[llvm] [RISCV] Move RISCVInsertVSETVLI to after phi elimination (PR #91440)

[Luke Lau via llvm-commits]

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@@ -541,9 +541,16 @@ void RISCVPassConfig::addPreRegAlloc() {
   addPass(createRISCVPreRAExpandPseudoPass());
   if (TM->getOptLevel() != CodeGenOptLevel::None)
     addPass(createRISCVMergeBaseOffsetOptPass());
+
   addPass(createRISCVInsertReadWriteCSRPass());
   addPass(createRISCVInsertWriteVXRMPass());
-  addPass(createRISCVInsertVSETVLIPass());
+
+  // Run RISCVInsertVSETVLI after PHI elimination. On O1 and above do it after
+  // register coalescing so needVSETVLIPHI doesn't need to look through COPYs.
+  if (TM->getOptLevel() == CodeGenOptLevel::None)
+    insertPass(&PHIEliminationID, createRISCVInsertVSETVLIPass());
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lukel97 wrote:

Thanks for catching this, I can reproduce this locally and your explanation makes sense.

I would presume all the addX hooks are called up front regardless of where the cut off point is. 

One possible fix is if we're deleting a pass because we've reached the cut off point, we could delete any other passes in InsertedPass that are waiting on it? Will try and prepare a patch for that.

https://github.com/llvm/llvm-project/pull/91440