[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:

Ok, so if I understand this would optimize for performance in your target and not for code-size.

In that case, have you thought about making the `enable-tail-merge` (and the value in the constructor of `BranchFolder`) into an `enum` covering your case ?

I'd not be against a target hook returning `true` if "unconditional-jumps-are-expensive" that helps control this behavior. 

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