[InlineCost] Correct the default branch cost for the switch statement (#85160)

Fixes #81723.

The earliest commit of the related code is:
https://github.com/llvm/llvm-project/commit/919f9e8d65ada6552b8b8a5ec12ea49db91c922a.
I tried to understand the following code with
https://github.com/llvm/llvm-project/pull/77856#issuecomment-1993499085.

https://github.com/llvm/llvm-project/blob/5932fcc47855fdd209784f38820422d2369b84b2/llvm/lib/Analysis/InlineCost.cpp#L709-L720

I think only scenarios where there is a default branch were considered.

1 files changed, 15 insertions, 10 deletions

diff --git a/llvm/lib/Analysis/InlineCost.cpp b/llvm/lib/Analysis/InlineCost.cpp
index c75460f..e4989db 100644
--- a/llvm/lib/Analysis/InlineCost.cpp
+++ b/llvm/lib/Analysis/InlineCost.cpp
@@ -701,21 +701,26 @@ class InlineCostCallAnalyzer final : public CallAnalyzer {
 
   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) {
+      // Suppose a default branch includes one compare and one conditional
+      // branch if it's reachable.
+      if (!DefaultDestUndefined)
+        addCost(2 * InstrCost);
+      // Suppose a jump table requires one load and one jump instruction.
       int64_t JTCost =
-          static_cast<int64_t>(JumpTableSize) * InstrCost + 4 * InstrCost;
+          static_cast<int64_t>(JumpTableSize) * InstrCost + 2 * InstrCost;
       addCost(JTCost);
       return;
     }
 
     if (NumCaseCluster <= 3) {
       // Suppose a comparison includes one compare and one conditional branch.
-      addCost(NumCaseCluster * 2 * InstrCost);
+      // We can reduce a set of instructions if the default branch is
+      // undefined.
+      addCost((NumCaseCluster - DefaultDestUndefined) * 2 * InstrCost);
       return;
     }
 
@@ -1152,7 +1157,7 @@ private:
   // FIXME: These constants are taken from the heuristic-based cost visitor.
   // These should be removed entirely in a later revision to avoid reliance on
   // heuristics in the ML inliner.
-  static constexpr int JTCostMultiplier = 4;
+  static constexpr int JTCostMultiplier = 2;
   static constexpr int CaseClusterCostMultiplier = 2;
   static constexpr int SwitchDefaultDestCostMultiplier = 2;
   static constexpr int SwitchCostMultiplier = 2;
@@ -1235,11 +1240,10 @@ private:
 
   void onFinalizeSwitch(unsigned JumpTableSize, unsigned NumCaseCluster,
                         bool DefaultDestUndefined) override {
-    if (!DefaultDestUndefined)
-      increment(InlineCostFeatureIndex::switch_default_dest_penalty,
-                SwitchDefaultDestCostMultiplier * InstrCost);
-
     if (JumpTableSize) {
+      if (!DefaultDestUndefined)
+        increment(InlineCostFeatureIndex::switch_default_dest_penalty,
+                  SwitchDefaultDestCostMultiplier * InstrCost);
       int64_t JTCost = static_cast<int64_t>(JumpTableSize) * InstrCost +
                        JTCostMultiplier * InstrCost;
       increment(InlineCostFeatureIndex::jump_table_penalty, JTCost);
@@ -1248,7 +1252,8 @@ private:
 
     if (NumCaseCluster <= 3) {
       increment(InlineCostFeatureIndex::case_cluster_penalty,
-                NumCaseCluster * CaseClusterCostMultiplier * InstrCost);
+                (NumCaseCluster - DefaultDestUndefined) *
+                    CaseClusterCostMultiplier * InstrCost);
       return;
     }