nudt-compiler-cpp/src/ir/passes/LoopFission.cpp

// 循环分裂：
// - 针对单块循环中两段彼此独立的 store 语句组做保守分裂
// - 仅处理单归纳变量、无其他 loop-carried phi 的情形

#include "ir/IR.h"

#include <algorithm>
#include <string>
#include <unordered_set>
#include <vector>

#include "LoopPassUtils.h"

namespace ir {
namespace passes {

namespace {

Value* StripPointerBase(Value* value) {
  while (auto* gep = dynamic_cast<GepInst*>(value)) {
    value = gep->GetBase();
  }
  return value;
}

bool IsFissionCandidate(const CanonicalLoopMatch& match) {
  if (match.loop->GetChildren().size() != 0) return false;
  if (match.loop->GetBlocks().size() != 2) return false;
  if (match.body != match.latch) return false;
  if (match.header_phis.size() != 1) return false;
  if (match.header_phis.front() != match.induction.phi) return false;
  if (match.induction.step <= 0) return false;
  auto* body_term =
      dynamic_cast<BranchInst*>(match.body->MutableInstructions().back().get());
  return body_term && body_term->GetTarget() == match.header;
}

bool DependsOnAny(Instruction* inst, const std::unordered_set<Instruction*>& defs) {
  for (size_t i = 0; i < inst->GetNumOperands(); ++i) {
    auto* def = dynamic_cast<Instruction*>(inst->GetOperand(i));
    if (def && defs.count(def) != 0) return true;
  }
  return false;
}

bool RunFissionOnLoop(Function& func, const CanonicalLoopMatch& match,
                      Context& ctx) {
  if (!IsFissionCandidate(match)) return false;

  std::vector<Instruction*> body_insts;
  for (const auto& inst_ptr : match.body->GetInstructions()) {
    if (!inst_ptr.get()->IsTerminator()) {
      body_insts.push_back(inst_ptr.get());
    }
  }
  if (body_insts.size() < 3) return false;

  auto* iv_next = dynamic_cast<Instruction*>(match.induction.next);
  if (!iv_next || iv_next->GetParent() != match.body) return false;

  std::vector<size_t> store_positions;
  for (size_t i = 0; i < body_insts.size(); ++i) {
    if (dynamic_cast<StoreInst*>(body_insts[i]) != nullptr) {
      store_positions.push_back(i);
    }
  }
  if (store_positions.size() != 2) return false;

  const size_t first_store_idx = store_positions[0];
  const size_t second_store_idx = store_positions[1];
  if (body_insts.back() != iv_next) return false;
  if (second_store_idx + 1 != body_insts.size() - 1) return false;

  auto* first_store = static_cast<StoreInst*>(body_insts[first_store_idx]);
  auto* second_store = static_cast<StoreInst*>(body_insts[second_store_idx]);
  if (StripPointerBase(first_store->GetPtr()) == StripPointerBase(second_store->GetPtr())) {
    return false;
  }

  std::vector<Instruction*> group1(body_insts.begin(),
                                   body_insts.begin() + first_store_idx + 1);
  std::vector<Instruction*> group2(body_insts.begin() + first_store_idx + 1,
                                   body_insts.begin() + second_store_idx + 1);

  std::unordered_set<Instruction*> group1_defs(group1.begin(), group1.end());
  std::unordered_set<Instruction*> group2_defs(group2.begin(), group2.end());
  group1_defs.erase(iv_next);
  group2_defs.erase(iv_next);

  for (auto* inst : group2) {
    if (DependsOnAny(inst, group1_defs)) return false;
  }
  for (auto* inst : group1) {
    if (DependsOnAny(inst, group2_defs)) return false;
  }

  auto* original_exit = match.exit;
  std::string block_suffix = ctx.NextTemp();
  if (!block_suffix.empty() && block_suffix.front() == '%') {
    block_suffix.erase(0, 1);
  }
  auto* preheader2 =
      func.CreateBlock(match.header->GetName() + ".fission.pre." + block_suffix);
  auto* header2 =
      func.CreateBlock(match.header->GetName() + ".fission.hdr." + block_suffix);
  auto* body2 =
      func.CreateBlock(match.body->GetName() + ".fission.body." + block_suffix);

  preheader2->Append<BranchInst>(Type::GetVoidType(), header2);

  auto* iv2 = header2->PrependPhi(Type::GetInt32Type(), ctx.NextTemp());
  iv2->AddIncoming(match.induction.init, preheader2);

  auto* cmp2 = header2->Append<CmpInst>(
      match.header_cmp->GetCmpOp(), Type::GetInt32Type(), iv2, match.bound,
      ctx.NextTemp());
  header2->Append<CondBranchInst>(Type::GetVoidType(), cmp2, body2, original_exit);

  ValueMap remap;
  remap.emplace(match.induction.phi, iv2);
  for (auto* inst : group2) {
    auto cloned = CloneInstruction(inst, remap, ".f2");
    if (!cloned) return false;
    auto* raw = cloned.get();
    body2->MutableInstructions().push_back(std::move(cloned));
    raw->SetParent(body2);
    remap[inst] = raw;
  }

  auto next2_cloned = CloneInstruction(iv_next, remap, ".f2");
  if (!next2_cloned) return false;
  auto* next2 = next2_cloned.get();
  body2->MutableInstructions().push_back(std::move(next2_cloned));
  next2->SetParent(body2);
  body2->Append<BranchInst>(Type::GetVoidType(), header2);
  iv2->AddIncoming(next2, body2);

  const bool exit_is_true = (match.header_branch->GetTrueBlock() == original_exit);
  match.header_branch->SetOperand(exit_is_true ? 1 : 2, preheader2);
  match.header->RemoveSuccessor(original_exit);
  match.header->AddSuccessor(preheader2);
  preheader2->AddPredecessor(match.header);
  original_exit->RemovePredecessor(match.header);

  for (auto* inst : group2) {
    match.body->RemoveInstruction(inst);
  }

  return true;
}

}  // namespace

bool RunLoopFission(Function& func, Context& ctx) {
  if (func.IsExternal()) return false;

  analysis::DominatorTree dom_tree(func);
  analysis::LoopInfo loop_info(func, dom_tree);

  for (const auto& loop_ptr : loop_info.GetLoops()) {
    auto match = MatchCanonicalLoop(loop_ptr.get());
    if (!match.has_value()) continue;
    if (RunFissionOnLoop(func, *match, ctx)) {
      return true;
    }
  }
  return false;
}

}  // namespace passes
}  // namespace ir