fixup! respond to review

Author: michaelmaitland

llvm/lib/Target/RISCV/RISCVVLOptimizer.cpp

@@ -658,26 +658,23 @@ 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) ||
-       VLOp.isReg())) {
-    bool UseTAPolicy = false;
+  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);
-    if (RISCVII::hasVecPolicyOp(Desc.TSFlags)) {
-      unsigned PolicyOpNum = RISCVII::getVecPolicyOpNum(Desc);
-      const MachineOperand &PolicyOp = MI.getOperand(PolicyOpNum);
-      uint64_t Policy = PolicyOp.getImm();
-      UseTAPolicy = Policy & RISCVII::TAIL_AGNOSTIC;
-      if (HasPassthru) {
-        unsigned PassthruOpIdx = MI.getNumExplicitDefs();
-        UseTAPolicy = UseTAPolicy || (MI.getOperand(PassthruOpIdx).getReg() ==
-                                      RISCV::NoRegister);
-      }
-    }
-    if (!UseTAPolicy) {
+    unsigned PassthruOpIdx = MI.getNumExplicitDefs();
+    if (HasPassthru &&
+        MI.getOperand(PassthruOpIdx).getReg() != RISCV::NoRegister) {
       LLVM_DEBUG(
-          dbgs() << "  Not a candidate because it uses tail-undisturbed policy"
+          dbgs() << "  Not a candidate because it uses non-undef passthru"
                     " with non-VLMAX VL\n");
       return false;
     }