[SME] Stop RA from coalescing COPY instructions that transcend beyond…

… smstart/smstop. (#78294)

This patch introduces a 'COALESCER_BARRIER' which is a pseudo node that
expands to
a 'nop', but which stops the register allocator from coalescing a COPY
node when
its use/def crosses a SMSTART or SMSTOP instruction.

For example:

    %0:fpr64 = COPY killed $d0
    undef %2.dsub:zpr = COPY %0       // <- Do not coalesce this COPY
    ADJCALLSTACKDOWN 0, 0
MSRpstatesvcrImm1 1, 0, csr_aarch64_smstartstop, implicit-def dead $d0
    $d0 = COPY killed %0
    BL @use_f64, csr_aarch64_aapcs

If the COPY would be coalesced, that would lead to:

    $d0 = COPY killed %0

being replaced by:

    $d0 = COPY killed %2.dsub

which means the whole ZPR reg would be live upto the call, causing the
MSRpstatesvcrImm1 (smstop) to spill/reload the ZPR register:

    str     q0, [sp]   // 16-byte Folded Spill
    smstop  sm
    ldr     z0, [sp]   // 16-byte Folded Reload
    bl      use_f64

which would be incorrect for two reasons:
1. The program may load more data than it has allocated.
2. If there are other SVE objects on the stack, the compiler might use
the
   'mul vl' addressing modes to access the spill location.

By disabling the coalescing, we get the desired results:

    str     d0, [sp, #8]  // 8-byte Folded Spill
    smstop  sm
    ldr     d0, [sp, #8]  // 8-byte Folded Reload
    bl      use_f64

llvm/lib/Target/AArch64/AArch64ExpandPseudoInsts.cpp

@@ -1544,6 +1544,12 @@ bool AArch64ExpandPseudo::expandMI(MachineBasicBlock &MBB,
        NextMBBI = MBB.end(); // The NextMBBI iterator is invalidated.
      return true;
    }
+   case AArch64::COALESCER_BARRIER_FPR16:
+   case AArch64::COALESCER_BARRIER_FPR32:
+   case AArch64::COALESCER_BARRIER_FPR64:
+   case AArch64::COALESCER_BARRIER_FPR128:
+     MI.eraseFromParent();
+     return true;
    case AArch64::LD1B_2Z_IMM_PSEUDO:
      return expandMultiVecPseudo(
          MBB, MBBI, AArch64::ZPR2RegClass, AArch64::ZPR2StridedRegClass,

llvm/lib/Target/AArch64/AArch64ISelLowering.cpp

@@ -2375,6 +2375,7 @@ const char *AArch64TargetLowering::getTargetNodeName(unsigned Opcode) const {
   switch ((AArch64ISD::NodeType)Opcode) {
   case AArch64ISD::FIRST_NUMBER:
     break;
+    MAKE_CASE(AArch64ISD::COALESCER_BARRIER)
     MAKE_CASE(AArch64ISD::SMSTART)
     MAKE_CASE(AArch64ISD::SMSTOP)
     MAKE_CASE(AArch64ISD::RESTORE_ZA)
@@ -7154,13 +7155,18 @@ void AArch64TargetLowering::saveVarArgRegisters(CCState &CCInfo,
   }
 }
 
+static bool isPassedInFPR(EVT VT) {
+  return VT.isFixedLengthVector() ||
+         (VT.isFloatingPoint() && !VT.isScalableVector());
+}
+
 /// LowerCallResult - Lower the result values of a call into the
 /// appropriate copies out of appropriate physical registers.
 SDValue AArch64TargetLowering::LowerCallResult(
     SDValue Chain, SDValue InGlue, CallingConv::ID CallConv, bool isVarArg,
     const SmallVectorImpl<CCValAssign> &RVLocs, const SDLoc &DL,
     SelectionDAG &DAG, SmallVectorImpl<SDValue> &InVals, bool isThisReturn,
-    SDValue ThisVal) const {
+    SDValue ThisVal, bool RequiresSMChange) const {
   DenseMap<unsigned, SDValue> CopiedRegs;
   // Copy all of the result registers out of their specified physreg.
   for (unsigned i = 0; i != RVLocs.size(); ++i) {
@@ -7205,6 +7211,10 @@ SDValue AArch64TargetLowering::LowerCallResult(
       break;
     }
 
+    if (RequiresSMChange && isPassedInFPR(VA.getValVT()))
+      Val = DAG.getNode(AArch64ISD::COALESCER_BARRIER, DL, Val.getValueType(),
+                        Val);
+
     InVals.push_back(Val);
   }
 
@@ -7915,6 +7925,12 @@ AArch64TargetLowering::LowerCall(CallLoweringInfo &CLI,
           return ArgReg.Reg == VA.getLocReg();
         });
       } else {
+        // Add an extra level of indirection for streaming mode changes by
+        // using a pseudo copy node that cannot be rematerialised between a
+        // smstart/smstop and the call by the simple register coalescer.
+        if (RequiresSMChange && isPassedInFPR(Arg.getValueType()))
+          Arg = DAG.getNode(AArch64ISD::COALESCER_BARRIER, DL,
+                            Arg.getValueType(), Arg);
         RegsToPass.emplace_back(VA.getLocReg(), Arg);
         RegsUsed.insert(VA.getLocReg());
         const TargetOptions &Options = DAG.getTarget().Options;
@@ -8159,9 +8175,9 @@ AArch64TargetLowering::LowerCall(CallLoweringInfo &CLI,
 
   // Handle result values, copying them out of physregs into vregs that we
   // return.
-  SDValue Result = LowerCallResult(Chain, InGlue, CallConv, IsVarArg, RVLocs,
-                                   DL, DAG, InVals, IsThisReturn,
-                                   IsThisReturn ? OutVals[0] : SDValue());
+  SDValue Result = LowerCallResult(
+      Chain, InGlue, CallConv, IsVarArg, RVLocs, DL, DAG, InVals, IsThisReturn,
+      IsThisReturn ? OutVals[0] : SDValue(), RequiresSMChange);
 
   if (!Ins.empty())
     InGlue = Result.getValue(Result->getNumValues() - 1);

llvm/lib/Target/AArch64/AArch64ISelLowering.h

@@ -58,6 +58,8 @@ enum NodeType : unsigned {
 
   CALL_BTI, // Function call followed by a BTI instruction.
 
+  COALESCER_BARRIER,
+
   SMSTART,
   SMSTOP,
   RESTORE_ZA,
@@ -1026,7 +1028,7 @@ class AArch64TargetLowering : public TargetLowering {
                           const SmallVectorImpl<CCValAssign> &RVLocs,
                           const SDLoc &DL, SelectionDAG &DAG,
                           SmallVectorImpl<SDValue> &InVals, bool isThisReturn,
-                          SDValue ThisVal) const;
+                          SDValue ThisVal, bool RequiresSMChange) const;
 
   SDValue LowerLOAD(SDValue Op, SelectionDAG &DAG) const;
   SDValue LowerSTORE(SDValue Op, SelectionDAG &DAG) const;

llvm/lib/Target/AArch64/AArch64RegisterInfo.cpp

@@ -1015,6 +1015,8 @@ bool AArch64RegisterInfo::shouldCoalesce(
     MachineInstr *MI, const TargetRegisterClass *SrcRC, unsigned SubReg,
     const TargetRegisterClass *DstRC, unsigned DstSubReg,
     const TargetRegisterClass *NewRC, LiveIntervals &LIS) const {
+  MachineRegisterInfo &MRI = MI->getMF()->getRegInfo();
+
   if (MI->isCopy() &&
       ((DstRC->getID() == AArch64::GPR64RegClassID) ||
        (DstRC->getID() == AArch64::GPR64commonRegClassID)) &&
@@ -1023,5 +1025,38 @@ bool AArch64RegisterInfo::shouldCoalesce(
     // which implements a 32 to 64 bit zero extension
     // which relies on the upper 32 bits being zeroed.
     return false;
+
+  auto IsCoalescerBarrier = [](const MachineInstr &MI) {
+    switch (MI.getOpcode()) {
+    case AArch64::COALESCER_BARRIER_FPR16:
+    case AArch64::COALESCER_BARRIER_FPR32:
+    case AArch64::COALESCER_BARRIER_FPR64:
+    case AArch64::COALESCER_BARRIER_FPR128:
+      return true;
+    default:
+      return false;
+    }
+  };
+
+  // For calls that temporarily have to toggle streaming mode as part of the
+  // call-sequence, we need to be more careful when coalescing copy instructions
+  // so that we don't end up coalescing the NEON/FP result or argument register
+  // with a whole Z-register, such that after coalescing the register allocator
+  // will try to spill/reload the entire Z register.
+  //
+  // We do this by checking if the node has any defs/uses that are
+  // COALESCER_BARRIER pseudos. These are 'nops' in practice, but they exist to
+  // instruct the coalescer to avoid coalescing the copy.
+  if (MI->isCopy() && SubReg != DstSubReg &&
+      (AArch64::ZPRRegClass.hasSubClassEq(DstRC) ||
+       AArch64::ZPRRegClass.hasSubClassEq(SrcRC))) {
+    unsigned SrcReg = MI->getOperand(1).getReg();
+    if (any_of(MRI.def_instructions(SrcReg), IsCoalescerBarrier))
+      return false;
+    unsigned DstReg = MI->getOperand(0).getReg();
+    if (any_of(MRI.use_nodbg_instructions(DstReg), IsCoalescerBarrier))
+      return false;
+  }
+
   return true;
 }

llvm/lib/Target/AArch64/AArch64SMEInstrInfo.td

@@ -28,6 +28,8 @@ def AArch64_restore_zt : SDNode<"AArch64ISD::RESTORE_ZT", SDTypeProfile<0, 2,
 def AArch64_save_zt : SDNode<"AArch64ISD::SAVE_ZT", SDTypeProfile<0, 2,
                              [SDTCisInt<0>, SDTCisPtrTy<1>]>,
                              [SDNPHasChain, SDNPSideEffect, SDNPMayStore]>;
+def AArch64CoalescerBarrier
+    : SDNode<"AArch64ISD::COALESCER_BARRIER", SDTypeProfile<1, 1, []>, []>;
 
 //===----------------------------------------------------------------------===//
 // Instruction naming conventions.
@@ -189,6 +191,26 @@ def : Pat<(int_aarch64_sme_set_tpidr2 i64:$val),
           (MSR 0xde85, GPR64:$val)>;
 def : Pat<(i64 (int_aarch64_sme_get_tpidr2)),
           (MRS 0xde85)>;
+
+multiclass CoalescerBarrierPseudo<RegisterClass rc, list<ValueType> vts> {
+  def NAME : Pseudo<(outs rc:$dst), (ins rc:$src), []>, Sched<[]> {
+    let Constraints = "$dst = $src";
+  }
+  foreach vt = vts in {
+    def : Pat<(vt (AArch64CoalescerBarrier (vt rc:$src))),
+              (!cast<Instruction>(NAME) rc:$src)>;
+  }
+}
+
+multiclass CoalescerBarriers {
+  defm _FPR16  : CoalescerBarrierPseudo<FPR16, [bf16, f16]>;
+  defm _FPR32  : CoalescerBarrierPseudo<FPR32, [f32]>;
+  defm _FPR64  : CoalescerBarrierPseudo<FPR64, [f64, v8i8, v4i16, v2i32, v1i64, v4f16, v2f32, v1f64, v4bf16]>;
+  defm _FPR128 : CoalescerBarrierPseudo<FPR128, [f128, v16i8, v8i16, v4i32, v2i64, v8f16, v4f32, v2f64, v8bf16]>;
+}
+
+defm COALESCER_BARRIER : CoalescerBarriers;
+
 } // End let Predicates = [HasSME]
 
 // Pseudo to match to smstart/smstop. This expands:

llvm/test/CodeGen/AArch64/sme-disable-gisel-fisel.ll

@@ -23,9 +23,9 @@ define double @nonstreaming_caller_streaming_callee(double %x) nounwind noinline
 ; CHECK-FISEL-NEXT:    bl streaming_callee
 ; CHECK-FISEL-NEXT:    str d0, [sp, #8] // 8-byte Folded Spill
 ; CHECK-FISEL-NEXT:    smstop sm
+; CHECK-FISEL-NEXT:    ldr d1, [sp, #8] // 8-byte Folded Reload
 ; CHECK-FISEL-NEXT:    adrp x8, .LCPI0_0
 ; CHECK-FISEL-NEXT:    ldr d0, [x8, :lo12:.LCPI0_0]
-; CHECK-FISEL-NEXT:    ldr d1, [sp, #8] // 8-byte Folded Reload
 ; CHECK-FISEL-NEXT:    fadd d0, d1, d0
 ; CHECK-FISEL-NEXT:    ldr x30, [sp, #80] // 8-byte Folded Reload
 ; CHECK-FISEL-NEXT:    ldp d9, d8, [sp, #64] // 16-byte Folded Reload
@@ -49,9 +49,9 @@ define double @nonstreaming_caller_streaming_callee(double %x) nounwind noinline
 ; CHECK-GISEL-NEXT:    bl streaming_callee
 ; CHECK-GISEL-NEXT:    str d0, [sp, #8] // 8-byte Folded Spill
 ; CHECK-GISEL-NEXT:    smstop sm
+; CHECK-GISEL-NEXT:    ldr d1, [sp, #8] // 8-byte Folded Reload
 ; CHECK-GISEL-NEXT:    mov x8, #4631107791820423168 // =0x4045000000000000
 ; CHECK-GISEL-NEXT:    fmov d0, x8
-; CHECK-GISEL-NEXT:    ldr d1, [sp, #8] // 8-byte Folded Reload
 ; CHECK-GISEL-NEXT:    fadd d0, d1, d0
 ; CHECK-GISEL-NEXT:    ldr x30, [sp, #80] // 8-byte Folded Reload
 ; CHECK-GISEL-NEXT:    ldp d9, d8, [sp, #64] // 16-byte Folded Reload
@@ -82,9 +82,9 @@ define double @streaming_caller_nonstreaming_callee(double %x) nounwind noinline
 ; CHECK-COMMON-NEXT:    bl normal_callee
 ; CHECK-COMMON-NEXT:    str d0, [sp, #8] // 8-byte Folded Spill
 ; CHECK-COMMON-NEXT:    smstart sm
+; CHECK-COMMON-NEXT:    ldr d1, [sp, #8] // 8-byte Folded Reload
 ; CHECK-COMMON-NEXT:    mov x8, #4631107791820423168 // =0x4045000000000000
 ; CHECK-COMMON-NEXT:    fmov d0, x8
-; CHECK-COMMON-NEXT:    ldr d1, [sp, #8] // 8-byte Folded Reload
 ; CHECK-COMMON-NEXT:    fadd d0, d1, d0
 ; CHECK-COMMON-NEXT:    ldr x30, [sp, #80] // 8-byte Folded Reload
 ; CHECK-COMMON-NEXT:    ldp d9, d8, [sp, #64] // 16-byte Folded Reload
@@ -110,14 +110,16 @@ define double @locally_streaming_caller_normal_callee(double %x) nounwind noinli
 ; CHECK-COMMON-NEXT:    str x30, [sp, #96] // 8-byte Folded Spill
 ; CHECK-COMMON-NEXT:    str d0, [sp, #24] // 8-byte Folded Spill
 ; CHECK-COMMON-NEXT:    smstart sm
+; CHECK-COMMON-NEXT:    ldr d0, [sp, #24] // 8-byte Folded Reload
+; CHECK-COMMON-NEXT:    str d0, [sp, #24] // 8-byte Folded Spill
 ; CHECK-COMMON-NEXT:    smstop sm
 ; CHECK-COMMON-NEXT:    ldr d0, [sp, #24] // 8-byte Folded Reload
 ; CHECK-COMMON-NEXT:    bl normal_callee
 ; CHECK-COMMON-NEXT:    str d0, [sp, #16] // 8-byte Folded Spill
 ; CHECK-COMMON-NEXT:    smstart sm
+; CHECK-COMMON-NEXT:    ldr d1, [sp, #16] // 8-byte Folded Reload
 ; CHECK-COMMON-NEXT:    mov x8, #4631107791820423168 // =0x4045000000000000
 ; CHECK-COMMON-NEXT:    fmov d0, x8
-; CHECK-COMMON-NEXT:    ldr d1, [sp, #16] // 8-byte Folded Reload
 ; CHECK-COMMON-NEXT:    fadd d0, d1, d0
 ; CHECK-COMMON-NEXT:    str d0, [sp, #8] // 8-byte Folded Spill
 ; CHECK-COMMON-NEXT:    smstop sm
@@ -329,9 +331,9 @@ define fp128 @f128_call_sm(fp128 %a, fp128 %b) "aarch64_pstate_sm_enabled" nounw
 ; CHECK-COMMON-NEXT:    stp d11, d10, [sp, #64] // 16-byte Folded Spill
 ; CHECK-COMMON-NEXT:    stp d9, d8, [sp, #80] // 16-byte Folded Spill
 ; CHECK-COMMON-NEXT:    str x30, [sp, #96] // 8-byte Folded Spill
-; CHECK-COMMON-NEXT:    stp q0, q1, [sp] // 32-byte Folded Spill
+; CHECK-COMMON-NEXT:    stp q1, q0, [sp] // 32-byte Folded Spill
 ; CHECK-COMMON-NEXT:    smstop sm
-; CHECK-COMMON-NEXT:    ldp q0, q1, [sp] // 32-byte Folded Reload
+; CHECK-COMMON-NEXT:    ldp q1, q0, [sp] // 32-byte Folded Reload
 ; CHECK-COMMON-NEXT:    bl __addtf3
 ; CHECK-COMMON-NEXT:    str q0, [sp, #16] // 16-byte Folded Spill
 ; CHECK-COMMON-NEXT:    smstart sm
@@ -390,9 +392,9 @@ define float @frem_call_sm(float %a, float %b) "aarch64_pstate_sm_enabled" nounw
 ; CHECK-COMMON-NEXT:    stp d11, d10, [sp, #48] // 16-byte Folded Spill
 ; CHECK-COMMON-NEXT:    stp d9, d8, [sp, #64] // 16-byte Folded Spill
 ; CHECK-COMMON-NEXT:    str x30, [sp, #80] // 8-byte Folded Spill
-; CHECK-COMMON-NEXT:    stp s0, s1, [sp, #8] // 8-byte Folded Spill
+; CHECK-COMMON-NEXT:    stp s1, s0, [sp, #8] // 8-byte Folded Spill
 ; CHECK-COMMON-NEXT:    smstop sm
-; CHECK-COMMON-NEXT:    ldp s0, s1, [sp, #8] // 8-byte Folded Reload
+; CHECK-COMMON-NEXT:    ldp s1, s0, [sp, #8] // 8-byte Folded Reload
 ; CHECK-COMMON-NEXT:    bl fmodf
 ; CHECK-COMMON-NEXT:    str s0, [sp, #12] // 4-byte Folded Spill
 ; CHECK-COMMON-NEXT:    smstart sm
@@ -420,7 +422,9 @@ define float @frem_call_sm_compat(float %a, float %b) "aarch64_pstate_sm_compati
 ; CHECK-COMMON-NEXT:    stp x30, x19, [sp, #80] // 16-byte Folded Spill
 ; CHECK-COMMON-NEXT:    stp s0, s1, [sp, #8] // 8-byte Folded Spill
 ; CHECK-COMMON-NEXT:    bl __arm_sme_state
+; CHECK-COMMON-NEXT:    ldp s2, s0, [sp, #8] // 8-byte Folded Reload
 ; CHECK-COMMON-NEXT:    and x19, x0, #0x1
+; CHECK-COMMON-NEXT:    stp s2, s0, [sp, #8] // 8-byte Folded Spill
 ; CHECK-COMMON-NEXT:    tbz w19, #0, .LBB12_2
 ; CHECK-COMMON-NEXT:  // %bb.1:
 ; CHECK-COMMON-NEXT:    smstop sm