[llvm] [AArch64][SME] Implement the SME ABI (ZA state management) in Machine IR (PR #149062)
Gaëtan Bossu via llvm-commits
llvm-commits at lists.llvm.org
Thu Aug 14 08:44:14 PDT 2025
================
@@ -0,0 +1,654 @@
+//===- MachineSMEABIPass.cpp ----------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// This pass implements the SME ABI requirements for ZA state. This includes
+// implementing the lazy ZA state save schemes around calls.
+//
+//===----------------------------------------------------------------------===//
+
+#include "AArch64InstrInfo.h"
+#include "AArch64MachineFunctionInfo.h"
+#include "AArch64Subtarget.h"
+#include "MCTargetDesc/AArch64AddressingModes.h"
+#include "llvm/ADT/BitmaskEnum.h"
+#include "llvm/ADT/SmallVector.h"
+#include "llvm/CodeGen/EdgeBundles.h"
+#include "llvm/CodeGen/LivePhysRegs.h"
+#include "llvm/CodeGen/MachineBasicBlock.h"
+#include "llvm/CodeGen/MachineFunctionPass.h"
+#include "llvm/CodeGen/MachineRegisterInfo.h"
+#include "llvm/CodeGen/TargetRegisterInfo.h"
+
+using namespace llvm;
+
+#define DEBUG_TYPE "aarch64-machine-sme-abi"
+
+namespace {
+
+enum ZAState {
+ // Any/unknown state (not valid)
+ ANY = 0,
+
+ // ZA is in use and active (i.e. within the accumulator)
+ ACTIVE,
+
+ // A ZA save has been set up or committed (i.e. ZA is dormant or off)
+ LOCAL_SAVED,
+
+ // ZA is off or a lazy save has been set up by the caller
+ CALLER_DORMANT,
+
+ // ZA is off
+ OFF,
+
+ // The number of ZA states (not a valid state)
+ NUM_ZA_STATE
+};
+
+/// A bitmask enum to record live physical registers that the "emit*" routines
+/// may need to preserve. Note: This only tracks registers we may clobber.
+enum LiveRegs : uint8_t {
+ None = 0,
+ NZCV = 1 << 0,
+ W0 = 1 << 1,
+ W0_HI = 1 << 2,
+ X0 = W0 | W0_HI,
+ LLVM_MARK_AS_BITMASK_ENUM(/* LargestValue = */ W0_HI)
+};
+
+/// Holds the virtual registers live physical registers have been saved to.
+struct PhysRegSave {
+ LiveRegs PhysLiveRegs;
+ Register StatusFlags = AArch64::NoRegister;
+ Register X0Save = AArch64::NoRegister;
+};
+
+static bool isLegalEdgeBundleZAState(ZAState State) {
+ switch (State) {
+ case ZAState::ACTIVE:
+ case ZAState::LOCAL_SAVED:
+ return true;
+ default:
+ return false;
+ }
+}
+struct TPIDR2State {
+ int FrameIndex = -1;
+};
+
+StringRef getZAStateString(ZAState State) {
+#define MAKE_CASE(V) \
+ case V: \
+ return #V;
+ switch (State) {
+ MAKE_CASE(ZAState::ANY)
+ MAKE_CASE(ZAState::ACTIVE)
+ MAKE_CASE(ZAState::LOCAL_SAVED)
+ MAKE_CASE(ZAState::CALLER_DORMANT)
+ MAKE_CASE(ZAState::OFF)
+ default:
+ llvm_unreachable("Unexpected ZAState");
+ }
+#undef MAKE_CASE
+}
+
+static bool isZAorZT0RegOp(const TargetRegisterInfo &TRI,
+ const MachineOperand &MO) {
+ if (!MO.isReg() || !MO.getReg().isPhysical())
+ return false;
+ return any_of(TRI.subregs_inclusive(MO.getReg()), [](const MCPhysReg &SR) {
+ return AArch64::MPR128RegClass.contains(SR) ||
+ AArch64::ZTRRegClass.contains(SR);
+ });
+}
+
+/// Returns the required ZA state needed before \p MI and an iterator pointing
+/// to where any code required to change the ZA state should be inserted.
+static std::pair<ZAState, MachineBasicBlock::iterator>
+getZAStateBeforeInst(const TargetRegisterInfo &TRI, MachineInstr &MI,
+ bool ZALiveAtReturn) {
+ MachineBasicBlock::iterator InsertPt(MI);
+
+ if (MI.getOpcode() == AArch64::InOutZAUsePseudo)
+ return {ZAState::ACTIVE, std::prev(InsertPt)};
+
+ if (MI.getOpcode() == AArch64::RequiresZASavePseudo)
+ return {ZAState::LOCAL_SAVED, std::prev(InsertPt)};
+
+ if (MI.isReturn())
+ return {ZALiveAtReturn ? ZAState::ACTIVE : ZAState::OFF, InsertPt};
+
+ for (auto &MO : MI.operands()) {
+ if (isZAorZT0RegOp(TRI, MO))
+ return {ZAState::ACTIVE, InsertPt};
+ }
+
+ return {ZAState::ANY, InsertPt};
+}
+
+struct MachineSMEABI : public MachineFunctionPass {
+ inline static char ID = 0;
+
+ MachineSMEABI() : MachineFunctionPass(ID) {}
+
+ bool runOnMachineFunction(MachineFunction &MF) override;
+
+ StringRef getPassName() const override { return "Machine SME ABI pass"; }
+
+ void getAnalysisUsage(AnalysisUsage &AU) const override {
+ AU.setPreservesCFG();
+ AU.addRequired<EdgeBundlesWrapperLegacy>();
+ AU.addPreservedID(MachineLoopInfoID);
+ AU.addPreservedID(MachineDominatorsID);
+ MachineFunctionPass::getAnalysisUsage(AU);
+ }
+
+ /// Collects the needed ZA state (and live registers) before each instruction
+ /// within the machine function.
+ void collectNeededZAStates(SMEAttrs);
+
+ /// Assigns each edge bundle a ZA state based on the needed states of blocks
+ /// that have incoming or outgoing edges in that bundle.
+ void assignBundleZAStates();
+
+ /// Inserts code to handle changes between ZA states within the function.
+ /// E.g., ACTIVE -> LOCAL_SAVED will insert code required to save ZA.
+ void insertStateChanges();
+
+ // Emission routines for private and shared ZA functions (using lazy saves).
+ void emitNewZAPrologue(MachineBasicBlock &MBB,
+ MachineBasicBlock::iterator MBBI);
+ void emitRestoreLazySave(MachineBasicBlock &MBB,
+ MachineBasicBlock::iterator MBBI,
+ LiveRegs PhysLiveRegs);
+ void emitSetupLazySave(MachineBasicBlock &MBB,
+ MachineBasicBlock::iterator MBBI);
+ void emitAllocateLazySaveBuffer(MachineBasicBlock &MBB,
+ MachineBasicBlock::iterator MBBI);
+ void emitZAOff(MachineBasicBlock &MBB, MachineBasicBlock::iterator MBBI,
+ bool ClearTPIDR2);
+
+ void emitStateChange(MachineBasicBlock &MBB, MachineBasicBlock::iterator MBBI,
+ ZAState From, ZAState To, LiveRegs PhysLiveRegs);
+
+ /// Save live physical registers to virtual registers.
+ PhysRegSave createPhysRegSave(LiveRegs PhysLiveRegs, MachineBasicBlock &MBB,
+ MachineBasicBlock::iterator MBBI, DebugLoc DL);
+ /// Restore physical registers from a save of their previous values.
+ void restorePhyRegSave(PhysRegSave const &RegSave, MachineBasicBlock &MBB,
+ MachineBasicBlock::iterator MBBI, DebugLoc DL);
+
+ /// Get or create a TPIDR2 block in this function.
+ TPIDR2State getTPIDR2Block();
+
+private:
+ /// Contains the needed ZA state (and live registers) at an instruction.
+ struct InstInfo {
+ ZAState NeededState{ZAState::ANY};
+ MachineBasicBlock::iterator InsertPt;
+ LiveRegs PhysLiveRegs = LiveRegs::None;
+ };
+
+ /// Contains the needed ZA state for each instruction in a block.
+ /// Instructions that do not require a ZA state are not recorded.
+ struct BlockInfo {
+ ZAState FixedEntryState{ZAState::ANY};
+ SmallVector<InstInfo> Insts;
+ LiveRegs PhysLiveRegsAtExit = LiveRegs::None;
+ };
+
+ // All pass state that must be cleared between functions.
+ struct PassState {
+ SmallVector<BlockInfo> Blocks;
+ SmallVector<ZAState> BundleStates;
+ std::optional<TPIDR2State> TPIDR2Block;
+ } State;
+
+ MachineFunction *MF = nullptr;
+ EdgeBundles *Bundles = nullptr;
+ const AArch64Subtarget *Subtarget = nullptr;
+ const AArch64RegisterInfo *TRI = nullptr;
+ const TargetInstrInfo *TII = nullptr;
+ MachineRegisterInfo *MRI = nullptr;
+};
+
+void MachineSMEABI::collectNeededZAStates(SMEAttrs SMEFnAttrs) {
+ assert((SMEFnAttrs.hasZT0State() || SMEFnAttrs.hasZAState()) &&
+ "Expected function to have ZA/ZT0 state!");
+
+ State.Blocks.resize(MF->getNumBlockIDs());
+ for (MachineBasicBlock &MBB : *MF) {
+ BlockInfo &Block = State.Blocks[MBB.getNumber()];
+ if (&MBB == &MF->front()) {
+ // Entry block:
+ Block.FixedEntryState = SMEFnAttrs.hasPrivateZAInterface()
+ ? ZAState::CALLER_DORMANT
+ : ZAState::ACTIVE;
+ } else if (MBB.isEHPad()) {
+ // EH entry block:
+ Block.FixedEntryState = ZAState::LOCAL_SAVED;
+ }
+
+ LiveRegUnits LiveUnits(*TRI);
+ LiveUnits.addLiveOuts(MBB);
+
+ auto GetPhysLiveRegs = [&] {
+ LiveRegs PhysLiveRegs = LiveRegs::None;
+ if (!LiveUnits.available(AArch64::NZCV))
+ PhysLiveRegs |= LiveRegs::NZCV;
+ // We have to track W0 and X0 separately as otherwise things can get
+ // confused if we attempt to preserve X0 but only W0 was defined.
+ if (!LiveUnits.available(AArch64::W0))
+ PhysLiveRegs |= LiveRegs::W0;
+ if (!LiveUnits.available(AArch64::W0_HI))
+ PhysLiveRegs |= LiveRegs::W0_HI;
+ return PhysLiveRegs;
+ };
+
+ Block.PhysLiveRegsAtExit = GetPhysLiveRegs();
+ auto FirstTerminatorInsertPt = MBB.getFirstTerminator();
+ for (MachineInstr &MI : reverse(MBB)) {
+ MachineBasicBlock::iterator MBBI(MI);
+ LiveUnits.stepBackward(MI);
+ LiveRegs PhysLiveRegs = GetPhysLiveRegs();
+ auto [NeededState, InsertPt] = getZAStateBeforeInst(
+ *TRI, MI, /*ZALiveAtReturn=*/SMEFnAttrs.hasSharedZAInterface());
+ assert((InsertPt == MBBI ||
+ InsertPt->getOpcode() == AArch64::ADJCALLSTACKDOWN) &&
+ "Unexpected state change insertion point!");
+ // TODO: Do something to avoid state changes where NZCV is live.
+ if (MBBI == FirstTerminatorInsertPt)
+ Block.PhysLiveRegsAtExit = PhysLiveRegs;
+ if (NeededState != ZAState::ANY)
+ Block.Insts.push_back({NeededState, InsertPt, PhysLiveRegs});
+ }
+
+ // Reverse vector (as we had to iterate backwards for liveness).
+ std::reverse(Block.Insts.begin(), Block.Insts.end());
+ }
+}
+
+void MachineSMEABI::assignBundleZAStates() {
+ State.BundleStates.resize(Bundles->getNumBundles());
+ for (unsigned I = 0, E = Bundles->getNumBundles(); I != E; ++I) {
+ LLVM_DEBUG(dbgs() << "Assigning ZA state for edge bundle: " << I << '\n');
+
+ // Attempt to assign a ZA state for this bundle that minimizes state
+ // transitions. Edges within loops are given a higher weight as we assume
+ // they will be executed more than once.
+ // TODO: We should propagate desired incoming/outgoing states through blocks
+ // that have the "ANY" state first to make better global decisions.
+ int EdgeStateCounts[ZAState::NUM_ZA_STATE] = {0};
+ for (unsigned BlockID : Bundles->getBlocks(I)) {
+ LLVM_DEBUG(dbgs() << "- bb." << BlockID);
+
+ BlockInfo &Block = State.Blocks[BlockID];
+ if (Block.Insts.empty()) {
+ LLVM_DEBUG(dbgs() << " (no state preference)\n");
+ continue;
+ }
+ bool InEdge = Bundles->getBundle(BlockID, /*Out=*/false) == I;
+ bool OutEdge = Bundles->getBundle(BlockID, /*Out=*/true) == I;
+
+ ZAState DesiredIncomingState = Block.Insts.front().NeededState;
+ if (InEdge && isLegalEdgeBundleZAState(DesiredIncomingState)) {
+ EdgeStateCounts[DesiredIncomingState]++;
+ LLVM_DEBUG(dbgs() << " DesiredIncomingState: "
+ << getZAStateString(DesiredIncomingState));
+ }
+ ZAState DesiredOutgoingState = Block.Insts.back().NeededState;
+ if (OutEdge && isLegalEdgeBundleZAState(DesiredOutgoingState)) {
+ EdgeStateCounts[DesiredOutgoingState]++;
+ LLVM_DEBUG(dbgs() << " DesiredOutgoingState: "
+ << getZAStateString(DesiredOutgoingState));
+ }
+ LLVM_DEBUG(dbgs() << '\n');
+ }
+
+ ZAState BundleState =
+ ZAState(max_element(EdgeStateCounts) - EdgeStateCounts);
+
+ // Force ZA to be active in bundles that don't have a preferred state.
+ // TODO: Something better here (to avoid extra mode switches).
+ if (BundleState == ZAState::ANY)
+ BundleState = ZAState::ACTIVE;
+
+ LLVM_DEBUG({
+ dbgs() << "Chosen ZA state: " << getZAStateString(BundleState) << '\n'
+ << "Edge counts:";
+ for (auto [State, Count] : enumerate(EdgeStateCounts))
+ dbgs() << " " << getZAStateString(ZAState(State)) << ": " << Count;
+ dbgs() << "\n\n";
+ });
+
+ State.BundleStates[I] = BundleState;
+ }
+}
+
+void MachineSMEABI::insertStateChanges() {
+ for (MachineBasicBlock &MBB : *MF) {
+ BlockInfo &Block = State.Blocks[MBB.getNumber()];
+ ZAState InState =
+ State.BundleStates[Bundles->getBundle(MBB.getNumber(), /*Out=*/false)];
+
+ ZAState CurrentState = Block.FixedEntryState;
+ if (CurrentState == ZAState::ANY)
+ CurrentState = InState;
+
+ for (auto &Inst : Block.Insts) {
+ if (CurrentState != Inst.NeededState)
+ emitStateChange(MBB, Inst.InsertPt, CurrentState, Inst.NeededState,
+ Inst.PhysLiveRegs);
+ CurrentState = Inst.NeededState;
+ }
----------------
gbossu wrote:
I like the approach, I think it's great to first collect all the required states, and them emit the right code if a transition is detected. :)
https://github.com/llvm/llvm-project/pull/149062
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