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

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.

Reviewed By: kito-cheng

Differential Revision: https://reviews.llvm.org/D129617

llvm/lib/Target/RISCV/RISCVFrameLowering.cpp

@@ -899,7 +899,8 @@ void RISCVFrameLowering::determineCalleeSaves(MachineFunction &MF,
 }
 
 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) {
@@ -912,10 +913,18 @@ RISCVFrameLowering::assignRVVStackObjectOffsets(MachineFrameInfo &MFI) const {
     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);
@@ -997,7 +1006,7 @@ void RISCVFrameLowering::processFunctionBeforeFrameFinalized(
 
   int64_t RVVStackSize;
   Align RVVStackAlign;
-  std::tie(RVVStackSize, RVVStackAlign) = assignRVVStackObjectOffsets(MFI);
+  std::tie(RVVStackSize, RVVStackAlign) = assignRVVStackObjectOffsets(MF);
 
   RVFI->setRVVStackSize(RVVStackSize);
   RVFI->setRVVStackAlign(RVVStackAlign);

llvm/lib/Target/RISCV/RISCVFrameLowering.h

@@ -84,7 +84,7 @@ class RISCVFrameLowering : public TargetFrameLowering {
                          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