[llvm] [RISCV][VLOPT] Look through PHI instructions (PR #132236)

[Craig Topper via llvm-commits]

================
@@ -1340,6 +1341,17 @@ RISCVVLOptimizer::checkUsers(const MachineInstr &MI) const {
       continue;
     }
 
+    if (UserMI.isPHI() && PHISeen.insert(&UserMI).second) {
+      LLVM_DEBUG(dbgs() << "    Peeking through uses of PHI\n");
+      for (auto &PhiUse : MRI->use_operands(UserMI.getOperand(0).getReg())) {
+        // If UserMI has a PHI cycle, don't analyze it.
+        if (PhiUse.getParent() == &UserMI)
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topperc wrote:

You can have a cycle without the phi directly using itself. The phi using itself is a more unusual case. The common case is there being some operation modifying the phi and then the result of that being used by the phi.

Example:

preheader:
  ...
loop:
  a = phi [preheader, 0], [loop, b]
  b = add a, 1
  ...
  branch to loop

https://github.com/llvm/llvm-project/pull/132236