FEX-Emu--FEX/FEXCore/Source/Interface/IR/Passes/RedundantFlagCalculationElimination.cpp

// SPDX-License-Identifier: MIT
/*
$info$
tags: ir|opts
desc: This is not used right now, possibly broken
$end_info$
*/

#include "FEXCore/Core/X86Enums.h"
#include "Interface/IR/IREmitter.h"

#include <FEXCore/IR/IR.h>
#include <FEXCore/IR/IntrusiveIRList.h>
#include <FEXCore/Utils/Profiler.h>

#include "Interface/IR/PassManager.h"

#include <array>
#include <memory>

// Flag bit flags
#define FLAG_V (1U << 0)
#define FLAG_C (1U << 1)
#define FLAG_Z (1U << 2)
#define FLAG_N (1U << 3)
#define FLAG_A (1U << 4)
#define FLAG_P (1U << 5)

#define FLAG_ZCV (FLAG_Z | FLAG_C | FLAG_V)
#define FLAG_NZCV (FLAG_N | FLAG_ZCV)
#define FLAG_ALL (FLAG_NZCV | FLAG_A | FLAG_P)

namespace FEXCore::IR {

struct FlagInfo {
  // If set, all following fields are zero, used for a quick exit.
  bool Trivial;

  // Set of flags read by the instruction.
  unsigned Read;

  // Set of flags written by the instruction. Happens AFTER the reads.
  unsigned Write;

  // If true, the instruction can be be eliminated if its flag writes can all be
  // eliminated.
  bool CanEliminate;

  // If true, the opcode can be replaced with Replacement if its flag writes can
  // all be eliminated.
  bool CanReplace;
  IROps Replacement;
};

class DeadFlagCalculationEliminination final : public FEXCore::IR::Pass {
public:
  bool Run(IREmitter *IREmit) override;

private:
  FlagInfo Classify(IROp_Header *Node);
  unsigned FlagForOffset(unsigned Offset);
  unsigned FlagsForCondClassType(CondClassType Cond);
};

unsigned
DeadFlagCalculationEliminination::FlagForOffset(unsigned Offset)
{
  return Offset == offsetof(FEXCore::Core::CPUState, pf_raw) ? FLAG_P :
         Offset == offsetof(FEXCore::Core::CPUState, af_raw) ? FLAG_A :
         0;
};

unsigned
DeadFlagCalculationEliminination::FlagsForCondClassType(CondClassType Cond)
{
  switch (Cond) {
  case COND_AL:
    return 0;

  case COND_MI:
  case COND_PL:
    return FLAG_N;

  case COND_EQ:
  case COND_NEQ:
    return FLAG_Z;

  case COND_UGE:
  case COND_ULT:
    return FLAG_C;

  case COND_VS:
  case COND_VC:
  case COND_FU:
  case COND_FNU:
    return FLAG_V;

  case COND_UGT:
  case COND_ULE:
    return FLAG_Z | FLAG_C;

  case COND_SGE:
  case COND_SLT:
  case COND_FLU:
  case COND_FGE:
    return FLAG_N | FLAG_V;

  case COND_SGT:
  case COND_SLE:
  case COND_FLEU:
  case COND_FGT:
    return FLAG_N | FLAG_Z | FLAG_V;

  default:
    LOGMAN_THROW_AA_FMT(false, "unknown cond class type");
    return FLAG_NZCV;
  }
}

FlagInfo
DeadFlagCalculationEliminination::Classify(IROp_Header *IROp)
{
  switch (IROp->Op) {
    case OP_ANDWITHFLAGS:
      return {
        .Write = FLAG_NZCV,
        .CanReplace = true,
        .Replacement = OP_AND,
      };

    case OP_ADDNZCV:
    case OP_SUBNZCV:
    case OP_TESTNZ:
    case OP_FCMP:
    case OP_STORENZCV:
      return {
        .Write = FLAG_NZCV,
        .CanEliminate = true,
      };

    case OP_AXFLAG:
      // Per the Arm spec, axflag reads Z/V/C but not N. It writes all flags.
      return {
        .Read = FLAG_ZCV,
        .Write = FLAG_NZCV,
        .CanEliminate = true,
      };

    case OP_CARRYINVERT:
      return {
        .Read = FLAG_C,
        .Write = FLAG_C,
        .CanEliminate = true,
      };

    case OP_LOADNZCV:
      return {.Read = FLAG_NZCV};

    case OP_ADCNZCV:
    case OP_SBBNZCV:
      return {
        .Read = FLAG_C,
        .Write = FLAG_NZCV,
        .CanEliminate = true,
      };

    case OP_NZCVSELECT: {
      auto Op = IROp->CW<IR::IROp_NZCVSelect>();
      return {.Read = FlagsForCondClassType(Op->Cond)};
    }

    case OP_CONDJUMP: {
      auto Op = IROp->CW<IR::IROp_CondJump>();
      if (!Op->FromNZCV)
        break;

      return {.Read = FlagsForCondClassType(Op->Cond)};
    }

    case OP_CONDADDNZCV: {
      auto Op = IROp->CW<IR::IROp_CondAddNZCV>();
      return {
        .Read = FlagsForCondClassType(Op->Cond),
        .Write = FLAG_NZCV,
        .CanEliminate = true,
      };
    }

    case OP_RMIFNZCV: {
      auto Op = IROp->CW<IR::IROp_RmifNZCV>();

      static_assert(FLAG_N == (1 << 3), "rmif mask lines up with our bits");
      static_assert(FLAG_Z == (1 << 2), "rmif mask lines up with our bits");
      static_assert(FLAG_C == (1 << 1), "rmif mask lines up with our bits");
      static_assert(FLAG_V == (1 << 0), "rmif mask lines up with our bits");

      return {
        .Write = Op->Mask,
        .CanEliminate = true,
      };
    }

    case OP_INVALIDATEFLAGS: {
      auto Op = IROp->CW<IR::IROp_InvalidateFlags>();
      unsigned Flags = 0;

      // TODO: Make this translation less silly
      if (Op->Flags & (1u << X86State::RFLAG_SF_RAW_LOC))
        Flags |= FLAG_N;

      if (Op->Flags & (1u << X86State::RFLAG_ZF_RAW_LOC))
        Flags |= FLAG_Z;

      if (Op->Flags & (1u << X86State::RFLAG_CF_RAW_LOC))
        Flags |= FLAG_C;

      if (Op->Flags & (1u << X86State::RFLAG_OF_RAW_LOC))
        Flags |= FLAG_V;

      if (Op->Flags & (1u << X86State::RFLAG_PF_RAW_LOC))
        Flags |= FLAG_P;

      if (Op->Flags & (1u << X86State::RFLAG_AF_RAW_LOC))
        Flags |= FLAG_A;

      // The mental model of InvalidateFlags is writing undefined values to all
      // of the selected flags, allowing the write-after-write optimizations to
      // optimize invalidate-after-write for free.
      return {
        .Write = Flags,
        .CanEliminate = true,
      };
    }

    case OP_LOADREGISTER: {
      auto Op = IROp->CW<IR::IROp_LoadRegister>();
      if (Op->Class != GPRClass || Op->StaticClass != GPRFixedClass)
        break;

      return {.Read = FlagForOffset(Op->Offset)};
    }

    case OP_STOREREGISTER: {
      auto Op = IROp->CW<IR::IROp_StoreRegister>();
      if (Op->Class != GPRClass || Op->StaticClass != GPRFixedClass)
        break;

      LOGMAN_THROW_A_FMT(!Op->IsPrewrite, "PF/AF writes are fixed-form");
      unsigned Flag = FlagForOffset(Op->Offset);

      return {
        .Write = Flag,
        .CanEliminate = Flag != 0,
      };
    }

    default:
      break;
  }

  return {.Trivial = true};
}

/**
 * @brief This pass removes flag calculations that will otherwise be unused INSIDE of that block
 */
bool DeadFlagCalculationEliminination::Run(IREmitter *IREmit) {
  FEXCORE_PROFILE_SCOPED("PassManager::DFE");

  bool Changed = false;
  auto CurrentIR = IREmit->ViewIR();

  for (auto [BlockNode, BlockHeader] : CurrentIR.GetBlocks()) {
    // We model all flags as read at the end of the block, since this pass is
    // presently purely local. Optimizing this requires global anslysis.
    uint32_t FlagsRead = FLAG_ALL;

    // Reverse iteration is not yet working with the iterators
    auto BlockIROp = BlockHeader->CW<FEXCore::IR::IROp_CodeBlock>();

    // We grab these nodes this way so we can iterate easily
    auto CodeBegin = CurrentIR.at(BlockIROp->Begin);
    auto CodeLast = CurrentIR.at(BlockIROp->Last);

    // Iterate the block in reverse
    while (1) {
      auto [CodeNode, IROp] = CodeLast();

      // Optimiation algorithm: For each flag written...
      //
      //  If the flag has a later read (per FlagsRead), remove the flag from
      //  FlagsRead, since the reader is covered by this write.
      //
      //  Else, there is no later read, so remove the flag write (if we can).
      //  This is the active part of the optimization.
      //
      // Then, add each flag read to FlagsRead.
      //
      // This order is important: instructions that read-modify-write flags
      // (like adcs) first read flags, then write flags. Since we're iterating
      // the block backwards, that means we handle the write first.
      struct FlagInfo Info = Classify(IROp);

      if (!Info.Trivial) {
        bool Eliminated = false;

        if ((FlagsRead & Info.Write) == 0) {
          if (Info.CanEliminate) {
            IREmit->Remove(CodeNode);
            Eliminated = true;
            Changed = true;
          } else if (Info.CanReplace) {
            IROp->Op = Info.Replacement;
            Changed = true;
          }
        } else {
          FlagsRead &= ~Info.Write;
        }

        // If we eliminated the instruction, we eliminate its read too. This
        // check is required to ensure the pass converges locally in a single
        // iteration.
        if (!Eliminated)
          FlagsRead |= Info.Read;
      }

      // Iterate in reverse
      if (CodeLast == CodeBegin) {
        break;
      }
      --CodeLast;
    }
  }

  return Changed;
}

fextl::unique_ptr<FEXCore::IR::Pass> CreateDeadFlagCalculationEliminination() {
  return fextl::make_unique<DeadFlagCalculationEliminination>();
}

}