[AArch64][FEAT_CMPBR] Codegen for Armv9.6-a compare-and-branch (llvm#116465)

This patch adds codegen for all Arm9.6-a compare-and-branch
instructions, that operate on full w or x registers. The instruction
variants operating on half-words (cbh) and bytes (cbb) are added in a
subsequent patch.

Since CB doesn't use standard 4-bit Arm condition codes but a reduced
set of conditions, encoded in 3 bits, some conditions are expressed by
modifying operands, namely incrementing or decrementing immediate
operands and swapping register operands. To invert a CB instruction it's
therefore not enough to just modify the condition code which doesn't
play particularly well with how the backend is currently organized. We
therefore introduce a number of pseudos which operate on the standard
4-bit condition codes and lower them late during codegen.

Author: dtellenbach

llvm/lib/Target/AArch64/AArch64AsmPrinter.cpp

@@ -208,6 +208,9 @@ class AArch64AsmPrinter : public AsmPrinter {
   void emitAttributes(unsigned Flags, uint64_t PAuthABIPlatform,
                       uint64_t PAuthABIVersion, AArch64TargetStreamer *TS);
 
+  // Emit expansion of Compare-and-branch pseudo instructions
+  void emitCBPseudoExpansion(const MachineInstr *MI);
+
   void EmitToStreamer(MCStreamer &S, const MCInst &Inst);
   void EmitToStreamer(const MCInst &Inst) {
     EmitToStreamer(*OutStreamer, Inst);
@@ -2589,6 +2592,124 @@ AArch64AsmPrinter::lowerBlockAddressConstant(const BlockAddress &BA) {
   return BAE;
 }
 
+void AArch64AsmPrinter::emitCBPseudoExpansion(const MachineInstr *MI) {
+  bool IsImm = false;
+  bool Is32Bit = false;
+
+  switch (MI->getOpcode()) {
+  default:
+    llvm_unreachable("This is not a CB pseudo instruction");
+  case AArch64::CBWPrr:
+    Is32Bit = true;
+    break;
+  case AArch64::CBXPrr:
+    Is32Bit = false;
+    break;
+  case AArch64::CBWPri:
+    IsImm = true;
+    Is32Bit = true;
+    break;
+  case AArch64::CBXPri:
+    IsImm = true;
+    break;
+  }
+
+  AArch64CC::CondCode CC =
+      static_cast<AArch64CC::CondCode>(MI->getOperand(0).getImm());
+  bool NeedsRegSwap = false;
+  bool NeedsImmDec = false;
+  bool NeedsImmInc = false;
+
+  // Decide if we need to either swap register operands or increment/decrement
+  // immediate operands
+  unsigned MCOpC;
+  switch (CC) {
+  default:
+    llvm_unreachable("Invalid CB condition code");
+  case AArch64CC::EQ:
+    MCOpC = IsImm ? (Is32Bit ? AArch64::CBEQWri : AArch64::CBEQXri)
+                  : (Is32Bit ? AArch64::CBEQWrr : AArch64::CBEQXrr);
+    break;
+  case AArch64CC::NE:
+    MCOpC = IsImm ? (Is32Bit ? AArch64::CBNEWri : AArch64::CBNEXri)
+                  : (Is32Bit ? AArch64::CBNEWrr : AArch64::CBNEXrr);
+    break;
+  case AArch64CC::HS:
+    MCOpC = IsImm ? (Is32Bit ? AArch64::CBHIWri : AArch64::CBHIXri)
+                  : (Is32Bit ? AArch64::CBHSWrr : AArch64::CBHSXrr);
+    NeedsImmDec = IsImm;
+    break;
+  case AArch64CC::LO:
+    MCOpC = IsImm ? (Is32Bit ? AArch64::CBLOWri : AArch64::CBLOXri)
+                  : (Is32Bit ? AArch64::CBHIWrr : AArch64::CBHIXrr);
+    NeedsRegSwap = !IsImm;
+    break;
+  case AArch64CC::HI:
+    MCOpC = IsImm ? (Is32Bit ? AArch64::CBHIWri : AArch64::CBHIXri)
+                  : (Is32Bit ? AArch64::CBHIWrr : AArch64::CBHIXrr);
+    break;
+  case AArch64CC::LS:
+    MCOpC = IsImm ? (Is32Bit ? AArch64::CBLOWri : AArch64::CBLOXri)
+                  : (Is32Bit ? AArch64::CBHSWrr : AArch64::CBHSXrr);
+    NeedsRegSwap = !IsImm;
+    NeedsImmInc = IsImm;
+    break;
+  case AArch64CC::GE:
+    MCOpC = IsImm ? (Is32Bit ? AArch64::CBGTWri : AArch64::CBGTXri)
+                  : (Is32Bit ? AArch64::CBGEWrr : AArch64::CBGEXrr);
+    NeedsImmDec = IsImm;
+    break;
+  case AArch64CC::LT:
+    MCOpC = IsImm ? (Is32Bit ? AArch64::CBLTWri : AArch64::CBLTXri)
+                  : (Is32Bit ? AArch64::CBGTWrr : AArch64::CBGTXrr);
+    NeedsRegSwap = !IsImm;
+    break;
+  case AArch64CC::GT:
+    MCOpC = IsImm ? (Is32Bit ? AArch64::CBGTWri : AArch64::CBGTXri)
+                  : (Is32Bit ? AArch64::CBGTWrr : AArch64::CBGTXrr);
+    break;
+  case AArch64CC::LE:
+    MCOpC = IsImm ? (Is32Bit ? AArch64::CBLTWri : AArch64::CBLTXri)
+                  : (Is32Bit ? AArch64::CBGEWrr : AArch64::CBGEXrr);
+    NeedsRegSwap = !IsImm;
+    NeedsImmInc = IsImm;
+    break;
+  }
+
+  MCInst Inst;
+  Inst.setOpcode(MCOpC);
+
+  MCOperand Lhs, Rhs, Trgt;
+  lowerOperand(MI->getOperand(1), Lhs);
+  lowerOperand(MI->getOperand(2), Rhs);
+  lowerOperand(MI->getOperand(3), Trgt);
+
+  // Now swap, increment or decrement
+  if (NeedsRegSwap) {
+    assert(Lhs.isReg() && "Expected register operand for CB");
+    assert(Rhs.isReg() && "Expected register operand for CB");
+    Inst.addOperand(Rhs);
+    Inst.addOperand(Lhs);
+  } else if (NeedsImmDec) {
+    Rhs.setImm(Rhs.getImm() - 1);
+    Inst.addOperand(Lhs);
+    Inst.addOperand(Rhs);
+  } else if (NeedsImmInc) {
+    Rhs.setImm(Rhs.getImm() + 1);
+    Inst.addOperand(Lhs);
+    Inst.addOperand(Rhs);
+  } else {
+    Inst.addOperand(Lhs);
+    Inst.addOperand(Rhs);
+  }
+
+  assert((!IsImm || (Rhs.getImm() >= 0 && Rhs.getImm() < 64)) &&
+         "CB immediate operand out-of-bounds");
+
+  Inst.addOperand(Trgt);
+  EmitToStreamer(*OutStreamer, Inst);
+}
+
 // Simple pseudo-instructions have their lowering (with expansion to real
 // instructions) auto-generated.
 #include "AArch64GenMCPseudoLowering.inc"
@@ -3155,13 +3276,20 @@ void AArch64AsmPrinter::emitInstruction(const MachineInstr *MI) {
     return;
 
   case AArch64::BLR:
-  case AArch64::BR:
+  case AArch64::BR: {
     recordIfImportCall(MI);
     MCInst TmpInst;
     MCInstLowering.Lower(MI, TmpInst);
     EmitToStreamer(*OutStreamer, TmpInst);
     return;
   }
+  case AArch64::CBWPri:
+  case AArch64::CBXPri:
+  case AArch64::CBWPrr:
+  case AArch64::CBXPrr:
+    emitCBPseudoExpansion(MI);
+    return;
+  }
 
   // Finally, do the automated lowerings for everything else.
   MCInst TmpInst;

llvm/lib/Target/AArch64/AArch64ISelDAGToDAG.cpp

@@ -507,6 +507,8 @@ class AArch64DAGToDAGISel : public SelectionDAGISel {
 
   bool SelectAllActivePredicate(SDValue N);
   bool SelectAnyPredicate(SDValue N);
+
+  bool SelectCmpBranchUImm6Operand(SDNode *P, SDValue N, SDValue &Imm);
 };
 
 class AArch64DAGToDAGISelLegacy : public SelectionDAGISelLegacy {
@@ -7489,3 +7491,52 @@ bool AArch64DAGToDAGISel::SelectSMETileSlice(SDValue N, unsigned MaxSize,
   Offset = CurDAG->getTargetConstant(0, SDLoc(N), MVT::i64);
   return true;
 }
+
+bool AArch64DAGToDAGISel::SelectCmpBranchUImm6Operand(SDNode *P, SDValue N,
+                                                      SDValue &Imm) {
+  AArch64CC::CondCode CC =
+      static_cast<AArch64CC::CondCode>(P->getConstantOperandVal(1));
+  if (auto *CN = dyn_cast<ConstantSDNode>(N)) {
+    // Check conservatively if the immediate fits the valid range [0, 64).
+    // Immediate variants for GE and HS definitely need to be decremented
+    // when lowering the pseudos later, so an immediate of 1 would become 0.
+    // For the inverse conditions LT and LO we don't know for sure if they
+    // will need a decrement but should the decision be made to reverse the
+    // branch condition, we again end up with the need to decrement.
+    // The same argument holds for LE, LS, GT and HI and possibly
+    // incremented immediates. This can lead to slightly less optimal
+    // codegen, e.g. we never codegen the legal case
+    //    cblt w0, #63, A
+    // because we could end up with the illegal case
+    //    cbge w0, #64, B
+    // should the decision to reverse the branch direction be made. For the
+    // lower bound cases this is no problem since we can express comparisons
+    // against 0 with either tbz/tnbz or using wzr/xzr.
+    uint64_t LowerBound = 0, UpperBound = 64;
+    switch (CC) {
+    case AArch64CC::GE:
+    case AArch64CC::HS:
+    case AArch64CC::LT:
+    case AArch64CC::LO:
+      LowerBound = 1;
+      break;
+    case AArch64CC::LE:
+    case AArch64CC::LS:
+    case AArch64CC::GT:
+    case AArch64CC::HI:
+      UpperBound = 63;
+      break;
+    default:
+      break;
+    }
+
+    if (CN->getAPIntValue().uge(LowerBound) &&
+        CN->getAPIntValue().ult(UpperBound)) {
+      SDLoc DL(N);
+      Imm = CurDAG->getTargetConstant(CN->getZExtValue(), DL, N.getValueType());
+      return true;
+    }
+  }
+
+  return false;
+}

llvm/lib/Target/AArch64/AArch64ISelLowering.cpp

@@ -2993,6 +2993,7 @@ const char *AArch64TargetLowering::getTargetNodeName(unsigned Opcode) const {
     MAKE_CASE(AArch64ISD::CTTZ_ELTS)
     MAKE_CASE(AArch64ISD::CALL_ARM64EC_TO_X64)
     MAKE_CASE(AArch64ISD::URSHR_I_PRED)
+    MAKE_CASE(AArch64ISD::CB)
   }
 #undef MAKE_CASE
   return nullptr;
@@ -10603,6 +10604,17 @@ SDValue AArch64TargetLowering::LowerBR_CC(SDValue Op, SelectionDAG &DAG) const {
                          DAG.getConstant(SignBitPos, dl, MVT::i64), Dest);
     }
 
+    // Try to emit Armv9.6 CB instructions. We prefer tb{n}z/cb{n}z due to their
+    // larger branch displacement but do prefer CB over cmp + br.
+    if (Subtarget->hasCMPBR() &&
+        AArch64CC::isValidCBCond(changeIntCCToAArch64CC(CC)) &&
+        ProduceNonFlagSettingCondBr) {
+      SDValue Cond =
+          DAG.getTargetConstant(changeIntCCToAArch64CC(CC), dl, MVT::i32);
+      return DAG.getNode(AArch64ISD::CB, dl, MVT::Other, Chain, Cond, LHS, RHS,
+                         Dest);
+    }
+
     SDValue CCVal;
     SDValue Cmp = getAArch64Cmp(LHS, RHS, CC, CCVal, DAG, dl);
     return DAG.getNode(AArch64ISD::BRCOND, dl, MVT::Other, Chain, Dest, CCVal,

llvm/lib/Target/AArch64/AArch64ISelLowering.h

@@ -529,6 +529,9 @@ enum NodeType : unsigned {
   // SME ZA loads and stores
   SME_ZA_LDR,
   SME_ZA_STR,
+
+  // Compare-and-branch
+  CB,
 };
 
 } // end namespace AArch64ISD

llvm/lib/Target/AArch64/AArch64InstrFormats.td

@@ -400,6 +400,16 @@ def uimm6_32b : Operand<i32>, ImmLeaf<i32, [{ return Imm >= 0 && Imm < 64; }]> {
    let ParserMatchClass = UImm6Operand;
 }
 
+def CmpBranchUImm6Operand_32b
+    : ComplexPattern<i32, 1, "SelectCmpBranchUImm6Operand", [imm]> {
+  let WantsParent = true;
+}
+
+def CmpBranchUImm6Operand_64b
+    : ComplexPattern<i64, 1, "SelectCmpBranchUImm6Operand", [imm]> {
+  let WantsParent = true;
+}
+
 def UImm6Plus1Operand : AsmOperandClass {
   let Name = "UImm6P1";
   let DiagnosticType = "InvalidImm1_64";
@@ -13225,6 +13235,21 @@ multiclass CmpBranchRegisterAlias<string mnemonic, string insn> {
   def : InstAlias<mnemonic # "\t$Rt, $Rm, $target",
                  (!cast<Instruction>(insn # "Xrr") GPR64:$Rm, GPR64:$Rt, am_brcmpcond:$target), 0>;
 }
+
+class CmpBranchRegisterPseudo<RegisterClass regtype>
+  : Pseudo<(outs), (ins ccode:$Cond, regtype:$Rt, regtype:$Rm, am_brcmpcond:$Target), []>,
+    Sched<[WriteBr]> {
+  let isBranch = 1;
+  let isTerminator = 1;
+}
+
+class CmpBranchImmediatePseudo<RegisterClass regtype, ImmLeaf imtype>
+  : Pseudo<(outs), (ins ccode:$Cond, regtype:$Rt, imtype:$Imm, am_brcmpcond:$Target), []>,
+    Sched<[WriteBr]> {
+  let isBranch = 1;
+  let isTerminator = 1;
+}
+
 //----------------------------------------------------------------------------
 // Allow the size specifier tokens to be upper case, not just lower.
 def : TokenAlias<".4B", ".4b">;  // Add dot product