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

[Luke Lau via llvm-commits]

================
@@ -759,29 +781,42 @@ bool RISCVVLOptimizer::tryReduceVL(MachineInstr &OrigMI) {
     MachineInstr &MI = *Worklist.pop_back_val();
     LLVM_DEBUG(dbgs() << "Trying to reduce VL for " << MI << "\n");
 
-    std::optional<Register> CommonVL;
+    const MachineOperand *CommonVL = nullptr;
     bool CanReduceVL = true;
     if (isVectorRegClass(MI.getOperand(0).getReg(), MRI))
       CanReduceVL = checkUsers(CommonVL, MI);
 
     if (!CanReduceVL || !CommonVL)
       continue;
 
-    if (!CommonVL->isVirtual()) {
-      LLVM_DEBUG(
-          dbgs() << "    Abort due to new VL is not virtual register.\n");
+    assert((CommonVL->isImm() || CommonVL->getReg().isVirtual()) &&
+           "Expected VL to be an Imm or virtual Reg");
+
+    unsigned VLOpNum = RISCVII::getVLOpNum(MI.getDesc());
+    MachineOperand &VLOp = MI.getOperand(VLOpNum);
+
+    if (!RISCV::isVLKnownLE(*CommonVL, VLOp)) {
+      LLVM_DEBUG(dbgs() << "    Abort due to no benefit.\n");
----------------
lukel97 wrote:

Nit, I think it would be a bit clearer if the debug message explicitly said "Abort due to CommonVL not <= VLOp" or something

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