intel-graphics-compiler/IGC/Compiler/CISACodeGen/PullConstantHeuristics.cpp

/*========================== begin_copyright_notice ============================

Copyright (C) 2017-2021 Intel Corporation

SPDX-License-Identifier: MIT

============================= end_copyright_notice ===========================*/

#include "PullConstantHeuristics.hpp"

#include "Platform.hpp"

using namespace llvm;
using namespace IGC;

static const unsigned EUInstCycleCount = 2;
static const unsigned SendInstCycleCount = 190;
static const unsigned RTWriteInstCycleCount = 190;

char PullConstantHeuristics::ID = 0;

#define PASS_FLAG "Analyse shader to determine push const threshold"
#define PASS_DESCRIPTION "This is an analysis pass for pulling constants for short shaders "
#define PASS_CFG_ONLY true
#define PASS_ANALYSIS true
IGC_INITIALIZE_PASS_BEGIN(PullConstantHeuristics, PASS_FLAG, PASS_DESCRIPTION, PASS_CFG_ONLY, PASS_ANALYSIS)
IGC_INITIALIZE_PASS_DEPENDENCY(CodeGenContextWrapper)
IGC_INITIALIZE_PASS_END(PullConstantHeuristics, PASS_FLAG, PASS_DESCRIPTION, PASS_CFG_ONLY, PASS_ANALYSIS)

// needs to be fine-tuned after obtaining feedback from visa
static unsigned estimateShaderLifetime(unsigned int EUCnt, unsigned SendMsgCnt, unsigned RTWriteCnt) {
  return EUCnt * EUInstCycleCount + SendMsgCnt * SendInstCycleCount + RTWriteCnt * RTWriteInstCycleCount;
}

static bool isSendMessage(const GenIntrinsicInst *inst) {
  if (isSampleInstruction(inst) || isSampleLoadGather4InfoInstruction(inst)) {
    return true;
  }
  return false;
}

// approximating EU insts count - TODO: need a better way to do this
static unsigned getEUInstEstimate(const Instruction *inst) {
  // handle ALU, Logical, and load-store insts
  // Presently we're restricting to shaders with 1 BB only, i.e, short shaders
  // But this handles branch insts as well (in case of multiple BB support in future)
  if (inst->getOpcode() <= Instruction::Fence) {
    return 1;
  }

  // handling remaining inst types
  switch (inst->getOpcode()) {
  case Instruction::FCmp:
  case Instruction::ICmp:
  case Instruction::Select:
  case Instruction::Ret:
    return 1;
  default:
    // bitcast insts don't make an EU inst in visa
    return 0;
  }
}

// estimate EU, SendMsg and RTWrite insts required by the PS
static std::tuple<unsigned, unsigned, unsigned> getInstStats(const Function &F) {
  unsigned EUInstCnt = 0;
  unsigned int SendMsgInstCnt = 0;
  unsigned int RTWriteInstCnt = 0;
  for (auto &BB : F) {
    for (auto &II : BB) {
      if (auto pIntrinsic = llvm::dyn_cast<llvm::GenIntrinsicInst>(&II)) {
        if (isSendMessage(pIntrinsic))
          SendMsgInstCnt++;
        else if (llvm::isa<llvm::RTWriteIntrinsic>(pIntrinsic))
          RTWriteInstCnt++;
      } else {
        EUInstCnt += getEUInstEstimate(&II);
      }
    }
  }
  return std::make_tuple(EUInstCnt, SendMsgInstCnt, RTWriteInstCnt);
}

// Pixel Shader Dispatch can be bottleneck if
//     thread_payload_size > max(simd4_sample_instr, simd4_eu_instr / 16, simd4_rt_write * 2, shader_lifetime / 56)
unsigned int PullConstantHeuristics::getPSDBottleNeckThreshold(const Function &F) {
  CodeGenContext *ctx = nullptr;
  ctx = getAnalysis<CodeGenContextWrapper>().getCodeGenContext();
  const unsigned numThreadsPerSubslice = ctx->platform.getMaxNumberThreadPerSubslice(); // read from ctx.platform
  const unsigned roofLineIPC = 16;
  const unsigned pixelRate = 2;

  auto [EstimatedEUInstCnt, SendMsgInstCnt, RTWriteInstCnt] = getInstStats(F);

  unsigned shaderLifetime = estimateShaderLifetime(EstimatedEUInstCnt, SendMsgInstCnt, RTWriteInstCnt);

  // calculate payload size threshold assuming SIMD16 to stop pushing constants
  unsigned SIMD4_EU_Cnt = EstimatedEUInstCnt * 4;
  unsigned SIMD4_Sample_Cnt = SendMsgInstCnt * 4;
  unsigned SIMD4_RT_Write_Cnt = RTWriteInstCnt * 4;
  unsigned payloadThreshold =
      std::max(std::max(SIMD4_Sample_Cnt, SIMD4_EU_Cnt / roofLineIPC),
               std::max(SIMD4_RT_Write_Cnt * pixelRate, shaderLifetime / numThreadsPerSubslice));
  return payloadThreshold;
}

// currentPayloadSize = payloadHeaderSIMD16 + payloadBarySIMD16 + inputSize;
//
static unsigned getCurrentPayloadSizeEstimate(const Function &F) {
  unsigned payloadHeaderSIMD16 = 3;
  unsigned payloadBarySIMD16 = 4;
  unsigned inputGRFSize = 1;

  // helper variables
  unsigned maxValueFromInputVec = 0;
  std::set<unsigned> countOfDifferentBary;
  for (auto &BB : F) {
    for (auto &II : BB) {
      if (auto pIntrinsic = llvm::dyn_cast<llvm::GenIntrinsicInst>(&II)) {
        if (pIntrinsic->getIntrinsicID() == GenISAIntrinsic::GenISA_DCL_inputVec) {
          countOfDifferentBary.insert((unsigned)llvm::cast<llvm::ConstantInt>(II.getOperand(1))->getZExtValue());
          maxValueFromInputVec =
              std::max(maxValueFromInputVec, (unsigned)llvm::cast<llvm::ConstantInt>(II.getOperand(0))->getZExtValue());
        }
      }
    }
  }
  payloadBarySIMD16 = countOfDifferentBary.size() * 4;
  inputGRFSize =
      (unsigned)(maxValueFromInputVec / 2) % 2 == 0 ? (maxValueFromInputVec / 2) : ((maxValueFromInputVec / 2) + 1);
  return payloadHeaderSIMD16 + payloadBarySIMD16 + inputGRFSize;
}

bool PullConstantHeuristics::runOnModule(Module &M) {
  if (IGC_IS_FLAG_ENABLED(DisablePullConstantHeuristics)) {
    return false;
  }
  CodeGenContext *ctx = nullptr;
  ctx = getAnalysis<CodeGenContextWrapper>().getCodeGenContext();
  if (ctx->type == ShaderType::PIXEL_SHADER && ctx->platform.hasPSDBottleneck()) {
    for (auto &F : M) {
      if (F.size() == 1) {
        BasicBlock *BB = &(*F.begin());
        if (BB->size() < 200) { // short shaders
          int PSDBottleNeckThreshold = getPSDBottleNeckThreshold(F);
          int PayloadWithoutConstants = getCurrentPayloadSizeEstimate(F);
          int maxPayload_Regkey = (IGC_GET_FLAG_VALUE(PayloadSizeThreshold));
          PSDBottleNeckThreshold =
              PSDBottleNeckThreshold > maxPayload_Regkey ? maxPayload_Regkey : PSDBottleNeckThreshold;

          int threshold = PSDBottleNeckThreshold - PayloadWithoutConstants;
          threshold = threshold < 0 ? 0 : threshold;
          thresholdMap.insert(std::make_pair(&F, threshold));
        }
      }
    }
  }
  return false; // does not modify IR
}