[llvm] [RISCV][VLOPT] Add support for checkUsers when UserMI is a Single-Width Integer Reduction (PR #120345)

[Luke Lau via llvm-commits]

================
@@ -963,46 +1031,58 @@ bool RISCVVLOptimizer::checkUsers(const MachineOperand *&CommonVL,
       break;
     }
 
-    const MCInstrDesc &Desc = UserMI.getDesc();
-    if (!RISCVII::hasVLOp(Desc.TSFlags) || !RISCVII::hasSEWOp(Desc.TSFlags)) {
-      LLVM_DEBUG(dbgs() << "    Abort due to lack of VL or SEW, assume that"
-                           " use VLMAX\n");
+    auto VLOp = getVLForUser(UserOp);
+    if (!VLOp) {
       CanReduceVL = false;
       break;
     }
 
-    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");
-
     // Use the largest VL among all the users. If we cannot determine this
     // statically, then we cannot optimize the VL.
-    if (!CommonVL || RISCV::isVLKnownLE(*CommonVL, VLOp)) {
-      CommonVL = &VLOp;
+    if (!CommonVL || RISCV::isVLKnownLE(*CommonVL, *VLOp)) {
+      CommonVL.emplace(*VLOp);
       LLVM_DEBUG(dbgs() << "    User VL is: " << VLOp << "\n");
-    } else if (!RISCV::isVLKnownLE(VLOp, *CommonVL)) {
+    } else if (!RISCV::isVLKnownLE(*VLOp, *CommonVL)) {
       LLVM_DEBUG(dbgs() << "    Abort because cannot determine a common VL\n");
       CanReduceVL = false;
       break;
     }
 
-    // The SEW and LMUL of destination and source registers need to match.
+    if (!RISCVII::hasSEWOp(UserMI.getDesc().TSFlags)) {
+      LLVM_DEBUG(dbgs() << "    Abort due to lack of SEW operand\n");
+      CanReduceVL = false;
+      break;
+    }
+
     OperandInfo ConsumerInfo = getOperandInfo(UserOp, MRI);
     OperandInfo ProducerInfo = getOperandInfo(MI.getOperand(0), MRI);
-    if (ConsumerInfo.isUnknown() || ProducerInfo.isUnknown() ||
-        !OperandInfo::EMULAndEEWAreEqual(ConsumerInfo, ProducerInfo)) {
-      LLVM_DEBUG(dbgs() << "    Abort due to incompatible or unknown "
-                           "information for EMUL or EEW.\n");
+    if (ConsumerInfo.isUnknown() || ProducerInfo.isUnknown()) {
+      LLVM_DEBUG(dbgs() << "    Abort due to unknown operand information.\n");
+      LLVM_DEBUG(dbgs() << "      ConsumerInfo is: " << ConsumerInfo << "\n");
+      LLVM_DEBUG(dbgs() << "      ProducerInfo is: " << ProducerInfo << "\n");
+      CanReduceVL = false;
+      break;
+    }
+
+    // If the operand is used as a scalar operand, then the EEW must be
+    // compatible. Otherwise, the EMUL *and* EEW must be compatible.
+    if ((isVectorOpUsedAsScalarOp(UserOp) &&
+         !OperandInfo::EEWAreEqual(ConsumerInfo, ProducerInfo)) ||
+        (!isVectorOpUsedAsScalarOp(UserOp) &&
+         !OperandInfo::EMULAndEEWAreEqual(ConsumerInfo, ProducerInfo))) {
+      LLVM_DEBUG(
+          dbgs()
+          << "    Abort due to incompatible information for EMUL or EEW.\n");
       LLVM_DEBUG(dbgs() << "      ConsumerInfo is: " << ConsumerInfo << "\n");
       LLVM_DEBUG(dbgs() << "      ProducerInfo is: " << ProducerInfo << "\n");
       CanReduceVL = false;
       break;
     }
   }
-  return CanReduceVL;
+
+  return CanReduceVL && CommonVL
+             ? std::make_optional<const MachineOperand>(*CommonVL)
+             : std::nullopt;
----------------
lukel97 wrote:

Nit, would it be simpler if we just returned std::nullopt where we have `CanReduceVL = false; break` and then returned CommonVL directly here

https://github.com/llvm/llvm-project/pull/120345