: runtime/vm/flow_graph_compiler_arm.cc
diff --git a/runtime/vm/flow_graph_compiler_arm.cc b/runtime/vm/flow_graph_compiler_arm.cc
index 787b5139a7dfc4e8770a729a4800949564330b3b..9c5804490349158e7f78429f3944acde2ce93b75 100644
--- a/runtime/vm/flow_graph_compiler_arm.cc
+++ b/runtime/vm/flow_graph_compiler_arm.cc
@@ -1319,7 +1319,30 @@ void FlowGraphCompiler::EmitMegamorphicInstanceCall(
       MegamorphicCacheTable::Lookup(isolate(), name, arguments_descriptor));
 
   __ Comment("MegamorphicCall");
+  // Load receiver into R0.
   __ LoadFromOffset(kWord, R0, SP, (argument_count - 1) * kWordSize);
+  Label done;
+  if (name.raw() == Symbols::hashCode().raw()) {
+    Label megamorphic_call;
+    __ Comment("Inlined get:hashCode for Smi and OneByteString");
+    __ tst(R0, Operand(kSmiTagMask));
+    __ b(&done, EQ);  // Is Smi (result is receiver).
+
+    // Use R9 (cache for megamorphic call) as scratch.
+    __ CompareClassId(R0, kOneByteStringCid, R9);
+    __ b(&megamorphic_call, NE);
+
+    __ mov(R9, Operand(R0));  // Preserve receiver in R9.
+    __ ldr(R0, FieldAddress(R0, String::hash_offset()));
+    ASSERT(Smi::New(0) == 0);
+    __ cmp(R0, Operand(0));
+
+    __ b(&done, NE);  // Return if already computed.
+    __ mov(R0, Operand(R9));  // Restore receiver in R0.
+
+    __ Bind(&megamorphic_call);
+    __ Comment("Slow case: megamorphic call");
+  }
   __ LoadObject(R9, cache);
   if (FLAG_use_megamorphic_stub) {
     __ BranchLink(*StubCode::MegamorphicLookup_entry());
@@ -1328,6 +1351,7 @@ void FlowGraphCompiler::EmitMegamorphicInstanceCall(
   }
   __ blx(R1);
 
+  __ Bind(&done);
   RecordSafepoint(locs, slow_path_argument_count);
   const intptr_t deopt_id_after = Thread::ToDeoptAfter(deopt_id);
   if (FLAG_precompiled_mode) {