[AArch64][SME] Propagate desired ZA states in the MachineSMEABIPass

This patch adds a propagation step to the MachineSMEABIPass that
propagates desired ZA states forwards/backwards (from predecessors to
successors, or vice versa).

The aim of this is to pick better ZA states for edge bundles, as when
many (or all) blocks in a bundle do not have a preferred ZA state, the
ZA state assigned to a bundle can be less than ideal.

An important case is nested loops, where only the inner loop has a
preferred ZA state. Here we'd like to propagate the ZA state up from the
inner loop to the outer loops (to avoid saves/restores in any loop).

Change-Id: I39f9c7d7608e2fa070be2fb88351b4d1d0079041

Author: MacDue

llvm/lib/Target/AArch64/MachineSMEABIPass.cpp

@@ -121,8 +121,10 @@ struct InstInfo {
 /// 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;
+  ZAState FixedEntryState{ZAState::ANY};
+  ZAState DesiredIncomingState{ZAState::ANY};
+  ZAState DesiredOutgoingState{ZAState::ANY};
   LiveRegs PhysLiveRegsAtEntry = LiveRegs::None;
   LiveRegs PhysLiveRegsAtExit = LiveRegs::None;
 };
@@ -268,6 +270,11 @@ struct MachineSMEABI : public MachineFunctionPass {
                           const EdgeBundles &Bundles,
                           ArrayRef<ZAState> BundleStates);
 
+  /// Propagates desired states forwards (from predecessors -> successors) if
+  /// \p Forwards, otherwise, propagates backwards (from successors ->
+  /// predecessors).
+  void propagateDesiredStates(FunctionInfo &FnInfo, bool Forwards = true);
+
   // Emission routines for private and shared ZA functions (using lazy saves).
   void emitNewZAPrologue(MachineBasicBlock &MBB,
                          MachineBasicBlock::iterator MBBI);
@@ -411,12 +418,70 @@ FunctionInfo MachineSMEABI::collectNeededZAStates(SMEAttrs SMEFnAttrs) {
 
     // Reverse vector (as we had to iterate backwards for liveness).
     std::reverse(Block.Insts.begin(), Block.Insts.end());
+
+    // Record the desired states on entry/exit of this block. These are the
+    // states that would not incur a state transition.
+    if (!Block.Insts.empty()) {
+      Block.DesiredIncomingState = Block.Insts.front().NeededState;
+      Block.DesiredOutgoingState = Block.Insts.back().NeededState;
+    }
   }
 
   return FunctionInfo{std::move(Blocks), AfterSMEProloguePt,
                       PhysLiveRegsAfterSMEPrologue};
 }
 
+void MachineSMEABI::propagateDesiredStates(FunctionInfo &FnInfo,
+                                           bool Forwards) {
+  // If `Forwards`, this propagates desired states from predecessors to
+  // successors, otherwise, this propagates states from successors to
+  // predecessors.
+  auto GetBlockState = [](BlockInfo &Block, bool Incoming) -> ZAState & {
+    return Incoming ? Block.DesiredIncomingState : Block.DesiredOutgoingState;
+  };
+
+  SmallVector<MachineBasicBlock *> Worklist;
+  for (auto [BlockID, BlockInfo] : enumerate(FnInfo.Blocks)) {
+    if (!isLegalEdgeBundleZAState(GetBlockState(BlockInfo, Forwards)))
+      Worklist.push_back(MF->getBlockNumbered(BlockID));
+  }
+
+  while (!Worklist.empty()) {
+    MachineBasicBlock *MBB = Worklist.pop_back_val();
+    auto &BlockInfo = FnInfo.Blocks[MBB->getNumber()];
+
+    // Pick a legal edge bundle state that matches the majority of
+    // predecessors/successors.
+    int StateCounts[ZAState::NUM_ZA_STATE] = {0};
+    for (MachineBasicBlock *PredOrSucc :
+         Forwards ? predecessors(MBB) : successors(MBB)) {
+      auto &PredOrSuccBlockInfo = FnInfo.Blocks[PredOrSucc->getNumber()];
+      auto ZAState = GetBlockState(PredOrSuccBlockInfo, !Forwards);
+      if (isLegalEdgeBundleZAState(ZAState))
+        StateCounts[ZAState]++;
+    }
+
+    ZAState PropagatedState = ZAState(max_element(StateCounts) - StateCounts);
+    auto &CurrentState = GetBlockState(BlockInfo, Forwards);
+    if (PropagatedState != CurrentState) {
+      CurrentState = PropagatedState;
+      auto &OtherState = GetBlockState(BlockInfo, !Forwards);
+      // Propagate to the incoming/outgoing state if that is also "ANY".
+      if (OtherState == ZAState::ANY)
+        OtherState = PropagatedState;
+      // Push any successors/predecessors that may need updating to the
+      // worklist.
+      for (MachineBasicBlock *SuccOrPred :
+           Forwards ? successors(MBB) : predecessors(MBB)) {
+        auto &SuccOrPredBlockInfo = FnInfo.Blocks[SuccOrPred->getNumber()];
+        if (!isLegalEdgeBundleZAState(
+                GetBlockState(SuccOrPredBlockInfo, Forwards)))
+          Worklist.push_back(SuccOrPred);
+      }
+    }
+  }
+}
+
 /// Assigns each edge bundle a ZA state based on the needed states of blocks
 /// that have incoming or outgoing edges in that bundle.
 SmallVector<ZAState>
@@ -429,40 +494,36 @@ MachineSMEABI::assignBundleZAStates(const EdgeBundles &Bundles,
     // 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);
 
       const BlockInfo &Block = FnInfo.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]++;
+      bool LegalInEdge =
+          InEdge && isLegalEdgeBundleZAState(Block.DesiredIncomingState);
+      bool LegalOutEgde =
+          OutEdge && isLegalEdgeBundleZAState(Block.DesiredOutgoingState);
+      if (LegalInEdge) {
         LLVM_DEBUG(dbgs() << " DesiredIncomingState: "
-                          << getZAStateString(DesiredIncomingState));
+                          << getZAStateString(Block.DesiredIncomingState));
+        EdgeStateCounts[Block.DesiredIncomingState]++;
       }
-      ZAState DesiredOutgoingState = Block.Insts.back().NeededState;
-      if (OutEdge && isLegalEdgeBundleZAState(DesiredOutgoingState)) {
-        EdgeStateCounts[DesiredOutgoingState]++;
+      if (LegalOutEgde) {
         LLVM_DEBUG(dbgs() << " DesiredOutgoingState: "
-                          << getZAStateString(DesiredOutgoingState));
+                          << getZAStateString(Block.DesiredOutgoingState));
+        EdgeStateCounts[Block.DesiredOutgoingState]++;
       }
+      if (!LegalInEdge && !LegalOutEgde)
+        LLVM_DEBUG(dbgs() << " (no state preference)");
       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;
 
@@ -858,6 +919,43 @@ bool MachineSMEABI::runOnMachineFunction(MachineFunction &MF) {
       getAnalysis<EdgeBundlesWrapperLegacy>().getEdgeBundles();
 
   FunctionInfo FnInfo = collectNeededZAStates(SMEFnAttrs);
+
+  if (OptLevel != CodeGenOptLevel::None) {
+    // Propagate desired states forwards then backwards. We propagate forwards
+    // first as this propagates desired states from inner to outer loops.
+    // Backwards propagation is then used to fill in any gaps. Note: Doing both
+    // in one step can give poor results. For example:
+    //
+    //    ┌─────┐
+    //  ┌─┤ BB0 ◄───┐
+    //  │ └─┬───┘   │
+    //  │ ┌─▼───◄──┐│
+    //  │ │ BB1 │  ││
+    //  │ └─┬┬──┘  ││
+    //  │   │└─────┘│
+    //  │ ┌─▼───┐   │
+    //  │ │ BB2 ├───┘
+    //  │ └─┬───┘
+    //  │ ┌─▼───┐
+    //  └─► BB3 │
+    //    └─────┘
+    //
+    // If:
+    // - "BB0" and "BB2" (outer loop) has no state preference
+    // - "BB1" (inner loop) desires the ACTIVE state on entry/exit
+    // - "BB3" desires the LOCAL_SAVED state on entry
+    //
+    // If we propagate forwards first, ACTIVE is propagated from BB1 to BB2,
+    // then from BB2 to BB0. Which results in the inner and outer loops having
+    // the "ACTIVE" state. This avoids any state changes in the loops.
+    //
+    // If we propagate backwards first, we _could_ propagate LOCAL_SAVED from
+    // BB3 to BB0, which would result in a transition from ACTIVE -> LOCAL_SAVED
+    // in the outer loop.
+    for (bool Forwards : {true, false})
+      propagateDesiredStates(FnInfo, Forwards);
+  }
+
   SmallVector<ZAState> BundleStates = assignBundleZAStates(Bundles, FnInfo);
 
   EmitContext Context;

llvm/test/CodeGen/AArch64/sme-agnostic-za.ll

@@ -361,7 +361,6 @@ define i64  @test_many_callee_arguments(
   ret i64 %ret
 }
 
-; FIXME: The new lowering should avoid saves/restores in the probing loop.
 define void @agnostic_za_buffer_alloc_with_stack_probes() nounwind "aarch64_za_state_agnostic" "probe-stack"="inline-asm" "stack-probe-size"="65536"{
 ; CHECK-LABEL: agnostic_za_buffer_alloc_with_stack_probes:
 ; CHECK:       // %bb.0:
@@ -399,18 +398,14 @@ define void @agnostic_za_buffer_alloc_with_stack_probes() nounwind "aarch64_za_s
 ; CHECK-NEWLOWERING-NEXT:    bl __arm_sme_state_size
 ; CHECK-NEWLOWERING-NEXT:    mov x8, sp
 ; CHECK-NEWLOWERING-NEXT:    sub x19, x8, x0
+; CHECK-NEWLOWERING-NEXT:    mov x0, x19
+; CHECK-NEWLOWERING-NEXT:    bl __arm_sme_save
 ; CHECK-NEWLOWERING-NEXT:  .LBB7_1: // =>This Inner Loop Header: Depth=1
 ; CHECK-NEWLOWERING-NEXT:    sub sp, sp, #16, lsl #12 // =65536
 ; CHECK-NEWLOWERING-NEXT:    cmp sp, x19
-; CHECK-NEWLOWERING-NEXT:    mov x0, x19
-; CHECK-NEWLOWERING-NEXT:    mrs x8, NZCV
-; CHECK-NEWLOWERING-NEXT:    bl __arm_sme_save
-; CHECK-NEWLOWERING-NEXT:    msr NZCV, x8
 ; CHECK-NEWLOWERING-NEXT:    b.le .LBB7_3
 ; CHECK-NEWLOWERING-NEXT:  // %bb.2: // in Loop: Header=BB7_1 Depth=1
-; CHECK-NEWLOWERING-NEXT:    mov x0, x19
 ; CHECK-NEWLOWERING-NEXT:    str xzr, [sp]
-; CHECK-NEWLOWERING-NEXT:    bl __arm_sme_restore
 ; CHECK-NEWLOWERING-NEXT:    b .LBB7_1
 ; CHECK-NEWLOWERING-NEXT:  .LBB7_3:
 ; CHECK-NEWLOWERING-NEXT:    mov sp, x19

llvm/test/CodeGen/AArch64/sme-za-control-flow.ll

@@ -228,65 +228,34 @@ exit:
   ret void
 }
 
-; FIXME: The codegen for this case could be improved (by tuning weights).
-; Here the ZA save has been hoisted out of the conditional, but would be better
-; to sink it.
 define void @cond_private_za_call(i1 %cond) "aarch64_inout_za" nounwind {
-; CHECK-LABEL: cond_private_za_call:
-; CHECK:       // %bb.0:
-; CHECK-NEXT:    stp x29, x30, [sp, #-16]! // 16-byte Folded Spill
-; CHECK-NEXT:    mov x29, sp
-; CHECK-NEXT:    sub sp, sp, #16
-; CHECK-NEXT:    rdsvl x8, #1
-; CHECK-NEXT:    mov x9, sp
-; CHECK-NEXT:    msub x9, x8, x8, x9
-; CHECK-NEXT:    mov sp, x9
-; CHECK-NEXT:    stp x9, x8, [x29, #-16]
-; CHECK-NEXT:    tbz w0, #0, .LBB3_4
-; CHECK-NEXT:  // %bb.1: // %private_za_call
-; CHECK-NEXT:    sub x8, x29, #16
-; CHECK-NEXT:    msr TPIDR2_EL0, x8
-; CHECK-NEXT:    bl private_za_call
-; CHECK-NEXT:    smstart za
-; CHECK-NEXT:    mrs x8, TPIDR2_EL0
-; CHECK-NEXT:    sub x0, x29, #16
-; CHECK-NEXT:    cbnz x8, .LBB3_3
-; CHECK-NEXT:  // %bb.2: // %private_za_call
-; CHECK-NEXT:    bl __arm_tpidr2_restore
-; CHECK-NEXT:  .LBB3_3: // %private_za_call
-; CHECK-NEXT:    msr TPIDR2_EL0, xzr
-; CHECK-NEXT:  .LBB3_4: // %exit
-; CHECK-NEXT:    mov sp, x29
-; CHECK-NEXT:    ldp x29, x30, [sp], #16 // 16-byte Folded Reload
-; CHECK-NEXT:    b shared_za_call
-;
-; CHECK-NEWLOWERING-LABEL: cond_private_za_call:
-; CHECK-NEWLOWERING:       // %bb.0:
-; CHECK-NEWLOWERING-NEXT:    stp x29, x30, [sp, #-16]! // 16-byte Folded Spill
-; CHECK-NEWLOWERING-NEXT:    mov x29, sp
-; CHECK-NEWLOWERING-NEXT:    sub sp, sp, #16
-; CHECK-NEWLOWERING-NEXT:    rdsvl x8, #1
-; CHECK-NEWLOWERING-NEXT:    mov x9, sp
-; CHECK-NEWLOWERING-NEXT:    msub x9, x8, x8, x9
-; CHECK-NEWLOWERING-NEXT:    mov sp, x9
-; CHECK-NEWLOWERING-NEXT:    sub x10, x29, #16
-; CHECK-NEWLOWERING-NEXT:    stp x9, x8, [x29, #-16]
-; CHECK-NEWLOWERING-NEXT:    msr TPIDR2_EL0, x10
-; CHECK-NEWLOWERING-NEXT:    tbz w0, #0, .LBB3_2
-; CHECK-NEWLOWERING-NEXT:  // %bb.1: // %private_za_call
-; CHECK-NEWLOWERING-NEXT:    bl private_za_call
-; CHECK-NEWLOWERING-NEXT:  .LBB3_2: // %exit
-; CHECK-NEWLOWERING-NEXT:    smstart za
-; CHECK-NEWLOWERING-NEXT:    mrs x8, TPIDR2_EL0
-; CHECK-NEWLOWERING-NEXT:    sub x0, x29, #16
-; CHECK-NEWLOWERING-NEXT:    cbnz x8, .LBB3_4
-; CHECK-NEWLOWERING-NEXT:  // %bb.3: // %exit
-; CHECK-NEWLOWERING-NEXT:    bl __arm_tpidr2_restore
-; CHECK-NEWLOWERING-NEXT:  .LBB3_4: // %exit
-; CHECK-NEWLOWERING-NEXT:    msr TPIDR2_EL0, xzr
-; CHECK-NEWLOWERING-NEXT:    mov sp, x29
-; CHECK-NEWLOWERING-NEXT:    ldp x29, x30, [sp], #16 // 16-byte Folded Reload
-; CHECK-NEWLOWERING-NEXT:    b shared_za_call
+; CHECK-COMMON-LABEL: cond_private_za_call:
+; CHECK-COMMON:       // %bb.0:
+; CHECK-COMMON-NEXT:    stp x29, x30, [sp, #-16]! // 16-byte Folded Spill
+; CHECK-COMMON-NEXT:    mov x29, sp
+; CHECK-COMMON-NEXT:    sub sp, sp, #16
+; CHECK-COMMON-NEXT:    rdsvl x8, #1
+; CHECK-COMMON-NEXT:    mov x9, sp
+; CHECK-COMMON-NEXT:    msub x9, x8, x8, x9
+; CHECK-COMMON-NEXT:    mov sp, x9
+; CHECK-COMMON-NEXT:    stp x9, x8, [x29, #-16]
+; CHECK-COMMON-NEXT:    tbz w0, #0, .LBB3_4
+; CHECK-COMMON-NEXT:  // %bb.1: // %private_za_call
+; CHECK-COMMON-NEXT:    sub x8, x29, #16
+; CHECK-COMMON-NEXT:    msr TPIDR2_EL0, x8
+; CHECK-COMMON-NEXT:    bl private_za_call
+; CHECK-COMMON-NEXT:    smstart za
+; CHECK-COMMON-NEXT:    mrs x8, TPIDR2_EL0
+; CHECK-COMMON-NEXT:    sub x0, x29, #16
+; CHECK-COMMON-NEXT:    cbnz x8, .LBB3_3
+; CHECK-COMMON-NEXT:  // %bb.2: // %private_za_call
+; CHECK-COMMON-NEXT:    bl __arm_tpidr2_restore
+; CHECK-COMMON-NEXT:  .LBB3_3: // %private_za_call
+; CHECK-COMMON-NEXT:    msr TPIDR2_EL0, xzr
+; CHECK-COMMON-NEXT:  .LBB3_4: // %exit
+; CHECK-COMMON-NEXT:    mov sp, x29
+; CHECK-COMMON-NEXT:    ldp x29, x30, [sp], #16 // 16-byte Folded Reload
+; CHECK-COMMON-NEXT:    b shared_za_call
   br i1 %cond, label %private_za_call, label %exit
 
 private_za_call:
@@ -910,7 +879,7 @@ define void @loop_with_external_entry(i1 %c1, i1 %c2) "aarch64_inout_za" nounwin
 ; CHECK-NEWLOWERING-LABEL: loop_with_external_entry:
 ; CHECK-NEWLOWERING:       // %bb.0: // %entry
 ; CHECK-NEWLOWERING-NEXT:    stp x29, x30, [sp, #-32]! // 16-byte Folded Spill
-; CHECK-NEWLOWERING-NEXT:    str x19, [sp, #16] // 8-byte Folded Spill
+; CHECK-NEWLOWERING-NEXT:    stp x20, x19, [sp, #16] // 16-byte Folded Spill
 ; CHECK-NEWLOWERING-NEXT:    mov x29, sp
 ; CHECK-NEWLOWERING-NEXT:    sub sp, sp, #16
 ; CHECK-NEWLOWERING-NEXT:    rdsvl x8, #1
@@ -923,23 +892,27 @@ define void @loop_with_external_entry(i1 %c1, i1 %c2) "aarch64_inout_za" nounwin
 ; CHECK-NEWLOWERING-NEXT:  // %bb.1: // %init
 ; CHECK-NEWLOWERING-NEXT:    bl shared_za_call
 ; CHECK-NEWLOWERING-NEXT:  .LBB11_2: // %loop.preheader
-; CHECK-NEWLOWERING-NEXT:    sub x8, x29, #16
-; CHECK-NEWLOWERING-NEXT:    msr TPIDR2_EL0, x8
+; CHECK-NEWLOWERING-NEXT:    sub x20, x29, #16
+; CHECK-NEWLOWERING-NEXT:    b .LBB11_4
 ; CHECK-NEWLOWERING-NEXT:  .LBB11_3: // %loop
+; CHECK-NEWLOWERING-NEXT:    // in Loop: Header=BB11_4 Depth=1
+; CHECK-NEWLOWERING-NEXT:    msr TPIDR2_EL0, xzr
+; CHECK-NEWLOWERING-NEXT:    tbz w19, #0, .LBB11_6
+; CHECK-NEWLOWERING-NEXT:  .LBB11_4: // %loop
 ; CHECK-NEWLOWERING-NEXT:    // =>This Inner Loop Header: Depth=1
+; CHECK-NEWLOWERING-NEXT:    msr TPIDR2_EL0, x20
 ; CHECK-NEWLOWERING-NEXT:    bl private_za_call
-; CHECK-NEWLOWERING-NEXT:    tbnz w19, #0, .LBB11_3
-; CHECK-NEWLOWERING-NEXT:  // %bb.4: // %exit
 ; CHECK-NEWLOWERING-NEXT:    smstart za
 ; CHECK-NEWLOWERING-NEXT:    mrs x8, TPIDR2_EL0
 ; CHECK-NEWLOWERING-NEXT:    sub x0, x29, #16
-; CHECK-NEWLOWERING-NEXT:    cbnz x8, .LBB11_6
-; CHECK-NEWLOWERING-NEXT:  // %bb.5: // %exit
+; CHECK-NEWLOWERING-NEXT:    cbnz x8, .LBB11_3
+; CHECK-NEWLOWERING-NEXT:  // %bb.5: // %loop
+; CHECK-NEWLOWERING-NEXT:    // in Loop: Header=BB11_4 Depth=1
 ; CHECK-NEWLOWERING-NEXT:    bl __arm_tpidr2_restore
+; CHECK-NEWLOWERING-NEXT:    b .LBB11_3
 ; CHECK-NEWLOWERING-NEXT:  .LBB11_6: // %exit
-; CHECK-NEWLOWERING-NEXT:    msr TPIDR2_EL0, xzr
 ; CHECK-NEWLOWERING-NEXT:    mov sp, x29
-; CHECK-NEWLOWERING-NEXT:    ldr x19, [sp, #16] // 8-byte Folded Reload
+; CHECK-NEWLOWERING-NEXT:    ldp x20, x19, [sp, #16] // 16-byte Folded Reload
 ; CHECK-NEWLOWERING-NEXT:    ldp x29, x30, [sp], #32 // 16-byte Folded Reload
 ; CHECK-NEWLOWERING-NEXT:    ret
 entry: