[AArch64][SME] Reuse ZT0 spill slot

Previously, we'd allocate a new spill slot each time we needed to spill
ZT0, which grows the stack size for each spill. Saving the spill slot
in FuncInfo will also allow us to reload the spill on entry to exception
handlers.

Author: MacDue

llvm/lib/Target/AArch64/AArch64ISelLowering.cpp

@@ -8023,6 +8023,17 @@ static bool isPassedInFPR(EVT VT) {
          (VT.isFloatingPoint() && !VT.isScalableVector());
 }
 
+static SDValue getZT0FrameIndex(MachineFrameInfo &MFI,
+                                AArch64FunctionInfo &FuncInfo,
+                                SelectionDAG &DAG) {
+  if (!FuncInfo.hasZT0SpillSlotIndex())
+    FuncInfo.setZT0SpillSlotIndex(MFI.CreateSpillStackObject(64, Align(16)));
+
+  return DAG.getFrameIndex(
+      FuncInfo.getZT0SpillSlotIndex(),
+      DAG.getTargetLoweringInfo().getFrameIndexTy(DAG.getDataLayout()));
+}
+
 SDValue AArch64TargetLowering::lowerEHPadEntry(SDValue Chain, SDLoc const &DL,
                                                SelectionDAG &DAG) const {
   assert(Chain.getOpcode() == ISD::EntryToken && "Unexpected Chain value");
@@ -9427,10 +9438,7 @@ AArch64TargetLowering::LowerCall(CallLoweringInfo &CLI,
   // If the caller has ZT0 state which will not be preserved by the callee,
   // spill ZT0 before the call.
   if (ShouldPreserveZT0) {
-    unsigned ZTObj = MFI.CreateSpillStackObject(64, Align(16));
-    ZTFrameIdx = DAG.getFrameIndex(
-        ZTObj,
-        DAG.getTargetLoweringInfo().getFrameIndexTy(DAG.getDataLayout()));
+    ZTFrameIdx = getZT0FrameIndex(MFI, *FuncInfo, DAG);
 
     Chain = DAG.getNode(AArch64ISD::SAVE_ZT, DL, DAG.getVTList(MVT::Other),
                         {Chain, DAG.getConstant(0, DL, MVT::i32), ZTFrameIdx});

llvm/lib/Target/AArch64/AArch64MachineFunctionInfo.h

@@ -239,6 +239,9 @@ class AArch64FunctionInfo final : public MachineFunctionInfo {
   // support).
   Register EarlyAllocSMESaveBuffer = AArch64::NoRegister;
 
+  // Holds the spill slot for ZT0.
+  int ZT0SpillSlotIndex = std::numeric_limits<int>::max();
+
   // Note: The following properties are only used for the old SME ABI lowering:
   /// The frame-index for the TPIDR2 object used for lazy saves.
   TPIDR2Object TPIDR2;
@@ -265,6 +268,15 @@ class AArch64FunctionInfo final : public MachineFunctionInfo {
     return EarlyAllocSMESaveBuffer;
   }
 
+  void setZT0SpillSlotIndex(int FI) { ZT0SpillSlotIndex = FI; }
+  int getZT0SpillSlotIndex() const {
+    assert(hasZT0SpillSlotIndex() && "ZT0 spill slot index not set!");
+    return ZT0SpillSlotIndex;
+  }
+  bool hasZT0SpillSlotIndex() const {
+    return ZT0SpillSlotIndex != std::numeric_limits<int>::max();
+  }
+
   // Old SME ABI lowering state getters/setters:
   Register getSMESaveBufferAddr() const { return SMESaveBufferAddr; };
   void setSMESaveBufferAddr(Register Reg) { SMESaveBufferAddr = Reg; };

llvm/test/CodeGen/AArch64/sme-peephole-opts.ll

@@ -224,22 +224,21 @@ define float @test6(float %f) nounwind "aarch64_pstate_sm_enabled" {
 define void @test7() nounwind "aarch64_inout_zt0" {
 ; CHECK-LABEL: test7:
 ; CHECK:       // %bb.0:
-; CHECK-NEXT:    sub sp, sp, #144
-; CHECK-NEXT:    stp x30, x19, [sp, #128] // 16-byte Folded Spill
-; CHECK-NEXT:    add x19, sp, #64
+; CHECK-NEXT:    sub sp, sp, #80
+; CHECK-NEXT:    stp x30, x19, [sp, #64] // 16-byte Folded Spill
+; CHECK-NEXT:    mov x19, sp
 ; CHECK-NEXT:    str zt0, [x19]
 ; CHECK-NEXT:    smstop za
 ; CHECK-NEXT:    bl callee
 ; CHECK-NEXT:    smstart za
 ; CHECK-NEXT:    ldr zt0, [x19]
-; CHECK-NEXT:    mov x19, sp
 ; CHECK-NEXT:    str zt0, [x19]
 ; CHECK-NEXT:    smstop za
 ; CHECK-NEXT:    bl callee
 ; CHECK-NEXT:    smstart za
 ; CHECK-NEXT:    ldr zt0, [x19]
-; CHECK-NEXT:    ldp x30, x19, [sp, #128] // 16-byte Folded Reload
-; CHECK-NEXT:    add sp, sp, #144
+; CHECK-NEXT:    ldp x30, x19, [sp, #64] // 16-byte Folded Reload
+; CHECK-NEXT:    add sp, sp, #80
 ; CHECK-NEXT:    ret
   call void @callee()
   call void @callee()

llvm/test/CodeGen/AArch64/sme-zt0-state.ll

@@ -391,37 +391,34 @@ define void @shared_za_new_zt0(ptr %callee) "aarch64_inout_za" "aarch64_new_zt0"
 define void @zt0_multiple_private_za_calls(ptr %callee) "aarch64_in_zt0" nounwind {
 ; CHECK-COMMON-LABEL: zt0_multiple_private_za_calls:
 ; CHECK-COMMON:       // %bb.0:
-; CHECK-COMMON-NEXT:    sub sp, sp, #288
-; CHECK-COMMON-NEXT:    stp x20, x19, [sp, #272] // 16-byte Folded Spill
-; CHECK-COMMON-NEXT:    add x20, sp, #192
+; CHECK-COMMON-NEXT:    sub sp, sp, #96
+; CHECK-COMMON-NEXT:    stp x20, x19, [sp, #80] // 16-byte Folded Spill
+; CHECK-COMMON-NEXT:    mov x20, sp
 ; CHECK-COMMON-NEXT:    mov x19, x0
-; CHECK-COMMON-NEXT:    stp x29, x30, [sp, #256] // 16-byte Folded Spill
+; CHECK-COMMON-NEXT:    str x30, [sp, #64] // 8-byte Folded Spill
 ; CHECK-COMMON-NEXT:    str zt0, [x20]
 ; CHECK-COMMON-NEXT:    smstop za
 ; CHECK-COMMON-NEXT:    blr x0
 ; CHECK-COMMON-NEXT:    smstart za
 ; CHECK-COMMON-NEXT:    ldr zt0, [x20]
-; CHECK-COMMON-NEXT:    add x20, sp, #128
 ; CHECK-COMMON-NEXT:    str zt0, [x20]
 ; CHECK-COMMON-NEXT:    smstop za
 ; CHECK-COMMON-NEXT:    blr x19
 ; CHECK-COMMON-NEXT:    smstart za
 ; CHECK-COMMON-NEXT:    ldr zt0, [x20]
-; CHECK-COMMON-NEXT:    add x20, sp, #64
 ; CHECK-COMMON-NEXT:    str zt0, [x20]
 ; CHECK-COMMON-NEXT:    smstop za
 ; CHECK-COMMON-NEXT:    blr x19
 ; CHECK-COMMON-NEXT:    smstart za
 ; CHECK-COMMON-NEXT:    ldr zt0, [x20]
-; CHECK-COMMON-NEXT:    mov x20, sp
 ; CHECK-COMMON-NEXT:    str zt0, [x20]
 ; CHECK-COMMON-NEXT:    smstop za
 ; CHECK-COMMON-NEXT:    blr x19
 ; CHECK-COMMON-NEXT:    smstart za
 ; CHECK-COMMON-NEXT:    ldr zt0, [x20]
-; CHECK-COMMON-NEXT:    ldp x20, x19, [sp, #272] // 16-byte Folded Reload
-; CHECK-COMMON-NEXT:    ldp x29, x30, [sp, #256] // 16-byte Folded Reload
-; CHECK-COMMON-NEXT:    add sp, sp, #288
+; CHECK-COMMON-NEXT:    ldp x20, x19, [sp, #80] // 16-byte Folded Reload
+; CHECK-COMMON-NEXT:    ldr x30, [sp, #64] // 8-byte Folded Reload
+; CHECK-COMMON-NEXT:    add sp, sp, #96
 ; CHECK-COMMON-NEXT:    ret
   call void %callee()
   call void %callee()