[PATCH] D101187: [MachineCSE] Prevent CSE of non-local convergent instrs

[Michael Kitzan via Phabricator via llvm-commits]

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At the moment, `MachineCSE` allows CSE-ing convergent instrs which are non-local to each other. This can cause illegal codegen as convergent instrs are control flow dependent. The patch prevents non-local CSE of convergent instrs by adding a check in `isProfitableToCSE` and rejecting CSE-ing if we're considering CSE-ing non-local convergent instrs. We can still CSE convergent instrs which are in the same control flow scope, so the patch purposely does not make all convergent instrs non-CSE candidates in `isCSECandidate`.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D101187

Files:
  llvm/lib/CodeGen/MachineCSE.cpp


Index: llvm/lib/CodeGen/MachineCSE.cpp
===================================================================
--- llvm/lib/CodeGen/MachineCSE.cpp
+++ llvm/lib/CodeGen/MachineCSE.cpp
@@ -433,6 +433,11 @@
                                    MachineBasicBlock *CSBB, MachineInstr *MI) {
   // FIXME: Heuristics that works around the lack the live range splitting.
 
+  MachineBasicBlock *BB = MI->getParent();
+  // Prevent CSE-ing non-local convergent instructions.
+  if (MI->isConvergent() && CSBB != BB)
+    return false;
+
   // If CSReg is used at all uses of Reg, CSE should not increase register
   // pressure of CSReg.
   bool MayIncreasePressure = true;
@@ -455,7 +460,6 @@
   // an immediate predecessor. We don't want to increase register pressure and
   // end up causing other computation to be spilled.
   if (TII->isAsCheapAsAMove(*MI)) {
-    MachineBasicBlock *BB = MI->getParent();
     if (CSBB != BB && !CSBB->isSuccessor(BB))
       return false;
   }


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