[llvm] [Mips][BranchFolding] Enable safe tail merging (PR #225382)

[Jiaxun Yang via llvm-commits]

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@@ -57,6 +59,23 @@ const MipsInstrInfo *MipsInstrInfo::create(MipsSubtarget &STI) {
   return createMipsSEInstrInfo(STI);
 }
 
+bool MipsInstrInfo::isLegalToSplitMBBAt(
+    MachineBasicBlock &MBB, MachineBasicBlock::iterator MBBI) const {
+  // Keep tail merging disabled for MIPS16, whose far branches clobber $ra.
+  if (Subtarget.inMips16Mode())
+    return false;
+
+  // Long branch expansion clobbers $at. Do not let tail merging introduce a
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FlyGoat wrote:

I just took a closer look and unfortunately, it's not trival to make `BranchFolding` respect liveness.

The tricky thing is to inspect the new branch before changing the block. Backends can can rewrite existing terminators in `insertBranch()`, i.e.  `insertBranch()` is not always reversible, so we can't simply inserting and removing a trial branch when it isn’t safe.

https://github.com/llvm/llvm-project/pull/225382