SPIRV-Tools/source/opt/aggressive_dead_code_elim_pass.cpp

// Copyright (c) 2017 The Khronos Group Inc.
// Copyright (c) 2017 Valve Corporation
// Copyright (c) 2017 LunarG Inc.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

#include "aggressive_dead_code_elim_pass.h"

#include "cfa.h"
#include "iterator.h"
#include "latest_version_glsl_std_450_header.h"

#include <stack>

namespace spvtools {
namespace opt {

namespace {

const uint32_t kTypePointerStorageClassInIdx = 0;
const uint32_t kEntryPointFunctionIdInIdx = 1;
const uint32_t kSelectionMergeMergeBlockIdInIdx = 0;
const uint32_t kLoopMergeMergeBlockIdInIdx = 0;
const uint32_t kLoopMergeContinueBlockIdInIdx = 1;

}  // namespace

bool AggressiveDCEPass::IsVarOfStorage(uint32_t varId, uint32_t storageClass) {
  if (varId == 0) return false;
  const ir::Instruction* varInst = get_def_use_mgr()->GetDef(varId);
  const SpvOp op = varInst->opcode();
  if (op != SpvOpVariable) return false;
  const uint32_t varTypeId = varInst->type_id();
  const ir::Instruction* varTypeInst = get_def_use_mgr()->GetDef(varTypeId);
  if (varTypeInst->opcode() != SpvOpTypePointer) return false;
  return varTypeInst->GetSingleWordInOperand(kTypePointerStorageClassInIdx) ==
         storageClass;
}

bool AggressiveDCEPass::IsLocalVar(uint32_t varId) {
  return IsVarOfStorage(varId, SpvStorageClassFunction) ||
         (IsVarOfStorage(varId, SpvStorageClassPrivate) && private_like_local_);
}

void AggressiveDCEPass::AddStores(uint32_t ptrId) {
  get_def_use_mgr()->ForEachUser(ptrId, [this](ir::Instruction* user) {
    switch (user->opcode()) {
      case SpvOpAccessChain:
      case SpvOpInBoundsAccessChain:
      case SpvOpCopyObject:
        this->AddStores(user->result_id());
        break;
      case SpvOpLoad:
        break;
      // If default, assume it stores e.g. frexp, modf, function call
      case SpvOpStore:
      default:
        if (!IsLive(user)) AddToWorklist(user);
        break;
    }
  });
}

bool AggressiveDCEPass::AllExtensionsSupported() const {
  // If any extension not in whitelist, return false
  for (auto& ei : get_module()->extensions()) {
    const char* extName =
        reinterpret_cast<const char*>(&ei.GetInOperand(0).words[0]);
    if (extensions_whitelist_.find(extName) == extensions_whitelist_.end())
      return false;
  }
  return true;
}

bool AggressiveDCEPass::IsTargetDead(ir::Instruction* inst) {
  const uint32_t tId = inst->GetSingleWordInOperand(0);
  const ir::Instruction* tInst = get_def_use_mgr()->GetDef(tId);
  if (dead_insts_.find(tInst) != dead_insts_.end()) {
    return true;
  }
  return false;
}

void AggressiveDCEPass::ProcessLoad(uint32_t varId) {
  // Only process locals
  if (!IsLocalVar(varId)) return;
  // Return if already processed
  if (live_local_vars_.find(varId) != live_local_vars_.end()) return;
  // Mark all stores to varId as live
  AddStores(varId);
  // Cache varId as processed
  live_local_vars_.insert(varId);
}

bool AggressiveDCEPass::IsStructuredIfOrLoopHeader(ir::BasicBlock* bp,
                                                   ir::Instruction** mergeInst,
                                                   ir::Instruction** branchInst,
                                                   uint32_t* mergeBlockId) {
  ir::Instruction* mi = bp->GetMergeInst();
  if (mi == nullptr) return false;
  ir::Instruction* bri = &*bp->tail();
  // Make sure it is not a Switch
  if (mi->opcode() == SpvOpSelectionMerge &&
      bri->opcode() != SpvOpBranchConditional)
    return false;
  if (branchInst != nullptr) *branchInst = bri;
  if (mergeInst != nullptr) *mergeInst = mi;
  if (mergeBlockId != nullptr) *mergeBlockId = mi->GetSingleWordInOperand(0);
  return true;
}

void AggressiveDCEPass::ComputeBlock2HeaderMaps(
    std::list<ir::BasicBlock*>& structuredOrder) {
  block2headerBranch_.clear();
  branch2merge_.clear();
  std::stack<ir::Instruction*> currentHeaderBranch;
  currentHeaderBranch.push(nullptr);
  uint32_t currentMergeBlockId = 0;
  for (auto bi = structuredOrder.begin(); bi != structuredOrder.end(); ++bi) {
    // If this block is the merge block of the current control construct,
    // we are leaving the current construct so we must update state
    if ((*bi)->id() == currentMergeBlockId) {
      currentHeaderBranch.pop();
      ir::Instruction* chb = currentHeaderBranch.top();
      if (chb != nullptr)
        currentMergeBlockId = branch2merge_[chb]->GetSingleWordInOperand(0);
    }
    ir::Instruction* mergeInst;
    ir::Instruction* branchInst;
    uint32_t mergeBlockId;
    bool is_header =
        IsStructuredIfOrLoopHeader(*bi, &mergeInst, &branchInst, &mergeBlockId);
    // If this is a loop header, update state first so the block will map to
    // the loop.
    if (is_header && mergeInst->opcode() == SpvOpLoopMerge) {
      currentHeaderBranch.push(branchInst);
      branch2merge_[branchInst] = mergeInst;
      currentMergeBlockId = mergeBlockId;
    }
    // Map the block to the current construct.
    block2headerBranch_[*bi] = currentHeaderBranch.top();
    // If this is an if header, update state so following blocks map to the if.
    if (is_header && mergeInst->opcode() == SpvOpSelectionMerge) {
      currentHeaderBranch.push(branchInst);
      branch2merge_[branchInst] = mergeInst;
      currentMergeBlockId = mergeBlockId;
    }
  }
}

void AggressiveDCEPass::ComputeInst2BlockMap(ir::Function* func) {
  for (auto& blk : *func) {
    blk.ForEachInst(
        [&blk, this](ir::Instruction* ip) { inst2block_[ip] = &blk; });
  }
}

void AggressiveDCEPass::AddBranch(uint32_t labelId, ir::BasicBlock* bp) {
  std::unique_ptr<ir::Instruction> newBranch(new ir::Instruction(
      context(), SpvOpBranch, 0, 0,
      {{spv_operand_type_t::SPV_OPERAND_TYPE_ID, {labelId}}}));
  get_def_use_mgr()->AnalyzeInstDefUse(&*newBranch);
  bp->AddInstruction(std::move(newBranch));
}

void AggressiveDCEPass::AddBreaksAndContinuesToWorklist(
    ir::Instruction* loopMerge) {
  const uint32_t mergeId =
      loopMerge->GetSingleWordInOperand(kLoopMergeMergeBlockIdInIdx);
  get_def_use_mgr()->ForEachUser(
      mergeId, [&loopMerge, this](ir::Instruction* user) {
        // A branch to the merge block can only be a break if it is nested in
        // the current loop
        SpvOp op = user->opcode();
        if (op != SpvOpBranchConditional && op != SpvOpBranch) return;
        ir::Instruction* branchInst = user;
        while (true) {
          ir::BasicBlock* blk = inst2block_[branchInst];
          ir::Instruction* hdrBranch = block2headerBranch_[blk];
          if (hdrBranch == nullptr) return;
          ir::Instruction* hdrMerge = branch2merge_[hdrBranch];
          if (hdrMerge == loopMerge) break;
          branchInst = hdrBranch;
        }
        if (!IsLive(user)) {
          AddToWorklist(user);
          // Add branch's merge if there is one
          ir::Instruction* userMerge = branch2merge_[user];
          if (userMerge != nullptr) AddToWorklist(userMerge);
        }
      });
  const uint32_t contId =
      loopMerge->GetSingleWordInOperand(kLoopMergeContinueBlockIdInIdx);
  get_def_use_mgr()->ForEachUser(contId, [&contId,
                                          this](ir::Instruction* user) {
    SpvOp op = user->opcode();
    if (op == SpvOpBranchConditional) {
      // A conditional branch can only be a continue if it does not have a merge
      // instruction or its merge block is not the continue block.
      ir::Instruction* hdrMerge = branch2merge_[user];
      if (hdrMerge != nullptr && hdrMerge->opcode() == SpvOpSelectionMerge) {
        uint32_t hdrMergeId =
            hdrMerge->GetSingleWordInOperand(kSelectionMergeMergeBlockIdInIdx);
        if (hdrMergeId == contId) return;
        // Need to mark merge instruction too
        if (!IsLive(hdrMerge)) AddToWorklist(hdrMerge);
      }
    } else if (op == SpvOpBranch) {
      // An unconditional branch can only be a continue if it is not
      // branching to its own merge block.
      ir::BasicBlock* blk = inst2block_[user];
      ir::Instruction* hdrBranch = block2headerBranch_[blk];
      if (hdrBranch == nullptr) return;
      ir::Instruction* hdrMerge = branch2merge_[hdrBranch];
      if (hdrMerge->opcode() == SpvOpLoopMerge) return;
      uint32_t hdrMergeId =
          hdrMerge->GetSingleWordInOperand(kSelectionMergeMergeBlockIdInIdx);
      if (contId == hdrMergeId) return;
    } else {
      return;
    }
    if (!IsLive(user)) AddToWorklist(user);
  });
}

bool AggressiveDCEPass::AggressiveDCE(ir::Function* func) {
  // Compute map from instruction to block
  ComputeInst2BlockMap(func);
  // Compute map from block to controlling conditional branch
  std::list<ir::BasicBlock*> structuredOrder;
  cfg()->ComputeStructuredOrder(func, &*func->begin(), &structuredOrder);
  ComputeBlock2HeaderMaps(structuredOrder);
  bool modified = false;
  // Add instructions with external side effects to worklist. Also add branches
  // EXCEPT those immediately contained in an "if" selection construct or a loop
  // or continue construct.
  // TODO(greg-lunarg): Handle Frexp, Modf more optimally
  call_in_func_ = false;
  func_is_entry_point_ = false;
  private_stores_.clear();
  // Stacks to keep track of when we are inside an if- or loop-construct.
  // When immediately inside an if- or loop-construct, we do not initially
  // mark branches live. All other branches must be marked live.
  std::stack<bool> assume_branches_live;
  std::stack<uint32_t> currentMergeBlockId;
  // Push sentinel values on stack for when outside of any control flow.
  assume_branches_live.push(true);
  currentMergeBlockId.push(0);
  for (auto bi = structuredOrder.begin(); bi != structuredOrder.end(); ++bi) {
    // If exiting if or loop, update stacks
    if ((*bi)->id() == currentMergeBlockId.top()) {
      assume_branches_live.pop();
      currentMergeBlockId.pop();
    }
    for (auto ii = (*bi)->begin(); ii != (*bi)->end(); ++ii) {
      SpvOp op = ii->opcode();
      switch (op) {
        case SpvOpStore: {
          uint32_t varId;
          (void)GetPtr(&*ii, &varId);
          // Mark stores as live if their variable is not function scope
          // and is not private scope. Remember private stores for possible
          // later inclusion
          if (IsVarOfStorage(varId, SpvStorageClassPrivate))
            private_stores_.push_back(&*ii);
          else if (!IsVarOfStorage(varId, SpvStorageClassFunction))
            AddToWorklist(&*ii);
        } break;
        case SpvOpLoopMerge: {
          assume_branches_live.push(false);
          currentMergeBlockId.push(
              ii->GetSingleWordInOperand(kLoopMergeMergeBlockIdInIdx));
        } break;
        case SpvOpSelectionMerge: {
          auto brii = ii;
          ++brii;
          bool is_structured_if = brii->opcode() == SpvOpBranchConditional;
          assume_branches_live.push(!is_structured_if);
          currentMergeBlockId.push(
              ii->GetSingleWordInOperand(kSelectionMergeMergeBlockIdInIdx));
          if (!is_structured_if) AddToWorklist(&*ii);
        } break;
        case SpvOpBranch:
        case SpvOpBranchConditional: {
          if (assume_branches_live.top()) AddToWorklist(&*ii);
        } break;
        default: {
          // Function calls, atomics, function params, function returns, etc.
          // TODO(greg-lunarg): function calls live only if write to non-local
          if (!context()->IsCombinatorInstruction(&*ii)) {
            AddToWorklist(&*ii);
          }
          // Remember function calls
          if (op == SpvOpFunctionCall) call_in_func_ = true;
        } break;
      }
    }
  }
  // See if current function is an entry point
  for (auto& ei : get_module()->entry_points()) {
    if (ei.GetSingleWordInOperand(kEntryPointFunctionIdInIdx) ==
        func->result_id()) {
      func_is_entry_point_ = true;
      break;
    }
  }
  // If the current function is an entry point and has no function calls,
  // we can optimize private variables as locals
  private_like_local_ = func_is_entry_point_ && !call_in_func_;
  // If privates are not like local, add their stores to worklist
  if (!private_like_local_)
    for (auto& ps : private_stores_) AddToWorklist(ps);
  // Add OpGroupDecorates to worklist because they are a pain to remove
  // ids from.
  // TODO(greg-lunarg): Handle dead ids in OpGroupDecorate
  for (auto& ai : get_module()->annotations()) {
    if (ai.opcode() == SpvOpGroupDecorate) AddToWorklist(&ai);
  }
  // Perform closure on live instruction set.
  while (!worklist_.empty()) {
    ir::Instruction* liveInst = worklist_.front();
    // Add all operand instructions if not already live
    liveInst->ForEachInId([&liveInst, this](const uint32_t* iid) {
      ir::Instruction* inInst = get_def_use_mgr()->GetDef(*iid);
      // Do not add label if an operand of a branch. This is not needed
      // as part of live code discovery and can create false live code,
      // for example, the branch to a header of a loop.
      if (inInst->opcode() == SpvOpLabel && liveInst->IsBranch()) return;
      if (!IsLive(inInst)) AddToWorklist(inInst);
    });
    // If in a structured if or loop construct, add the controlling
    // conditional branch and its merge. Any containing control construct
    // is marked live when the merge and branch are processed out of the
    // worklist.
    ir::BasicBlock* blk = inst2block_[liveInst];
    ir::Instruction* branchInst = block2headerBranch_[blk];
    if (branchInst != nullptr && !IsLive(branchInst)) {
      AddToWorklist(branchInst);
      ir::Instruction* mergeInst = branch2merge_[branchInst];
      AddToWorklist(mergeInst);
      // If in a loop, mark all its break and continue instructions live
      if (mergeInst->opcode() == SpvOpLoopMerge)
        AddBreaksAndContinuesToWorklist(mergeInst);
    }
    // If local load, add all variable's stores if variable not already live
    if (liveInst->opcode() == SpvOpLoad) {
      uint32_t varId;
      (void)GetPtr(liveInst, &varId);
      if (varId != 0) {
        ProcessLoad(varId);
      }
    }
    // If function call, treat as if it loads from all pointer arguments
    else if (liveInst->opcode() == SpvOpFunctionCall) {
      liveInst->ForEachInId([this](const uint32_t* iid) {
        // Skip non-ptr args
        if (!IsPtr(*iid)) return;
        uint32_t varId;
        (void)GetPtr(*iid, &varId);
        ProcessLoad(varId);
      });
    }
    // If function parameter, treat as if it's result id is loaded from
    else if (liveInst->opcode() == SpvOpFunctionParameter) {
      ProcessLoad(liveInst->result_id());
    }
    worklist_.pop();
  }
  // Mark all non-live instructions dead except non-structured branches, which
  // now should be considered live unless their block is deleted.
  for (auto bi = structuredOrder.begin(); bi != structuredOrder.end(); ++bi) {
    for (auto ii = (*bi)->begin(); ii != (*bi)->end(); ++ii) {
      if (IsLive(&*ii)) continue;
      if (ii->IsBranch() &&
          !IsStructuredIfOrLoopHeader(*bi, nullptr, nullptr, nullptr))
        continue;
      dead_insts_.insert(&*ii);
    }
  }
  // Remove debug and annotation statements referencing dead instructions.
  // This must be done before killing the instructions, otherwise there are
  // dead objects in the def/use database.
  ir::Instruction* instruction = &*get_module()->debug2_begin();
  while (instruction) {
    if (instruction->opcode() != SpvOpName) {
      instruction = instruction->NextNode();
      continue;
    }

    if (IsTargetDead(instruction)) {
      instruction = context()->KillInst(instruction);
      modified = true;
    } else {
      instruction = instruction->NextNode();
    }
  }

  instruction = &*get_module()->annotation_begin();
  while (instruction) {
    if (instruction->opcode() != SpvOpDecorate &&
        instruction->opcode() != SpvOpDecorateId) {
      instruction = instruction->NextNode();
      continue;
    }

    if (IsTargetDead(instruction)) {
      instruction = context()->KillInst(instruction);
      modified = true;
    } else {
      instruction = instruction->NextNode();
    }
  }

  // Kill dead instructions and remember dead blocks
  for (auto bi = structuredOrder.begin(); bi != structuredOrder.end();) {
    uint32_t mergeBlockId = 0;
    (*bi)->ForEachInst([this, &modified, &mergeBlockId](ir::Instruction* inst) {
      if (dead_insts_.find(inst) == dead_insts_.end()) return;
      // If dead instruction is selection merge, remember merge block
      // for new branch at end of block
      if (inst->opcode() == SpvOpSelectionMerge ||
          inst->opcode() == SpvOpLoopMerge)
        mergeBlockId = inst->GetSingleWordInOperand(0);
      context()->KillInst(inst);
      modified = true;
    });
    // If a structured if or loop was deleted, add a branch to its merge
    // block, and traverse to the merge block and continue processing there.
    // We know the block still exists because the label is not deleted.
    if (mergeBlockId != 0) {
      AddBranch(mergeBlockId, *bi);
      for (++bi; (*bi)->id() != mergeBlockId; ++bi) {
      }
    } else {
      ++bi;
    }
  }
  // Cleanup all CFG including all unreachable blocks
  CFGCleanup(func);

  return modified;
}

void AggressiveDCEPass::Initialize(ir::IRContext* c) {
  InitializeProcessing(c);

  // Clear collections
  worklist_ = std::queue<ir::Instruction*>{};
  live_insts_.clear();
  live_local_vars_.clear();
  dead_insts_.clear();

  // Initialize extensions whitelist
  InitExtensions();
}

Pass::Status AggressiveDCEPass::ProcessImpl() {
  // Current functionality assumes shader capability
  // TODO(greg-lunarg): Handle additional capabilities
  if (!context()->get_feature_mgr()->HasCapability(SpvCapabilityShader))
    return Status::SuccessWithoutChange;
  // Current functionality assumes relaxed logical addressing (see
  // instruction.h)
  // TODO(greg-lunarg): Handle non-logical addressing
  if (context()->get_feature_mgr()->HasCapability(SpvCapabilityAddresses))
    return Status::SuccessWithoutChange;
  // If any extensions in the module are not explicitly supported,
  // return unmodified.
  if (!AllExtensionsSupported()) return Status::SuccessWithoutChange;
  // Process all entry point functions
  ProcessFunction pfn = [this](ir::Function* fp) { return AggressiveDCE(fp); };
  bool modified = ProcessEntryPointCallTree(pfn, get_module());
  return modified ? Status::SuccessWithChange : Status::SuccessWithoutChange;
}

AggressiveDCEPass::AggressiveDCEPass() {}

Pass::Status AggressiveDCEPass::Process(ir::IRContext* c) {
  Initialize(c);
  return ProcessImpl();
}

void AggressiveDCEPass::InitExtensions() {
  extensions_whitelist_.clear();
  extensions_whitelist_.insert({
      "SPV_AMD_shader_explicit_vertex_parameter",
      "SPV_AMD_shader_trinary_minmax",
      "SPV_AMD_gcn_shader",
      "SPV_KHR_shader_ballot",
      "SPV_AMD_shader_ballot",
      "SPV_AMD_gpu_shader_half_float",
      "SPV_KHR_shader_draw_parameters",
      "SPV_KHR_subgroup_vote",
      "SPV_KHR_16bit_storage",
      "SPV_KHR_device_group",
      "SPV_KHR_multiview",
      "SPV_NVX_multiview_per_view_attributes",
      "SPV_NV_viewport_array2",
      "SPV_NV_stereo_view_rendering",
      "SPV_NV_sample_mask_override_coverage",
      "SPV_NV_geometry_shader_passthrough",
      "SPV_AMD_texture_gather_bias_lod",
      "SPV_KHR_storage_buffer_storage_class",
      // SPV_KHR_variable_pointers
      //   Currently do not support extended pointer expressions
      "SPV_AMD_gpu_shader_int16",
      "SPV_KHR_post_depth_coverage",
      "SPV_KHR_shader_atomic_counter_ops",
  });
}

}  // namespace opt
}  // namespace spvtools