[InlineCost] Consider the default branch when calculating cost (#77856)

First step in fixing #76772.

This PR considers the default branch as a case branch. This will give
the unreachable default branch fair consideration.

Author: dianqk

llvm/include/llvm/Analysis/InlineModelFeatureMaps.h

@@ -39,6 +39,8 @@ namespace llvm {
   M(int64_t, {1}, jump_table_penalty, "Accumulation of costs for jump tables") \
   M(int64_t, {1}, case_cluster_penalty,                                        \
     "Accumulation of costs for case clusters")                                 \
+  M(int64_t, {1}, switch_default_dest_penalty,                                 \
+    "Accumulation of costs for switch default destination")                    \
   M(int64_t, {1}, switch_penalty,                                              \
     "Accumulation of costs for switch statements")                             \
   M(int64_t, {1}, unsimplified_common_instructions,                            \

llvm/include/llvm/IR/Instructions.h

@@ -49,6 +49,7 @@ class DataLayout;
 class StringRef;
 class Type;
 class Value;
+class UnreachableInst;
 
 //===----------------------------------------------------------------------===//
 //                                AllocaInst Class
@@ -3505,6 +3506,12 @@ class SwitchInst : public Instruction {
     return cast<BasicBlock>(getOperand(1));
   }
 
+  /// Returns true if the default branch must result in immediate undefined
+  /// behavior, false otherwise.
+  bool defaultDestUndefined() const {
+    return isa<UnreachableInst>(getDefaultDest()->getFirstNonPHIOrDbg());
+  }
+
   void setDefaultDest(BasicBlock *DefaultCase) {
     setOperand(1, reinterpret_cast<Value*>(DefaultCase));
   }

llvm/lib/Analysis/InlineCost.cpp

@@ -336,8 +336,8 @@ class CallAnalyzer : public InstVisitor<CallAnalyzer, bool> {
 
   /// Called at the end of processing a switch instruction, with the given
   /// number of case clusters.
-  virtual void onFinalizeSwitch(unsigned JumpTableSize,
-                                unsigned NumCaseCluster) {}
+  virtual void onFinalizeSwitch(unsigned JumpTableSize, unsigned NumCaseCluster,
+                                bool DefaultDestUndefined) {}
 
   /// Called to account for any other instruction not specifically accounted
   /// for.
@@ -699,15 +699,16 @@ class InlineCostCallAnalyzer final : public CallAnalyzer {
                                        CallPenalty));
   }
 
-  void onFinalizeSwitch(unsigned JumpTableSize,
-                        unsigned NumCaseCluster) override {
+  void onFinalizeSwitch(unsigned JumpTableSize, unsigned NumCaseCluster,
+                        bool DefaultDestUndefined) override {
+    if (!DefaultDestUndefined)
+      addCost(2 * InstrCost);
     // If suitable for a jump table, consider the cost for the table size and
     // branch to destination.
     // Maximum valid cost increased in this function.
     if (JumpTableSize) {
       int64_t JTCost =
           static_cast<int64_t>(JumpTableSize) * InstrCost + 4 * InstrCost;
-
       addCost(JTCost);
       return;
     }
@@ -1153,6 +1154,7 @@ class InlineCostFeaturesAnalyzer final : public CallAnalyzer {
   // heuristics in the ML inliner.
   static constexpr int JTCostMultiplier = 4;
   static constexpr int CaseClusterCostMultiplier = 2;
+  static constexpr int SwitchDefaultDestCostMultiplier = 2;
   static constexpr int SwitchCostMultiplier = 2;
 
   // FIXME: These are taken from the heuristic-based cost visitor: we should
@@ -1231,8 +1233,11 @@ class InlineCostFeaturesAnalyzer final : public CallAnalyzer {
     }
   }
 
-  void onFinalizeSwitch(unsigned JumpTableSize,
-                        unsigned NumCaseCluster) override {
+  void onFinalizeSwitch(unsigned JumpTableSize, unsigned NumCaseCluster,
+                        bool DefaultDestUndefined) override {
+    if (!DefaultDestUndefined)
+      increment(InlineCostFeatureIndex::switch_default_dest_penalty,
+                SwitchDefaultDestCostMultiplier * InstrCost);
 
     if (JumpTableSize) {
       int64_t JTCost = static_cast<int64_t>(JumpTableSize) * InstrCost +
@@ -2461,7 +2466,7 @@ bool CallAnalyzer::visitSwitchInst(SwitchInst &SI) {
   unsigned NumCaseCluster =
       TTI.getEstimatedNumberOfCaseClusters(SI, JumpTableSize, PSI, BFI);
 
-  onFinalizeSwitch(JumpTableSize, NumCaseCluster);
+  onFinalizeSwitch(JumpTableSize, NumCaseCluster, SI.defaultDestUndefined());
   return false;
 }