[BranchFolding] Add a hook to override tail merge size (#99025)

A new hook `TargetInstrInfo::getTailMergeSize()` is added so that
targets can override it.

This removes an existing TODO.

Author: wangpc-pp

llvm/include/llvm/CodeGen/TargetInstrInfo.h

@@ -2226,6 +2226,12 @@ class TargetInstrInfo : public MCInstrInfo {
     return OptLevel >= CodeGenOptLevel::Aggressive ? 4 : 2;
   }
 
+  /// Returns the target-specific default value for tail merging.
+  /// This value will be used if the tail-merge-size argument is not provided.
+  virtual unsigned getTailMergeSize(const MachineFunction &MF) const {
+    return 3;
+  }
+
   /// Returns the callee operand from the given \p MI.
   virtual const MachineOperand &getCalleeOperand(const MachineInstr &MI) const {
     return MI.getOperand(0);

llvm/lib/CodeGen/BranchFolding.cpp

@@ -80,7 +80,6 @@ TailMergeThreshold("tail-merge-threshold",
           cl::init(150), cl::Hidden);
 
 // Heuristic for tail merging (and, inversely, tail duplication).
-// TODO: This should be replaced with a target query.
 static cl::opt<unsigned>
 TailMergeSize("tail-merge-size",
               cl::desc("Min number of instructions to consider tail merging"),
@@ -145,8 +144,6 @@ BranchFolder::BranchFolder(bool DefaultEnableTailMerge, bool CommonHoist,
                            ProfileSummaryInfo *PSI, unsigned MinTailLength)
     : EnableHoistCommonCode(CommonHoist), MinCommonTailLength(MinTailLength),
       MBBFreqInfo(FreqInfo), MBPI(ProbInfo), PSI(PSI) {
-  if (MinCommonTailLength == 0)
-    MinCommonTailLength = TailMergeSize;
   switch (FlagEnableTailMerge) {
   case cl::BOU_UNSET:
     EnableTailMerge = DefaultEnableTailMerge;
@@ -195,6 +192,12 @@ bool BranchFolder::OptimizeFunction(MachineFunction &MF,
   MLI = mli;
   this->MRI = &MRI;
 
+  if (MinCommonTailLength == 0) {
+    MinCommonTailLength = TailMergeSize.getNumOccurrences() > 0
+                              ? TailMergeSize
+                              : TII->getTailMergeSize(MF);
+  }
+
   UpdateLiveIns = MRI.tracksLiveness() && TRI->trackLivenessAfterRegAlloc(MF);
   if (!UpdateLiveIns)
     MRI.invalidateLiveness();