VM: Avoid smi-check when loading class-ids from known non-smi values

runtime/vm/intermediate_language_x64.cc

Index: runtime/vm/intermediate_language_x64.cc
diff --git a/runtime/vm/intermediate_language_x64.cc b/runtime/vm/intermediate_language_x64.cc
index 8253b0cbdee7a1a09cb307744d7862a17355733a..e1a73e86f6f71c0c71cca773398d89d2df2f59f1 100644
--- a/runtime/vm/intermediate_language_x64.cc
+++ b/runtime/vm/intermediate_language_x64.cc
@@ -926,19 +926,25 @@ LocationSummary* LoadClassIdInstr::MakeLocationSummary(Zone* zone,
 void LoadClassIdInstr::EmitNativeCode(FlowGraphCompiler* compiler) {
   const Register object = locs()->in(0).reg();
   const Register result = locs()->out(0).reg();
-  Label load, done;
-
-  // We don't use Assembler::LoadTaggedClassIdMayBeSmi() here---which uses
-  // a conditional move instead---because it is slower, probably due to
-  // branch prediction usually working just fine in this case.
-  __ testq(object, Immediate(kSmiTagMask));
-  __ j(NOT_ZERO, &load, Assembler::kNearJump);
-  __ LoadImmediate(result, Immediate(Smi::RawValue(kSmiCid)));
-  __ jmp(&done);
-  __ Bind(&load);
-  __ LoadClassId(result, object);
-  __ SmiTag(result);
-  __ Bind(&done);
+  const AbstractType& value_type = *this->object()->Type()->ToAbstractType();
+  if (CompileType::Smi().IsAssignableTo(value_type) ||
+      value_type.IsTypeParameter()) {
+    // We don't use Assembler::LoadTaggedClassIdMayBeSmi() here---which uses
+    // a conditional move instead---because it is slower, probably due to
+    // branch prediction usually working just fine in this case.
+    Label load, done;
+    __ testq(object, Immediate(kSmiTagMask));
+    __ j(NOT_ZERO, &load, Assembler::kNearJump);
+    __ LoadImmediate(result, Immediate(Smi::RawValue(kSmiCid)));
+    __ jmp(&done);
+    __ Bind(&load);
+    __ LoadClassId(result, object);
+    __ SmiTag(result);
+    __ Bind(&done);
+  } else {
+    __ LoadClassId(result, object);
+    __ SmiTag(result);
+  }
 }