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

runtime/vm/flow_graph_compiler_arm64.cc

Index: runtime/vm/flow_graph_compiler_arm64.cc
diff --git a/runtime/vm/flow_graph_compiler_arm64.cc b/runtime/vm/flow_graph_compiler_arm64.cc
index 290fa6ad28e3d880378526af3bd79daf941e1c8e..d62a23a3059960a3c784cc5511c0a7be77c02b73 100644
--- a/runtime/vm/flow_graph_compiler_arm64.cc
+++ b/runtime/vm/flow_graph_compiler_arm64.cc
@@ -1300,7 +1300,29 @@ void FlowGraphCompiler::EmitMegamorphicInstanceCall(
       MegamorphicCacheTable::Lookup(isolate(), name, arguments_descriptor));
 
   __ Comment("MegamorphicCall");
+  // Load receiver into R0.
   __ LoadFromOffset(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");
+    __ tsti(R0, Immediate(kSmiTagMask));
+    __ b(&done, EQ);  // Is Smi (result is receiver).
+
+    __ CompareClassId(R0, kOneByteStringCid);
+    __ b(&megamorphic_call, NE);
+
+    // Use R5 (cache for megamorphic call) as scratch.
+    __ mov(R5, R0);  // Preserve receiver in R5, result in R0.
+    __ ldr(R0, FieldAddress(R0, String::hash_offset()));
+    __ CompareRegisters(R0, ZR);
+    __ b(&done, NE);
+    __ mov(R0, R5);  // Restore receiver in R0,
+
+    __ Bind(&megamorphic_call);
+    __ Comment("Slow case: megamorphic call");
+  }
+
   __ LoadObject(R5, cache);
   if (FLAG_use_megamorphic_stub) {
     __ BranchLink(*StubCode::MegamorphicLookup_entry());
@@ -1309,6 +1331,7 @@ void FlowGraphCompiler::EmitMegamorphicInstanceCall(
   }
   __ blr(R1);
 
+  __ Bind(&done);
   RecordSafepoint(locs, slow_path_argument_count);
   const intptr_t deopt_id_after = Thread::ToDeoptAfter(deopt_id);
   if (FLAG_precompiled_mode) {