[RISCV] Model FFLAGS accesses

3 files changed, 100 insertions, 1 deletions

diff --git a/backend/lib/CodeGen/PostRAScheduler.cpp b/backend/lib/CodeGen/PostRAScheduler.cpp
index 026f763..204f05e 100644
--- a/backend/lib/CodeGen/PostRAScheduler.cpp
+++ b/backend/lib/CodeGen/PostRAScheduler.cpp
@@ -130,6 +130,13 @@ void removeUnits(RegisterUnitLaneSet &Live, const std::vector<PhysicalUnitAccess
     Live.remove(Unit.Unit, Unit.Lanes);
 }
 
+[[nodiscard]] bool isReservedPhysicalRegister(Register Reg, const MachineFunction &Function,
+                                              const TargetDescription &Target) noexcept {
+  const std::optional<PhysicalRegister> Physical = Reg.physicalRegister();
+  return Physical && (Function.registerInfo().isReserved(*Physical) ||
+                      Target.registers().isReservedRegister(*Physical));
+}
+
 [[nodiscard]] OperandFlagUpdate &getFlagUpdate(std::vector<OperandFlagUpdate> &Updates,
                                                MachineOperand &Operand) {
   const auto Position = std::ranges::find_if(
@@ -175,7 +182,7 @@ buildBlockLivenessRepairPlan(MachineBasicBlock &Block, const TargetDescription &
       auto Units = physicalOperandUnits(*Operand, Target);
       if (!Units)
         return std::unexpected(Units.error());
-      if (!intersects(Live, *Units))
+      if (!isReservedPhysicalRegister(*Reg, Block.function(), Target) && !intersects(Live, *Units))
         Update.Flags.set(RegisterOperandFlag::Dead);
     }
 
@@ -218,6 +225,8 @@ buildBlockLivenessRepairPlan(MachineBasicBlock &Block, const TargetDescription &
       const Register *Reg = Operand->getIf<Register>();
       if (Reg == nullptr || Reg->isNoRegister())
         continue;
+      if (isReservedPhysicalRegister(*Reg, Block.function(), Target))
+        continue;
       auto Units = physicalOperandUnits(*Operand, Target);
       if (!Units)
         return std::unexpected(Units.error());
diff --git a/backend/lib/Target/RISCV/RISCV.rccdesc b/backend/lib/Target/RISCV/RISCV.rccdesc
index 9943724..c589424 100644
--- a/backend/lib/Target/RISCV/RISCV.rccdesc
+++ b/backend/lib/Target/RISCV/RISCV.rccdesc
@@ -75,6 +75,7 @@ register_units:
   - { id: 62, symbol: U_F29, name: f29.unit }
   - { id: 63, symbol: U_F30, name: f30.unit }
   - { id: 64, symbol: U_F31, name: f31.unit }
+  - { id: 65, symbol: U_FFLAGS, name: fflags.unit }
 
 registers:
   - { id: 1, symbol: X0, name: x0, encoding: 0, dwarf_register: 0, units: [{ unit: U_X0, lanes: 1 }], constant: true, reserved: true, aliases: [zero] }
@@ -173,10 +174,12 @@ registers:
   - { id: 94, symbol: F29_F, name: f29_f, encoding: 29, dwarf_register: 61, units: [{ unit: U_F29, lanes: 1 }] }
   - { id: 95, symbol: F30_F, name: f30_f, encoding: 30, dwarf_register: 62, units: [{ unit: U_F30, lanes: 1 }] }
   - { id: 96, symbol: F31_F, name: f31_f, encoding: 31, dwarf_register: 63, units: [{ unit: U_F31, lanes: 1 }] }
+  - { id: 97, symbol: FFLAGS, name: fflags, encoding: 0, dwarf_register: 65, units: [{ unit: U_FFLAGS, lanes: 1 }], artificial: true, reserved: true }
 
 register_banks:
   - { id: 1, symbol: GPR, name: GPRB, maximum_size_bits: 64 }
   - { id: 2, symbol: FPR, name: FPRB, maximum_size_bits: 64 }
+  - { id: 3, symbol: CSR, name: CSRB, maximum_size_bits: 32 }
 
 register_classes:
   -
@@ -267,6 +270,18 @@ register_classes:
     lanes: 1
     registers: [X1, X3, X4, X5, X6, X7, X8, X9, X10, X11, X12, X13, X14, X15, X16, X17, X18, X19, X20, X21, X22, X23, X24, X25, X26, X27, X28, X29, X30, X31]
     allocation_order: [X5, X6, X7, X28, X29, X30, X31, X10, X11, X12, X13, X14, X15, X16, X17, X8, X9, X18, X19, X20, X21, X22, X23, X24, X25, X26, X27, X1]
+  -
+    id: 9
+    symbol: FFLAGS
+    name: FFLAGS
+    bank: CSR
+    size_bits: 32
+    spill_size_bits: 32
+    spill_alignment_bits: 32
+    lanes: 1
+    registers: [FFLAGS]
+    allocation_order: []
+    allocatable: false
 
 register_masks:
   -
@@ -1007,6 +1022,7 @@ instructions:
       - { kinds: [immediate], immediate_predicate: UIMM12, name: CSR }
       - { kinds: [register], role: use, register_class: GPR, register_bank: GPR, name: RS1 }
     num_explicit_defs: 1
+    allow_extra_implicit_operands: true
     flags: [has_side_effects, produces_code]
     scheduling_class: SYSTEM
     encoding_category: SYSTEM
@@ -1022,6 +1038,7 @@ instructions:
       - { kinds: [immediate], immediate_predicate: UIMM12, name: CSR }
       - { kinds: [register], role: use, register_class: GPR, register_bank: GPR, name: RS1 }
     num_explicit_defs: 1
+    allow_extra_implicit_operands: true
     flags: [has_side_effects, produces_code]
     scheduling_class: SYSTEM
     encoding_category: SYSTEM
@@ -3520,6 +3537,28 @@ instructions:
     owner: instruction_select
     deadline: selected
   -
+    id: 1245
+    symbol: ReadFFLAGS
+    name: ReadFFLAGS
+    operands:
+      - { kinds: [register], role: def, register_class: GPR, register_bank: GPR }
+    num_explicit_defs: 1
+    implicit_uses: [{ register: FFLAGS }]
+    flags: [pseudo, produces_code, allowed_pre_legalizer, has_side_effects]
+    owner: post_ra_expansion
+    deadline: post_ra_finalized
+  -
+    id: 1246
+    symbol: WriteFFLAGS
+    name: WriteFFLAGS
+    operands:
+      - { kinds: [register], role: use, register_class: GPR, register_bank: GPR }
+    num_explicit_defs: 0
+    implicit_defs: [{ register: FFLAGS }]
+    flags: [pseudo, produces_code, allowed_pre_legalizer, has_side_effects]
+    owner: post_ra_expansion
+    deadline: post_ra_finalized
+  -
     id: 1216
     symbol: PseudoCC_COPY
     name: PseudoCC_COPY
diff --git a/backend/lib/Target/RISCV/RISCVPseudoExpansion.cpp b/backend/lib/Target/RISCV/RISCVPseudoExpansion.cpp
index ae5e51d..d0de28e 100644
--- a/backend/lib/Target/RISCV/RISCVPseudoExpansion.cpp
+++ b/backend/lib/Target/RISCV/RISCVPseudoExpansion.cpp
@@ -334,6 +334,53 @@ copyRegisters(const MachineInstr &Instruction) {
   return {};
 }
 
+[[nodiscard]] Expected<bool> expandFFLAGSAccess(MachineInstr &Instruction,
+                                                MachineFunction &Function,
+                                                const TargetDescription &Target, bool IsRead) {
+  const MachineOperand *ValueOperand = Instruction.operandAt(0);
+  const Register *Value = ValueOperand == nullptr ? nullptr : ValueOperand->getIf<Register>();
+  if (Value == nullptr || !Value->isPhysical()) {
+    return makeUnexpected(ErrorCode::VerificationError,
+                          "RISC-V FFLAGS pseudo retains a nonphysical value register");
+  }
+  auto Zero = physical(PhysicalRegisterID::X0);
+  if (!Zero)
+    return std::unexpected(Zero.error());
+
+  MachineIRBuilder Builder(Function, Target);
+  auto Point = Builder.setInsertPointBefore(Instruction);
+  if (!Point)
+    return std::unexpected(Point.error());
+  auto Pending = Builder.buildInstr(
+      targetOpcode(IsRead ? InstructionOpcode::CSRRS : InstructionOpcode::CSRRW));
+  if (!Pending)
+    return std::unexpected(Pending.error());
+  Expected<void> Added = IsRead ? Pending->addOperand(ValueOperand->cloneDetached())
+                                : Pending->addDef(Register::fromPhysical(*Zero));
+  if (!Added)
+    return std::unexpected(Added.error());
+  Added = Pending->addImmediate(1);
+  if (!Added)
+    return std::unexpected(Added.error());
+  Added = IsRead ? Pending->addUse(Register::fromPhysical(*Zero))
+                 : Pending->addOperand(ValueOperand->cloneDetached());
+  if (!Added)
+    return std::unexpected(Added.error());
+  auto Implicit = copyExtraImplicitOperands(Instruction, 1, *Pending, std::nullopt, std::nullopt);
+  if (!Implicit)
+    return std::unexpected(Implicit.error());
+  auto Replacement = Pending->build();
+  if (!Replacement)
+    return std::unexpected(Replacement.error());
+  auto Decoration = copyCommonDecoration(Instruction, **Replacement, true, true, true);
+  if (!Decoration)
+    return std::unexpected(Decoration.error());
+  auto Removed = removeOldInstruction(Instruction);
+  if (!Removed)
+    return std::unexpected(Removed.error());
+  return true;
+}
+
 [[nodiscard]] Expected<bool> expandCall(MachineInstr &Instruction, MachineFunction &Function,
                                         const TargetDescription &Target, bool IsTail) {
   MachineBasicBlock *Block = Instruction.parent();
@@ -534,6 +581,10 @@ public:
       return expandReturn(Instruction, Function, Target);
     if (Instruction.opcode() == targetOpcode(InstructionOpcode::PseudoBR))
       return expandBranch(Instruction, Function, Target);
+    if (Instruction.opcode() == targetOpcode(InstructionOpcode::ReadFFLAGS))
+      return expandFFLAGSAccess(Instruction, Function, Target, true);
+    if (Instruction.opcode() == targetOpcode(InstructionOpcode::WriteFFLAGS))
+      return expandFFLAGSAccess(Instruction, Function, Target, false);
     return false;
   }
 };