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

[Vladimir Radosavljevic 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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vladimirradosavljevic wrote:

Sorry for the confusion, but this option can reduce code size when comparing application size when `-enable-tail-merge=false` is passed, not in usual cases.
I'm working on a downstream target where jumps are not cheap and BranchFolding can create a lot of them in hot paths. For applications where performance is pivotal, I found that disabling tail merge (`-enable-tail-merge=false`) is boosting performance a lot, and saw that we can reduce code size if tail merge was performed for returning BBs, which in our case is a cold path. Because of this, I introduced this option to cover that case.

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