[llvm] [RISCV][VLOPT] Allow propagation even when VL isn't VLMAX (PR #112228)

[Michael Maitland via llvm-commits]

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@@ -641,10 +641,33 @@ bool RISCVVLOptimizer::isCandidate(const MachineInstr &MI) const {
   if (MI.getNumDefs() != 1)
     return false;
 
+  // If we're not using VLMAX, then we need to be careful whether we are using
+  // TA/TU when there is a non-undef Passthru. But when we are using VLMAX, it
+  // does not matter whether we are using TA/TU with a non-undef Passthru, since
+  // there are no tail elements to be perserved.
   unsigned VLOpNum = RISCVII::getVLOpNum(Desc);
   const MachineOperand &VLOp = MI.getOperand(VLOpNum);
-  if (!VLOp.isImm() || VLOp.getImm() != RISCV::VLMaxSentinel)
+  if (VLOp.isReg() || VLOp.getImm() != RISCV::VLMaxSentinel) {
+    // If MI has a non-undef passthru, we will not try to optimize it since
+    // that requires us to preserve tail elements according to TA/TU.
+    // Otherwise, The MI has an undef Passthru, so it doesn't matter whether we
+    // are using TA/TU.
+    bool HasPassthru = RISCVII::isFirstDefTiedToFirstUse(Desc);
+    unsigned PassthruOpIdx = MI.getNumExplicitDefs();
+    if (HasPassthru &&
+        MI.getOperand(PassthruOpIdx).getReg() != RISCV::NoRegister) {
+      LLVM_DEBUG(
+          dbgs() << "  Not a candidate because it uses non-undef passthru"
+                    " with non-VLMAX VL\n");
+      return false;
+    }
+  }
+
+  // If the VL is 1, then there is no need to reduce it.
+  if (VLOp.isImm() && VLOp.getImm() == 1) {
+    LLVM_DEBUG(dbgs() << "  Not a candidate because VL is already 1\n");
     return false;
+  }
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michaelmaitland wrote:

An optimization, not needed for correctness.

https://github.com/llvm/llvm-project/pull/112228