[RISCV] Use vleff's AVL when output VL doesn't dominate in RISCVVLOptimizer

If an instruction's demanded VL is a virtual register defined by a vleff instruction, it might not dominate and fail to have its VL reduced.

In leiu of the output VL, we can try and use the AVL passed to the vleff itself since it will be at least greater than or equal the original VL.

I tried to create an LLVM IR test for this in but didn't have any luck because the scheduler kept on moving the instruction past the vleff, so it always dominated. So I've just included some mir tests instead.

Author: lukel97

llvm/lib/Target/RISCV/RISCVVLOptimizer.cpp

@@ -1464,6 +1464,15 @@ bool RISCVVLOptimizer::tryReduceVL(MachineInstr &MI) const {
   assert((CommonVL->isImm() || CommonVL->getReg().isVirtual()) &&
          "Expected VL to be an Imm or virtual Reg");
 
+  // If the VL is defined by a vleff that doesn't dominate MI, try using the
+  // vleff's AVL. It will be greater than or equal to the output VL.
+  if (CommonVL->isReg()) {
+    const MachineInstr *VLMI = MRI->getVRegDef(CommonVL->getReg());
+    if (RISCVInstrInfo::isFaultOnlyFirstLoad(*VLMI) &&
+        !MDT->dominates(VLMI, &MI))
+      CommonVL = VLMI->getOperand(4);
+  }
+
   if (!RISCV::isVLKnownLE(*CommonVL, VLOp)) {
     LLVM_DEBUG(dbgs() << "  Abort due to CommonVL not <= VLOp.\n");
     return false;

llvm/test/CodeGen/RISCV/rvv/vl-opt.mir

@@ -608,7 +608,7 @@ name: vleff_imm
 body: |
   bb.0:
     ; CHECK-LABEL: name: vleff_imm
-    ; CHECK: %x:vr = PseudoVADD_VV_M1 $noreg, $noreg, $noreg, -1, 3 /* e8 */, 3 /* ta, ma */
+    ; CHECK: %x:vr = PseudoVADD_VV_M1 $noreg, $noreg, $noreg, 1, 3 /* e8 */, 3 /* ta, ma */
     ; CHECK-NEXT: %y:vr, %vl:gprnox0 = PseudoVLE8FF_V_M1 $noreg, $noreg, 1, 3 /* e8 */, 3 /* ta, ma */
     ; CHECK-NEXT: PseudoVSE8_V_M1 %x, $noreg, %vl, 3 /* e8 */
     %x:vr = PseudoVADD_VV_M1 $noreg, $noreg, $noreg, -1, 3 /* e8 */, 3 /* ta, ma */
@@ -624,7 +624,7 @@ body: |
     ; CHECK: liveins: $x8
     ; CHECK-NEXT: {{  $}}
     ; CHECK-NEXT: %avl:gprnox0 = COPY $x8
-    ; CHECK-NEXT: %x:vr = PseudoVADD_VV_M1 $noreg, $noreg, $noreg, -1, 3 /* e8 */, 3 /* ta, ma */
+    ; CHECK-NEXT: %x:vr = PseudoVADD_VV_M1 $noreg, $noreg, $noreg, %avl, 3 /* e8 */, 3 /* ta, ma */
     ; CHECK-NEXT: %y:vr, %vl:gprnox0 = PseudoVLE8FF_V_M1 $noreg, $noreg, %avl, 3 /* e8 */, 3 /* ta, ma */
     ; CHECK-NEXT: PseudoVSE8_V_M1 %x, $noreg, %vl, 3 /* e8 */
     %avl:gprnox0 = COPY $x8