[RISCV] Add early-exit to RVV stack computation. NFCI.

[frasercrmck]

Summary

This patch was split off from D126465, where an early-exit is necessary
as it checks the VLEN and that asserts that V instructions are present.

Since this makes logical sense on its own, I think it's worth landing
regardless of D126465.

[kito-cheng]

LGTM

llvm/lib/Target/RISCV/RISCVFrameLowering.h

private:
   void determineFrameLayout(MachineFunction &MF) const;
   void adjustReg(MachineBasicBlock &MBB, MachineBasicBlock::iterator MBBI,
                  const DebugLoc &DL, Register DestReg, Register SrcReg,
                  int64_t Val, MachineInstr::MIFlag Flag) const;
   void adjustStackForRVV(MachineFunction &MF, MachineBasicBlock &MBB,
                          MachineBasicBlock::iterator MBBI, const DebugLoc &DL,
                          int64_t Amount, MachineInstr::MIFlag Flag) const;
   std::pair<int64_t, Align>
-  assignRVVStackObjectOffsets(MachineFrameInfo &MFI) const;
+  assignRVVStackObjectOffsets(MachineFunction &MF) const;
 };
-}
+} // namespace llvm
 #endif

llvm/lib/Target/RISCV/RISCVFrameLowering.cpp

if (MF.getSubtarget<RISCVSubtarget>().hasStdExtF()) {
             RISCV::FPR32RegClass.contains(Regs[i]) ||
             RISCV::FPR64RegClass.contains(Regs[i]))
           SavedRegs.set(Regs[i]);
     }
   }
 }
 
 std::pair<int64_t, Align>
-RISCVFrameLowering::assignRVVStackObjectOffsets(MachineFrameInfo &MFI) const {
+RISCVFrameLowering::assignRVVStackObjectOffsets(MachineFunction &MF) const {
+  MachineFrameInfo &MFI = MF.getFrameInfo();
   // Create a buffer of RVV objects to allocate.
   SmallVector<int, 8> ObjectsToAllocate;
   for (int I = 0, E = MFI.getObjectIndexEnd(); I != E; ++I) {
     unsigned StackID = MFI.getStackID(I);
     if (StackID != TargetStackID::ScalableVector)
       continue;
     if (MFI.isDeadObjectIndex(I))
       continue;
 
     ObjectsToAllocate.push_back(I);
   }
 
-  // Allocate all RVV locals and spills
-  int64_t Offset = 0;
   // The minimum alignment is 16 bytes.
   Align RVVStackAlign(16);
+  const auto &ST = MF.getSubtarget<RISCVSubtarget>();
+
+  if (!ST.hasVInstructions()) {
+    assert(ObjectsToAllocate.empty() &&
+           "Can't allocate scalable-vector objects without V instructions");
+    return std::make_pair(0, RVVStackAlign);
+  }
+
+  // Allocate all RVV locals and spills
+  int64_t Offset = 0;
   for (int FI : ObjectsToAllocate) {
     // ObjectSize in bytes.
     int64_t ObjectSize = MFI.getObjectSize(FI);
     auto ObjectAlign = std::max(Align(8), MFI.getObjectAlign(FI));
     // If the data type is the fractional vector type, reserve one vector
     // register for it.
     if (ObjectSize < 8)
       ObjectSize = 8;
void RISCVFrameLowering::processFunctionBeforeFrameFinalized(
   const RISCVRegisterInfo *RegInfo =
       MF.getSubtarget<RISCVSubtarget>().getRegisterInfo();
   MachineFrameInfo &MFI = MF.getFrameInfo();
   const TargetRegisterClass *RC = &RISCV::GPRRegClass;
   auto *RVFI = MF.getInfo<RISCVMachineFunctionInfo>();
 
   int64_t RVVStackSize;
   Align RVVStackAlign;
-  std::tie(RVVStackSize, RVVStackAlign) = assignRVVStackObjectOffsets(MFI);
+  std::tie(RVVStackSize, RVVStackAlign) = assignRVVStackObjectOffsets(MF);
 
   RVFI->setRVVStackSize(RVVStackSize);
   RVFI->setRVVStackAlign(RVVStackAlign);
 
   // Ensure the entire stack is aligned to at least the RVV requirement: some
   // scalable-vector object alignments are not considered by the
   // target-independent code.
   MFI.ensureMaxAlignment(RVVStackAlign);