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

[Quentin Dian via llvm-commits]

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@@ -536,7 +536,13 @@ class CallAnalyzer : public InstVisitor<CallAnalyzer, bool> {
 // Considering comparisons from leaf and non-leaf nodes, we can estimate the
 // number of comparisons in a simple closed form :
 //   n + n / 2 - 1 = n * 3 / 2 - 1
-int64_t getExpectedNumberOfCompare(int NumCaseCluster) {
+int64_t getExpectedNumberOfCompare(int NumCaseCluster,
+                                   bool DefaultDestUndefined) {
+  // The compare instruction count should be less than the branch count
+  // when default branch is undefined.
+  if (DefaultDestUndefined) {
+    return static_cast<int64_t>(NumCaseCluster) - 1;
+  }
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DianQK wrote:

Here is an example: https://llvm.godbolt.org/z/x6ETdfY79. If there are common target branches, the number of compare instructions will decrease. I haven't started learning about instruction selection, so I will leave a note here.

https://github.com/llvm/llvm-project/pull/85160