[RISCV] Add scalable offset under very large stack size.

If the stack size is larger than 12 bits, we have to use a scratch
register to store the stack size. Before we introduce the scalable stack
offset, we could simplify

%0 = ADDI %stack.0, 0

=>

%scratch = ... # sequence of instructions to move the offset into
%%scratch
%0 = ADD %fp, %scratch

However, if the offset contains scalable part, we need to consider it.

%0 = ADDI %stack.0, 0

=>

%scratch = ... # sequence of instructions to move the offset into
%%scratch
%scratch = ADD %fp, %scratch
%scalable_offset = ... # sequence of instructions for vscaled-offset.
%0 = ADD/SUB %scratch, %scalable_offset

Differential Revision: https://reviews.llvm.org/D100035

llvm/lib/Target/RISCV/RISCVRegisterInfo.cpp

@@ -213,7 +213,7 @@ void RISCVRegisterInfo::eliminateFrameIndex(MachineBasicBlock::iterator II,
     // Modify Offset and FrameReg appropriately
     Register ScratchReg = MRI.createVirtualRegister(&RISCV::GPRRegClass);
     TII->movImm(MBB, II, DL, ScratchReg, Offset.getFixed());
-    if (MI.getOpcode() == RISCV::ADDI) {
+    if (MI.getOpcode() == RISCV::ADDI && !Offset.getScalable()) {
       BuildMI(MBB, II, DL, TII->get(RISCV::ADD), MI.getOperand(0).getReg())
         .addReg(FrameReg)
         .addReg(ScratchReg, RegState::Kill);
@@ -258,6 +258,13 @@ void RISCVRegisterInfo::eliminateFrameIndex(MachineBasicBlock::iterator II,
         TII->getVLENFactoredAmount(MF, MBB, II, ScalableValue);
 
     // 2. Calculate address: FrameReg + result of multiply
+    if (MI.getOpcode() == RISCV::ADDI && !Offset.getFixed()) {
+      BuildMI(MBB, II, DL, TII->get(Opc), MI.getOperand(0).getReg())
+          .addReg(FrameReg, getKillRegState(FrameRegIsKill))
+          .addReg(FactorRegister, RegState::Kill);
+      MI.eraseFromParent();
+      return;
+    }
     Register VL = MRI.createVirtualRegister(&RISCV::GPRRegClass);
     BuildMI(MBB, II, DL, TII->get(Opc), VL)
         .addReg(FrameReg, getKillRegState(FrameRegIsKill))

llvm/test/CodeGen/RISCV/rvv/addi-scalable-offset.mir

@@ -43,6 +43,8 @@ body: |
     ; CHECK: $x10 = LUI 1048575
     ; CHECK: $x10 = ADDIW killed $x10, 1824
     ; CHECK: $x10 = ADD $x8, killed $x10
+    ; CHECK: $x11 = PseudoReadVLENB
+    ; CHECK: $x10 = SUB killed $x10, killed $x11
     ; CHECK: VS1R_V killed renamable $v25, killed renamable $x10
     ; CHECK: $x10 = PseudoReadVLENB
     ; CHECK: $x2 = ADD $x2, killed $x10