[RISCV] Add a query for exact VLEN to RISCVSubtarget [nfc]

We've now got enough of these in tree that we can see which patterns
appear to be idiomatic.  As such, extract a helper for checking
if we know the exact VLEN.

Author: preames

llvm/lib/Target/RISCV/RISCVFrameLowering.cpp

@@ -399,9 +399,9 @@ void RISCVFrameLowering::adjustStackForRVV(MachineFunction &MF,
 
   // Optimize compile time offset case
   StackOffset Offset = StackOffset::getScalable(Amount);
-  if (STI.getRealMinVLen() == STI.getRealMaxVLen()) {
+  if (auto VLEN = STI.getRealVLen()) {
     // 1. Multiply the number of v-slots by the (constant) length of register
-    const int64_t VLENB = STI.getRealMinVLen() / 8;
+    const int64_t VLENB = *VLEN / 8;
     assert(Amount % 8 == 0 &&
            "Reserve the stack by the multiple of one vector size.");
     const int64_t NumOfVReg = Amount / 8;

llvm/lib/Target/RISCV/RISCVISelDAGToDAG.cpp

@@ -577,9 +577,8 @@ void RISCVDAGToDAGISel::selectVSETVLI(SDNode *Node) {
   SDValue VLOperand;
   unsigned Opcode = RISCV::PseudoVSETVLI;
   if (auto *C = dyn_cast<ConstantSDNode>(Node->getOperand(1))) {
-    const unsigned VLEN = Subtarget->getRealMinVLen();
-    if (VLEN == Subtarget->getRealMaxVLen())
-      if (VLEN / RISCVVType::getSEWLMULRatio(SEW, VLMul) == C->getZExtValue())
+    if (auto VLEN = Subtarget->getRealVLen())
+      if (*VLEN / RISCVVType::getSEWLMULRatio(SEW, VLMul) == C->getZExtValue())
         VLMax = true;
   }
   if (VLMax || isAllOnesConstant(Node->getOperand(1))) {

llvm/lib/Target/RISCV/RISCVISelLowering.cpp

@@ -8092,12 +8092,11 @@ SDValue RISCVTargetLowering::lowerINSERT_VECTOR_ELT(SDValue Op,
     // If we're compiling for an exact VLEN value, we can always perform
     // the insert in m1 as we can determine the register corresponding to
     // the index in the register group.
-    const unsigned MinVLen = Subtarget.getRealMinVLen();
-    const unsigned MaxVLen = Subtarget.getRealMaxVLen();
     const MVT M1VT = getLMUL1VT(ContainerVT);
-    if (MinVLen == MaxVLen && ContainerVT.bitsGT(M1VT)) {
+    if (auto VLEN = Subtarget.getRealVLen();
+        VLEN && ContainerVT.bitsGT(M1VT)) {
       EVT ElemVT = VecVT.getVectorElementType();
-      unsigned ElemsPerVReg = MinVLen / ElemVT.getFixedSizeInBits();
+      unsigned ElemsPerVReg = *VLEN / ElemVT.getFixedSizeInBits();
       unsigned RemIdx = OrigIdx % ElemsPerVReg;
       unsigned SubRegIdx = OrigIdx / ElemsPerVReg;
       unsigned ExtractIdx =

llvm/lib/Target/RISCV/RISCVRegisterInfo.cpp

@@ -283,8 +283,8 @@ void RISCVRegisterInfo::lowerVSPILL(MachineBasicBlock::iterator II) const {
 
   Register VL = MRI.createVirtualRegister(&RISCV::GPRRegClass);
   // Optimize for constant VLEN.
-  if (STI.getRealMinVLen() == STI.getRealMaxVLen()) {
-    const int64_t VLENB = STI.getRealMinVLen() / 8;
+  if (auto VLEN = STI.getRealVLen()) {
+    const int64_t VLENB = *VLEN / 8;
     int64_t Offset = VLENB * LMUL;
     STI.getInstrInfo()->movImm(MBB, II, DL, VL, Offset);
   } else {
@@ -360,8 +360,8 @@ void RISCVRegisterInfo::lowerVRELOAD(MachineBasicBlock::iterator II) const {
 
   Register VL = MRI.createVirtualRegister(&RISCV::GPRRegClass);
   // Optimize for constant VLEN.
-  if (STI.getRealMinVLen() == STI.getRealMaxVLen()) {
-    const int64_t VLENB = STI.getRealMinVLen() / 8;
+  if (auto VLEN = STI.getRealVLen()) {
+    const int64_t VLENB = *VLEN / 8;
     int64_t Offset = VLENB * LMUL;
     STI.getInstrInfo()->movImm(MBB, II, DL, VL, Offset);
   } else {

llvm/lib/Target/RISCV/RISCVSubtarget.h

@@ -188,6 +188,14 @@ class RISCVSubtarget : public RISCVGenSubtargetInfo {
     unsigned VLen = getMaxRVVVectorSizeInBits();
     return VLen == 0 ? 65536 : VLen;
   }
+  // If we know the exact VLEN, return it.  Otherwise, return std::nullopt.
+  std::optional<unsigned> getRealVLen() const {
+    unsigned Min = getRealMinVLen();
+    if (Min != getRealMaxVLen())
+      return std::nullopt;
+    return Min;
+  }
+
   RISCVABI::ABI getTargetABI() const { return TargetABI; }
   bool isSoftFPABI() const {
     return TargetABI == RISCVABI::ABI_LP64 ||