VM: Improve single-target polymorphic calls.

Use the existing class-id check for dense ranges in more places.
Previously we would fall back to megamorphic lookup with >4 cids,
even though we can check cid ranges up to word-size efficiently.

Next step is to generalize the dense class id checks to multiple
word-sizes.

BUG=
R=vegorov@google.com

Review URL: https://codereview.chromium.org/1847293002 .

runtime/vm/intermediate_language.cc

@@ -133,6 +133,7 @@ CheckClassInstr::CheckClassInstr(Value* value,
       unary_checks_(unary_checks),
       cids_(unary_checks.NumberOfChecks()),
       licm_hoisted_(false),
+      is_dense_switch_(IsDenseCidRange(unary_checks)),
       token_pos_(token_pos) {
   ASSERT(unary_checks.IsZoneHandle());
   // Expected useful check data.
@@ -231,14 +232,22 @@ bool CheckClassInstr::DeoptIfNotNull() const {
 }
 
 
+bool CheckClassInstr::IsDenseCidRange(const ICData& unary_checks) {
+  if (unary_checks.GetReceiverClassIdAt(0) == kSmiCid) return false;
+  if (unary_checks.NumberOfChecks() <= 2) return false;
+  intptr_t max = 0;
+  intptr_t min = kIntptrMax;
+  for (intptr_t i = 0; i < unary_checks.NumberOfChecks(); ++i) {
+    intptr_t cid = unary_checks.GetCidAt(i);
+    if (cid < min) min = cid;
+    if (cid > max) max = cid;
+  }
+  return (max - min) < kBitsPerWord;
+}
+
 
 bool CheckClassInstr::IsDenseSwitch() const {
-  if (unary_checks().GetReceiverClassIdAt(0) == kSmiCid) return false;
-  if (cids_.length() > 2 &&
-      cids_[cids_.length() - 1] - cids_[0] < kBitsPerWord) {
-    return true;
-  }
-  return false;
+  return is_dense_switch_;
 }
 
 

runtime/vm/intermediate_language.h

@@ -7805,6 +7805,7 @@ class CheckClassInstr : public TemplateInstruction<1, NoThrow> {
   bool DeoptIfNotNull() const;
 
   bool IsDenseSwitch() const;
+  static bool IsDenseCidRange(const ICData& unary_checks);
   intptr_t ComputeCidMask() const;
   static bool IsDenseMask(intptr_t mask);
 
@@ -7823,6 +7824,7 @@ class CheckClassInstr : public TemplateInstruction<1, NoThrow> {
   const ICData& unary_checks_;
   GrowableArray<intptr_t> cids_;  // Sorted, lowest first.
   bool licm_hoisted_;
+  bool is_dense_switch_;
   const TokenPosition token_pos_;
 
   DISALLOW_COPY_AND_ASSIGN(CheckClassInstr);

runtime/vm/jit_optimizer.cc

@@ -2690,10 +2690,12 @@ void JitOptimizer::VisitInstanceCall(InstanceCallInstr* instr) {
   const ICData& unary_checks =
       ICData::ZoneHandle(Z, instr->ic_data()->AsUnaryClassChecks());
 
+  const bool is_dense = CheckClassInstr::IsDenseCidRange(unary_checks);
   const intptr_t max_checks = (op_kind == Token::kEQ)
       ? FLAG_max_equality_polymorphic_checks
       : FLAG_max_polymorphic_checks;
   if ((unary_checks.NumberOfChecks() > max_checks) &&
+      !is_dense &&
       flow_graph()->InstanceCallNeedsClassCheck(
           instr, RawFunction::kRegularFunction)) {
     // Too many checks, it will be megamorphic which needs unary checks.
@@ -2759,7 +2761,8 @@ void JitOptimizer::VisitInstanceCall(InstanceCallInstr* instr) {
     }
   }
 
-  if (unary_checks.NumberOfChecks() <= FLAG_max_polymorphic_checks) {
+  if ((unary_checks.NumberOfChecks() <= FLAG_max_polymorphic_checks) ||
+      (has_one_target && is_dense)) {
     bool call_with_checks;
     if (has_one_target && FLAG_polymorphic_with_deopt) {
       // Type propagation has not run yet, we cannot eliminate the check.