[RISCV] Extract a utility for computing bounds on VLMAX [nfc]

Simplifying an upcoming change...

Author: preames

llvm/lib/Target/RISCV/RISCVISelLowering.cpp

@@ -2624,6 +2624,25 @@ SDValue RISCVTargetLowering::computeVLMax(MVT VecVT, const SDLoc &DL,
                              VecVT.getVectorElementCount());
 }
 
+std::pair<unsigned, unsigned>
+RISCVTargetLowering::computeVLMAXBounds(MVT VecVT,
+                                        const RISCVSubtarget &Subtarget) {
+  assert(VecVT.isScalableVector() && "Expected scalable vector");
+
+  unsigned EltSize = VecVT.getScalarSizeInBits();
+  unsigned MinSize = VecVT.getSizeInBits().getKnownMinValue();
+
+  unsigned VectorBitsMax = Subtarget.getRealMaxVLen();
+  unsigned MaxVLMAX =
+      RISCVTargetLowering::computeVLMAX(VectorBitsMax, EltSize, MinSize);
+
+  unsigned VectorBitsMin = Subtarget.getRealMinVLen();
+  unsigned MinVLMAX =
+      RISCVTargetLowering::computeVLMAX(VectorBitsMin, EltSize, MinSize);
+
+  return std::make_pair(MinVLMAX, MaxVLMAX);
+}
+
 // The state of RVV BUILD_VECTOR and VECTOR_SHUFFLE lowering is that very few
 // of either is (currently) supported. This can get us into an infinite loop
 // where we try to lower a BUILD_VECTOR as a VECTOR_SHUFFLE as a BUILD_VECTOR
@@ -8123,16 +8142,8 @@ static SDValue lowerVectorIntrinsicScalars(SDValue Op, SelectionDAG &DAG,
 
     // Optimize for constant AVL
     if (isa<ConstantSDNode>(AVL)) {
-      unsigned EltSize = VT.getScalarSizeInBits();
-      unsigned MinSize = VT.getSizeInBits().getKnownMinValue();
-
-      unsigned VectorBitsMax = Subtarget.getRealMaxVLen();
-      unsigned MaxVLMAX =
-          RISCVTargetLowering::computeVLMAX(VectorBitsMax, EltSize, MinSize);
-
-      unsigned VectorBitsMin = Subtarget.getRealMinVLen();
-      unsigned MinVLMAX =
-          RISCVTargetLowering::computeVLMAX(VectorBitsMin, EltSize, MinSize);
+      const auto [MinVLMAX, MaxVLMAX] =
+          RISCVTargetLowering::computeVLMAXBounds(VT, Subtarget);
 
       uint64_t AVLInt = cast<ConstantSDNode>(AVL)->getZExtValue();
       if (AVLInt <= MinVLMAX) {

llvm/lib/Target/RISCV/RISCVISelLowering.h

@@ -744,7 +744,13 @@ class RISCVTargetLowering : public TargetLowering {
     // The following equations have been reordered to prevent loss of precision
     // when calculating fractional LMUL.
     return ((VectorBits / EltSize) * MinSize) / RISCV::RVVBitsPerBlock;
-  };
+  }
+
+  // Return inclusive (low, high) bounds on the value of VLMAX for the
+  // given scalable container type given known bounds on VLEN.
+  static std::pair<unsigned, unsigned>
+  computeVLMAXBounds(MVT ContainerVT, const RISCVSubtarget &Subtarget);
+
   static unsigned getRegClassIDForLMUL(RISCVII::VLMUL LMul);
   static unsigned getSubregIndexByMVT(MVT VT, unsigned Index);
   static unsigned getRegClassIDForVecVT(MVT VT);