source/opt/inline_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 "inline_pass.h"

#include "cfa.h"

// Indices of operands in SPIR-V instructions

static const int kSpvFunctionCallFunctionId = 2;
static const int kSpvFunctionCallArgumentId = 3;
static const int kSpvReturnValueId = 0;
static const int kSpvLoopMergeMergeBlockId = 0;
static const int kSpvLoopMergeContinueTargetIdInIdx = 1;

namespace spvtools {
namespace opt {

uint32_t InlinePass::AddPointerToType(uint32_t type_id,
                                      SpvStorageClass storage_class) {
  uint32_t resultId = TakeNextId();
  std::unique_ptr<ir::Instruction> type_inst(new ir::Instruction(
      context(), SpvOpTypePointer, 0, resultId,
      {{spv_operand_type_t::SPV_OPERAND_TYPE_STORAGE_CLASS,
        {uint32_t(storage_class)}},
       {spv_operand_type_t::SPV_OPERAND_TYPE_ID, {type_id}}}));
  context()->AddType(std::move(type_inst));
  analysis::Type* pointeeTy;
  std::unique_ptr<analysis::Pointer> pointerTy;
  std::tie(pointeeTy, pointerTy) =
      context()->get_type_mgr()->GetTypeAndPointerType(type_id,
                                                       SpvStorageClassFunction);
  context()->get_type_mgr()->RegisterType(resultId, *pointerTy);
  return resultId;
}

void InlinePass::AddBranch(uint32_t label_id,
                           std::unique_ptr<ir::BasicBlock>* block_ptr) {
  std::unique_ptr<ir::Instruction> newBranch(new ir::Instruction(
      context(), SpvOpBranch, 0, 0,
      {{spv_operand_type_t::SPV_OPERAND_TYPE_ID, {label_id}}}));
  (*block_ptr)->AddInstruction(std::move(newBranch));
}

void InlinePass::AddBranchCond(uint32_t cond_id, uint32_t true_id,
                               uint32_t false_id,
                               std::unique_ptr<ir::BasicBlock>* block_ptr) {
  std::unique_ptr<ir::Instruction> newBranch(new ir::Instruction(
      context(), SpvOpBranchConditional, 0, 0,
      {{spv_operand_type_t::SPV_OPERAND_TYPE_ID, {cond_id}},
       {spv_operand_type_t::SPV_OPERAND_TYPE_ID, {true_id}},
       {spv_operand_type_t::SPV_OPERAND_TYPE_ID, {false_id}}}));
  (*block_ptr)->AddInstruction(std::move(newBranch));
}

void InlinePass::AddLoopMerge(uint32_t merge_id, uint32_t continue_id,
                              std::unique_ptr<ir::BasicBlock>* block_ptr) {
  std::unique_ptr<ir::Instruction> newLoopMerge(new ir::Instruction(
      context(), SpvOpLoopMerge, 0, 0,
      {{spv_operand_type_t::SPV_OPERAND_TYPE_ID, {merge_id}},
       {spv_operand_type_t::SPV_OPERAND_TYPE_ID, {continue_id}},
       {spv_operand_type_t::SPV_OPERAND_TYPE_LOOP_CONTROL, {0}}}));
  (*block_ptr)->AddInstruction(std::move(newLoopMerge));
}

void InlinePass::AddStore(uint32_t ptr_id, uint32_t val_id,
                          std::unique_ptr<ir::BasicBlock>* block_ptr) {
  std::unique_ptr<ir::Instruction> newStore(new ir::Instruction(
      context(), SpvOpStore, 0, 0,
      {{spv_operand_type_t::SPV_OPERAND_TYPE_ID, {ptr_id}},
       {spv_operand_type_t::SPV_OPERAND_TYPE_ID, {val_id}}}));
  (*block_ptr)->AddInstruction(std::move(newStore));
}

void InlinePass::AddLoad(uint32_t type_id, uint32_t resultId, uint32_t ptr_id,
                         std::unique_ptr<ir::BasicBlock>* block_ptr) {
  std::unique_ptr<ir::Instruction> newLoad(new ir::Instruction(
      context(), SpvOpLoad, type_id, resultId,
      {{spv_operand_type_t::SPV_OPERAND_TYPE_ID, {ptr_id}}}));
  (*block_ptr)->AddInstruction(std::move(newLoad));
}

std::unique_ptr<ir::Instruction> InlinePass::NewLabel(uint32_t label_id) {
  std::unique_ptr<ir::Instruction> newLabel(
      new ir::Instruction(context(), SpvOpLabel, 0, label_id, {}));
  return newLabel;
}

uint32_t InlinePass::GetFalseId() {
  if (false_id_ != 0) return false_id_;
  false_id_ = get_module()->GetGlobalValue(SpvOpConstantFalse);
  if (false_id_ != 0) return false_id_;
  uint32_t boolId = get_module()->GetGlobalValue(SpvOpTypeBool);
  if (boolId == 0) {
    boolId = TakeNextId();
    get_module()->AddGlobalValue(SpvOpTypeBool, boolId, 0);
  }
  false_id_ = TakeNextId();
  get_module()->AddGlobalValue(SpvOpConstantFalse, false_id_, boolId);
  return false_id_;
}

void InlinePass::MapParams(
    ir::Function* calleeFn, ir::BasicBlock::iterator call_inst_itr,
    std::unordered_map<uint32_t, uint32_t>* callee2caller) {
  int param_idx = 0;
  calleeFn->ForEachParam(
      [&call_inst_itr, &param_idx, &callee2caller](const ir::Instruction* cpi) {
        const uint32_t pid = cpi->result_id();
        (*callee2caller)[pid] = call_inst_itr->GetSingleWordOperand(
            kSpvFunctionCallArgumentId + param_idx);
        ++param_idx;
      });
}

void InlinePass::CloneAndMapLocals(
    ir::Function* calleeFn,
    std::vector<std::unique_ptr<ir::Instruction>>* new_vars,
    std::unordered_map<uint32_t, uint32_t>* callee2caller) {
  auto callee_block_itr = calleeFn->begin();
  auto callee_var_itr = callee_block_itr->begin();
  while (callee_var_itr->opcode() == SpvOp::SpvOpVariable) {
    std::unique_ptr<ir::Instruction> var_inst(
        callee_var_itr->Clone(callee_var_itr->context()));
    uint32_t newId = TakeNextId();
    get_decoration_mgr()->CloneDecorations(callee_var_itr->result_id(), newId,
                                           update_def_use_mgr_);
    var_inst->SetResultId(newId);
    (*callee2caller)[callee_var_itr->result_id()] = newId;
    new_vars->push_back(std::move(var_inst));
    ++callee_var_itr;
  }
}

uint32_t InlinePass::CreateReturnVar(
    ir::Function* calleeFn,
    std::vector<std::unique_ptr<ir::Instruction>>* new_vars) {
  uint32_t returnVarId = 0;
  const uint32_t calleeTypeId = calleeFn->type_id();
  const ir::Instruction* calleeType =
      get_def_use_mgr()->id_to_defs().find(calleeTypeId)->second;
  if (calleeType->opcode() != SpvOpTypeVoid) {
    // Find or create ptr to callee return type.
    uint32_t returnVarTypeId = context()->get_type_mgr()->FindPointerToType(
        calleeTypeId, SpvStorageClassFunction);
    if (returnVarTypeId == 0)
      returnVarTypeId = AddPointerToType(calleeTypeId, SpvStorageClassFunction);
    // Add return var to new function scope variables.
    returnVarId = TakeNextId();
    std::unique_ptr<ir::Instruction> var_inst(new ir::Instruction(
        context(), SpvOpVariable, returnVarTypeId, returnVarId,
        {{spv_operand_type_t::SPV_OPERAND_TYPE_STORAGE_CLASS,
          {SpvStorageClassFunction}}}));
    new_vars->push_back(std::move(var_inst));
  }
  get_decoration_mgr()->CloneDecorations(calleeFn->result_id(), returnVarId,
                                         update_def_use_mgr_);
  return returnVarId;
}

bool InlinePass::IsSameBlockOp(const ir::Instruction* inst) const {
  return inst->opcode() == SpvOpSampledImage || inst->opcode() == SpvOpImage;
}

void InlinePass::CloneSameBlockOps(
    std::unique_ptr<ir::Instruction>* inst,
    std::unordered_map<uint32_t, uint32_t>* postCallSB,
    std::unordered_map<uint32_t, ir::Instruction*>* preCallSB,
    std::unique_ptr<ir::BasicBlock>* block_ptr) {
  (*inst)->ForEachInId([&postCallSB, &preCallSB, &block_ptr,
                        this](uint32_t* iid) {
    const auto mapItr = (*postCallSB).find(*iid);
    if (mapItr == (*postCallSB).end()) {
      const auto mapItr2 = (*preCallSB).find(*iid);
      if (mapItr2 != (*preCallSB).end()) {
        // Clone pre-call same-block ops, map result id.
        const ir::Instruction* inInst = mapItr2->second;
        std::unique_ptr<ir::Instruction> sb_inst(
            inInst->Clone(inInst->context()));
        CloneSameBlockOps(&sb_inst, postCallSB, preCallSB, block_ptr);
        const uint32_t rid = sb_inst->result_id();
        const uint32_t nid = this->TakeNextId();
        get_decoration_mgr()->CloneDecorations(rid, nid, update_def_use_mgr_);
        sb_inst->SetResultId(nid);
        (*postCallSB)[rid] = nid;
        *iid = nid;
        (*block_ptr)->AddInstruction(std::move(sb_inst));
      }
    } else {
      // Reset same-block op operand.
      *iid = mapItr->second;
    }
  });
}

void InlinePass::GenInlineCode(
    std::vector<std::unique_ptr<ir::BasicBlock>>* new_blocks,
    std::vector<std::unique_ptr<ir::Instruction>>* new_vars,
    ir::BasicBlock::iterator call_inst_itr,
    ir::UptrVectorIterator<ir::BasicBlock> call_block_itr) {
  // Map from all ids in the callee to their equivalent id in the caller
  // as callee instructions are copied into caller.
  std::unordered_map<uint32_t, uint32_t> callee2caller;
  // Pre-call same-block insts
  std::unordered_map<uint32_t, ir::Instruction*> preCallSB;
  // Post-call same-block op ids
  std::unordered_map<uint32_t, uint32_t> postCallSB;

  ir::Function* calleeFn = id2function_[call_inst_itr->GetSingleWordOperand(
      kSpvFunctionCallFunctionId)];

  // Check for multiple returns in the callee.
  auto fi = multi_return_funcs_.find(calleeFn->result_id());
  const bool multiReturn = fi != multi_return_funcs_.end();

  // Map parameters to actual arguments.
  MapParams(calleeFn, call_inst_itr, &callee2caller);

  // Define caller local variables for all callee variables and create map to
  // them.
  CloneAndMapLocals(calleeFn, new_vars, &callee2caller);

  // Create return var if needed.
  uint32_t returnVarId = CreateReturnVar(calleeFn, new_vars);

  // Create set of callee result ids. Used to detect forward references
  std::unordered_set<uint32_t> callee_result_ids;
  calleeFn->ForEachInst([&callee_result_ids](const ir::Instruction* cpi) {
    const uint32_t rid = cpi->result_id();
    if (rid != 0) callee_result_ids.insert(rid);
  });

  // If the caller is in a single-block loop, and the callee has multiple
  // blocks, then the normal inlining logic will place the OpLoopMerge in
  // the last of several blocks in the loop.  Instead, it should be placed
  // at the end of the first block.  First determine if the caller is in a
  // single block loop.  We'll wait to move the OpLoopMerge until the end
  // of the regular inlining logic, and only if necessary.
  bool caller_is_single_block_loop = false;
  bool caller_is_loop_header = false;
  if (auto* loop_merge = call_block_itr->GetLoopMergeInst()) {
    caller_is_loop_header = true;
    caller_is_single_block_loop =
        call_block_itr->id() ==
        loop_merge->GetSingleWordInOperand(kSpvLoopMergeContinueTargetIdInIdx);
  }

  bool callee_begins_with_structured_header =
      (*(calleeFn->begin())).GetMergeInst() != nullptr;

  // Clone and map callee code. Copy caller block code to beginning of
  // first block and end of last block.
  bool prevInstWasReturn = false;
  uint32_t singleTripLoopHeaderId = 0;
  uint32_t singleTripLoopContinueId = 0;
  uint32_t returnLabelId = 0;
  bool multiBlocks = false;
  const uint32_t calleeTypeId = calleeFn->type_id();
  // new_blk_ptr is a new basic block in the caller.  New instructions are
  // written to it.  It is created when we encounter the OpLabel
  // of the first callee block.  It is appended to new_blocks only when
  // it is complete.
  std::unique_ptr<ir::BasicBlock> new_blk_ptr;
  calleeFn->ForEachInst([&new_blocks, &callee2caller, &call_block_itr,
                         &call_inst_itr, &new_blk_ptr, &prevInstWasReturn,
                         &returnLabelId, &returnVarId, caller_is_loop_header,
                         callee_begins_with_structured_header, &calleeTypeId,
                         &multiBlocks, &postCallSB, &preCallSB, multiReturn,
                         &singleTripLoopHeaderId, &singleTripLoopContinueId,
                         &callee_result_ids, this](const ir::Instruction* cpi) {
    switch (cpi->opcode()) {
      case SpvOpFunction:
      case SpvOpFunctionParameter:
      case SpvOpVariable:
        // Already processed
        break;
      case SpvOpLabel: {
        // If previous instruction was early return, insert branch
        // instruction to return block.
        if (prevInstWasReturn) {
          if (returnLabelId == 0) returnLabelId = this->TakeNextId();
          AddBranch(returnLabelId, &new_blk_ptr);
          prevInstWasReturn = false;
        }
        // Finish current block (if it exists) and get label for next block.
        uint32_t labelId;
        bool firstBlock = false;
        if (new_blk_ptr != nullptr) {
          new_blocks->push_back(std::move(new_blk_ptr));
          // If result id is already mapped, use it, otherwise get a new
          // one.
          const uint32_t rid = cpi->result_id();
          const auto mapItr = callee2caller.find(rid);
          labelId = (mapItr != callee2caller.end()) ? mapItr->second
                                                    : this->TakeNextId();
        } else {
          // First block needs to use label of original block
          // but map callee label in case of phi reference.
          labelId = call_block_itr->id();
          callee2caller[cpi->result_id()] = labelId;
          firstBlock = true;
        }
        // Create first/next block.
        new_blk_ptr.reset(new ir::BasicBlock(NewLabel(labelId)));
        if (firstBlock) {
          // Copy contents of original caller block up to call instruction.
          for (auto cii = call_block_itr->begin(); cii != call_inst_itr;
               ++cii) {
            std::unique_ptr<ir::Instruction> cp_inst(cii->Clone(context()));
            // Remember same-block ops for possible regeneration.
            if (IsSameBlockOp(&*cp_inst)) {
              auto* sb_inst_ptr = cp_inst.get();
              preCallSB[cp_inst->result_id()] = sb_inst_ptr;
            }
            new_blk_ptr->AddInstruction(std::move(cp_inst));
          }
          if (caller_is_loop_header && callee_begins_with_structured_header) {
            // We can't place both the caller's merge instruction and another
            // merge instruction in the same block.  So split the calling block.
            // Insert an unconditional branch to a new guard block.  Later,
            // once we know the ID of the last block,  we will move the caller's
            // OpLoopMerge from the last generated block into the first block.
            // We also wait to avoid invalidating various iterators.
            const auto guard_block_id = this->TakeNextId();
            AddBranch(guard_block_id, &new_blk_ptr);
            new_blocks->push_back(std::move(new_blk_ptr));
            // Start the next block.
            new_blk_ptr.reset(new ir::BasicBlock(NewLabel(guard_block_id)));
            // Reset the mapping of the callee's entry block to point to
            // the guard block.  Do this so we can fix up phis later on to
            // satisfy dominance.
            callee2caller[cpi->result_id()] = guard_block_id;
          }
          // If callee has multiple returns, insert a header block for
          // single-trip loop that will encompass callee code.  Start postheader
          // block.
          //
          // Note: Consider the following combination:
          //  - the caller is a single block loop
          //  - the callee does not begin with a structure header
          //  - the callee has multiple returns.
          // We still need to split the caller block and insert a guard block.
          // But we only need to do it once. We haven't done it yet, but the
          // single-trip loop header will serve the same purpose.
          if (multiReturn) {
            singleTripLoopHeaderId = this->TakeNextId();
            AddBranch(singleTripLoopHeaderId, &new_blk_ptr);
            new_blocks->push_back(std::move(new_blk_ptr));
            new_blk_ptr.reset(
                new ir::BasicBlock(NewLabel(singleTripLoopHeaderId)));
            returnLabelId = this->TakeNextId();
            singleTripLoopContinueId = this->TakeNextId();
            AddLoopMerge(returnLabelId, singleTripLoopContinueId, &new_blk_ptr);
            uint32_t postHeaderId = this->TakeNextId();
            AddBranch(postHeaderId, &new_blk_ptr);
            new_blocks->push_back(std::move(new_blk_ptr));
            new_blk_ptr.reset(new ir::BasicBlock(NewLabel(postHeaderId)));
            multiBlocks = true;
            // Reset the mapping of the callee's entry block to point to
            // the post-header block.  Do this so we can fix up phis later
            // on to satisfy dominance.
            callee2caller[cpi->result_id()] = postHeaderId;
          }
        } else {
          multiBlocks = true;
        }
      } break;
      case SpvOpReturnValue: {
        // Store return value to return variable.
        assert(returnVarId != 0);
        uint32_t valId = cpi->GetInOperand(kSpvReturnValueId).words[0];
        const auto mapItr = callee2caller.find(valId);
        if (mapItr != callee2caller.end()) {
          valId = mapItr->second;
        }
        AddStore(returnVarId, valId, &new_blk_ptr);

        // Remember we saw a return; if followed by a label, will need to
        // insert branch.
        prevInstWasReturn = true;
      } break;
      case SpvOpReturn: {
        // Remember we saw a return; if followed by a label, will need to
        // insert branch.
        prevInstWasReturn = true;
      } break;
      case SpvOpFunctionEnd: {
        // If there was an early return, we generated a return label id
        // for it.  Now we have to generate the return block with that Id.
        if (returnLabelId != 0) {
          // If previous instruction was return, insert branch instruction
          // to return block.
          if (prevInstWasReturn) AddBranch(returnLabelId, &new_blk_ptr);
          if (multiReturn) {
            // If we generated a loop header to for the single-trip loop
            // to accommodate multiple returns, insert the continue
            // target block now, with a false branch back to the loop header.
            new_blocks->push_back(std::move(new_blk_ptr));
            new_blk_ptr.reset(
                new ir::BasicBlock(NewLabel(singleTripLoopContinueId)));
            AddBranchCond(GetFalseId(), singleTripLoopHeaderId, returnLabelId,
                          &new_blk_ptr);
          }
          // Generate the return block.
          new_blocks->push_back(std::move(new_blk_ptr));
          new_blk_ptr.reset(new ir::BasicBlock(NewLabel(returnLabelId)));
          multiBlocks = true;
        }
        // Load return value into result id of call, if it exists.
        if (returnVarId != 0) {
          const uint32_t resId = call_inst_itr->result_id();
          assert(resId != 0);
          AddLoad(calleeTypeId, resId, returnVarId, &new_blk_ptr);
        }
        // Copy remaining instructions from caller block.
        auto cii = call_inst_itr;
        for (++cii; cii != call_block_itr->end(); ++cii) {
          std::unique_ptr<ir::Instruction> cp_inst(cii->Clone(context()));
          // If multiple blocks generated, regenerate any same-block
          // instruction that has not been seen in this last block.
          if (multiBlocks) {
            CloneSameBlockOps(&cp_inst, &postCallSB, &preCallSB, &new_blk_ptr);
            // Remember same-block ops in this block.
            if (IsSameBlockOp(&*cp_inst)) {
              const uint32_t rid = cp_inst->result_id();
              postCallSB[rid] = rid;
            }
          }
          new_blk_ptr->AddInstruction(std::move(cp_inst));
        }
        // Finalize inline code.
        new_blocks->push_back(std::move(new_blk_ptr));
      } break;
      default: {
        // Copy callee instruction and remap all input Ids.
        std::unique_ptr<ir::Instruction> cp_inst(cpi->Clone(context()));
        cp_inst->ForEachInId([&callee2caller, &callee_result_ids,
                              this](uint32_t* iid) {
          const auto mapItr = callee2caller.find(*iid);
          if (mapItr != callee2caller.end()) {
            *iid = mapItr->second;
          } else if (callee_result_ids.find(*iid) != callee_result_ids.end()) {
            // Forward reference. Allocate a new id, map it,
            // use it and check for it when remapping result ids
            const uint32_t nid = this->TakeNextId();
            callee2caller[*iid] = nid;
            *iid = nid;
          }
        });
        // If result id is non-zero, remap it. If already mapped, use mapped
        // value, else use next id.
        const uint32_t rid = cp_inst->result_id();
        if (rid != 0) {
          const auto mapItr = callee2caller.find(rid);
          uint32_t nid;
          if (mapItr != callee2caller.end()) {
            nid = mapItr->second;
          } else {
            nid = this->TakeNextId();
            callee2caller[rid] = nid;
          }
          cp_inst->SetResultId(nid);
          get_decoration_mgr()->CloneDecorations(rid, nid, update_def_use_mgr_);
        }
        new_blk_ptr->AddInstruction(std::move(cp_inst));
      } break;
    }
  });

  if (caller_is_loop_header && (new_blocks->size() > 1)) {
    // Move the OpLoopMerge from the last block back to the first, where
    // it belongs.
    auto& first = new_blocks->front();
    auto& last = new_blocks->back();
    assert(first != last);

    // Insert a modified copy of the loop merge into the first block.
    auto loop_merge_itr = last->tail();
    --loop_merge_itr;
    assert(loop_merge_itr->opcode() == SpvOpLoopMerge);
    std::unique_ptr<ir::Instruction> cp_inst(loop_merge_itr->Clone(context()));
    if (caller_is_single_block_loop) {
      // Also, update its continue target to point to the last block.
      cp_inst->SetInOperand(kSpvLoopMergeContinueTargetIdInIdx, {last->id()});
    }
    first->tail().InsertBefore(std::move(cp_inst));

    // Remove the loop merge from the last block.
    loop_merge_itr->RemoveFromList();
    delete &*loop_merge_itr;
  }

  // Update block map given replacement blocks.
  for (auto& blk : *new_blocks) {
    id2block_[blk->id()] = &*blk;
  }
}

bool InlinePass::IsInlinableFunctionCall(const ir::Instruction* inst) {
  if (inst->opcode() != SpvOp::SpvOpFunctionCall) return false;
  const uint32_t calleeFnId =
      inst->GetSingleWordOperand(kSpvFunctionCallFunctionId);
  const auto ci = inlinable_.find(calleeFnId);
  return ci != inlinable_.cend();
}

void InlinePass::UpdateSucceedingPhis(
    std::vector<std::unique_ptr<ir::BasicBlock>>& new_blocks) {
  const auto firstBlk = new_blocks.begin();
  const auto lastBlk = new_blocks.end() - 1;
  const uint32_t firstId = (*firstBlk)->id();
  const uint32_t lastId = (*lastBlk)->id();
  (*lastBlk)->ForEachSuccessorLabel([&firstId, &lastId, this](uint32_t succ) {
    ir::BasicBlock* sbp = this->id2block_[succ];
    sbp->ForEachPhiInst([&firstId, &lastId](ir::Instruction* phi) {
      phi->ForEachInId([&firstId, &lastId](uint32_t* id) {
        if (*id == firstId) *id = lastId;
      });
    });
  });
}

bool InlinePass::HasMultipleReturns(ir::Function* func) {
  bool seenReturn = false;
  bool multipleReturns = false;
  for (auto& blk : *func) {
    auto terminal_ii = blk.cend();
    --terminal_ii;
    if (terminal_ii->opcode() == SpvOpReturn ||
        terminal_ii->opcode() == SpvOpReturnValue) {
      if (seenReturn) {
        multipleReturns = true;
        break;
      }
      seenReturn = true;
    }
  }
  return multipleReturns;
}

void InlinePass::ComputeStructuredSuccessors(ir::Function* func) {
  // If header, make merge block first successor.
  for (auto& blk : *func) {
    uint32_t mbid = blk.MergeBlockIdIfAny();
    if (mbid != 0) {
      block2structured_succs_[&blk].push_back(id2block_[mbid]);
    }

    // Add true successors.
    blk.ForEachSuccessorLabel([&blk, this](uint32_t sbid) {
      block2structured_succs_[&blk].push_back(id2block_[sbid]);
    });
  }
}

InlinePass::GetBlocksFunction InlinePass::StructuredSuccessorsFunction() {
  return [this](const ir::BasicBlock* block) {
    return &(block2structured_succs_[block]);
  };
}

bool InlinePass::HasNoReturnInLoop(ir::Function* func) {
  // If control not structured, do not do loop/return analysis
  // TODO: Analyze returns in non-structured control flow
  if (!context()->get_feature_mgr()->HasCapability(SpvCapabilityShader))
    return false;
  // Compute structured block order. This order has the property
  // that dominators are before all blocks they dominate and merge blocks
  // are after all blocks that are in the control constructs of their header.
  ComputeStructuredSuccessors(func);
  auto ignore_block = [](cbb_ptr) {};
  auto ignore_edge = [](cbb_ptr, cbb_ptr) {};
  std::list<const ir::BasicBlock*> structuredOrder;
  spvtools::CFA<ir::BasicBlock>::DepthFirstTraversal(
      &*func->begin(), StructuredSuccessorsFunction(), ignore_block,
      [&](cbb_ptr b) { structuredOrder.push_front(b); }, ignore_edge);
  // Search for returns in loops. Only need to track outermost loop
  bool return_in_loop = false;
  uint32_t outerLoopMergeId = 0;
  for (auto& blk : structuredOrder) {
    // Exiting current outer loop
    if (blk->id() == outerLoopMergeId) outerLoopMergeId = 0;
    // Return block
    auto terminal_ii = blk->cend();
    --terminal_ii;
    if (terminal_ii->opcode() == SpvOpReturn ||
        terminal_ii->opcode() == SpvOpReturnValue) {
      if (outerLoopMergeId != 0) {
        return_in_loop = true;
        break;
      }
    } else if (terminal_ii != blk->cbegin()) {
      auto merge_ii = terminal_ii;
      --merge_ii;
      // Entering outermost loop
      if (merge_ii->opcode() == SpvOpLoopMerge && outerLoopMergeId == 0)
        outerLoopMergeId =
            merge_ii->GetSingleWordOperand(kSpvLoopMergeMergeBlockId);
    }
  }
  return !return_in_loop;
}

void InlinePass::AnalyzeReturns(ir::Function* func) {
  // Look for multiple returns
  if (!HasMultipleReturns(func)) {
    no_return_in_loop_.insert(func->result_id());
    return;
  }
  multi_return_funcs_.insert(func->result_id());
  // If multiple returns, see if any are in a loop
  if (HasNoReturnInLoop(func)) no_return_in_loop_.insert(func->result_id());
}

bool InlinePass::IsInlinableFunction(ir::Function* func) {
  // We can only inline a function if it has blocks.
  if (func->cbegin() == func->cend()) return false;
  // Do not inline functions with returns in loops. Currently early return
  // functions are inlined by wrapping them in a one trip loop and implementing
  // the returns as a branch to the loop's merge block. However, this can only
  // done validly if the return was not in a loop in the original function.
  // Also remember functions with multiple (early) returns.
  AnalyzeReturns(func);
  return no_return_in_loop_.find(func->result_id()) !=
         no_return_in_loop_.cend();
}

void InlinePass::InitializeInline(ir::IRContext* c) {
  InitializeProcessing(c);

  // Don't bother updating the DefUseManger
  update_def_use_mgr_ = [](ir::Instruction&, bool) {};

  false_id_ = 0;

  // clear collections
  id2function_.clear();
  id2block_.clear();
  block2structured_succs_.clear();
  inlinable_.clear();
  no_return_in_loop_.clear();
  multi_return_funcs_.clear();

  for (auto& fn : *get_module()) {
    // Initialize function and block maps.
    id2function_[fn.result_id()] = &fn;
    for (auto& blk : fn) {
      id2block_[blk.id()] = &blk;
    }
    // Compute inlinability
    if (IsInlinableFunction(&fn)) inlinable_.insert(fn.result_id());
  }
};

InlinePass::InlinePass() {}

}  // namespace opt
}  // namespace spvtools