[llvm] [RISCV][VLOPT] Allow users that are passthrus if tail elements aren't demanded (PR #124066)

[Luke Lau via llvm-commits]

================
@@ -1188,6 +1188,25 @@ RISCVVLOptimizer::getMinimumVLForUser(MachineOperand &UserOp) {
     return std::nullopt;
   }
 
+  unsigned VLOpNum = RISCVII::getVLOpNum(Desc);
+  const MachineOperand &VLOp = UserMI.getOperand(VLOpNum);
+  // Looking for an immediate or a register VL that isn't X0.
+  assert((!VLOp.isReg() || VLOp.getReg() != RISCV::X0) &&
+         "Did not expect X0 VL");
+
+  // If the user is a passthru it will read the elements past VL, so
+  // abort if any of the elements past VL are demanded.
+  if (UserOp.isTied()) {
+    assert(UserOp.getOperandNo() == UserMI.getNumExplicitDefs() &&
+           RISCVII::isFirstDefTiedToFirstUse(UserMI.getDesc()));
+    auto DemandedVL = DemandedVLs[&UserMI];
+    if (!DemandedVL || !RISCV::isVLKnownLE(*DemandedVL, VLOp)) {
----------------
lukel97 wrote:

Yeah, it should be read as "we couldn't compute the demanded VL for UserMI", so assume it's VLMAX. 

https://github.com/llvm/llvm-project/pull/124066