[llvm] [BranchFolding] Add an option to tail merge only bbs without successors (PR #101969)

[Juan Manuel Martinez Caamaño via llvm-commits]

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@@ -1035,6 +1040,9 @@ bool BranchFolder::TailMergeBlocks(MachineFunction &MF) {
   if (MergePotentials.size() >= 2)
     MadeChange |= TryTailMergeBlocks(nullptr, nullptr, MinCommonTailLength);
 
+  if (TailMergeOnlyBBsWithoutSucc)
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jmmartinez wrote:

Hello !

> this option can reduce code size in case function has multiple return points that can be merged.

Is the driver that sets the option when optimizing for size?

Could we instead check if _optimizing for code size_ and _the function has multiple return points that can be merged_ instead of using an option?

I'd also expect that if a branch edge [is hot](https://llvm.org/doxygen/classllvm_1_1MachineBranchProbabilityInfo.html#a6cd25f432a77253606cc6387d0057833) this transformation would still be applied (even if optimizing for size).

https://github.com/llvm/llvm-project/pull/101969