thread_storage_sync.cc

/*
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you 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.
 */

/*!
 * \file thread_storage_sync.cc
 */
#include <tvm/runtime/registry.h>
#include <tvm/tir/analysis.h>
#include <tvm/tir/builtin.h>
#include <tvm/tir/expr.h>
#include <tvm/tir/stmt_functor.h>
#include <tvm/tir/transform.h>

#include <unordered_map>
#include <unordered_set>

#include "./storage_access.h"
#include "runtime/thread_storage_scope.h"
#include "tir/transforms/ir_utils.h"

namespace tvm {
namespace tl {

using namespace tir;

class TileLangThreadSyncPlanner : public TileLangStorageAccessVisitor {
public:
  explicit TileLangThreadSyncPlanner(StorageScope sync_scope)
      : sync_scope_(sync_scope) {}

  // The syncs inserted before each statement
  std::unordered_set<const Object *> syncs_inserted_;
  std::unordered_map<const Object *, int> partial_syncs_inserted_;

protected:
  bool Enabled(const VarNode *buf, const StorageScope &scope) const final {
    return in_device_env() && scope == sync_scope_;
  }
  // Plan the sync
  std::vector<AccessEntry> Summarize(std::vector<StmtEntry> seq,
                                     const ForNode *loop) final {
    // Redirect all "shared.dyn" buffer access to the same buffer var
    // so that the accesses can be planned together.
    Var shared_dyn_buf;
    // for (StmtEntry& entry : seq) {
    //   for (AccessEntry& access : entry.access) {
    //     if (access.scope.rank == StorageRank::kShared && access.scope.tag ==
    //     ".dyn" &&
    //         access.buffer.defined()) {
    //       if (!shared_dyn_buf.defined()) {
    //         shared_dyn_buf = access.buffer;
    //       } else {
    //         access.buffer = shared_dyn_buf;
    //       }
    //     }
    //   }
    // }

    // Unsynced reads and writes
    std::vector<AccessEntry> reads;
    std::vector<AccessEntry> writes;
    // if it is a loop, rotate two times to consider effect of loop.
    // simulation based approach to find dependencies
    for (size_t i = 0; i < seq.size(); ++i) {
      const StmtEntry &s = seq[i];
      // check if sync before statement is needed.
      bool sync_before_stmt = (syncs_inserted_.count(s.stmt) != 0);
      // Apply the syncs added already.
      if (sync_before_stmt) {
        reads.clear();
        writes.clear();
      }
      for (const AccessEntry &acc : s.access) {
        if (acc.type == kRead) {
          if (FindConflict(writes, acc, false)) {
            sync_before_stmt = true;
            break;
          }
        } else if (acc.type == kWrite) {
          if (FindConflict(reads, acc, false)) {
            sync_before_stmt = true;
            break;
          }
        } else if (acc.type == kSync) {
          reads.clear();
          writes.clear();
        }
      }
      // If sync is inserted. remove the irrelevant things.
      if (sync_before_stmt) {
        reads.clear();
        writes.clear();
      }
      // Add the read/write of current statement
      for (const AccessEntry &acc : s.access) {
        if (acc.type == kRead) {
          reads.push_back(acc);
        } else if (acc.type == kWrite) {
          writes.push_back(acc);
        } else if (acc.type == kSync) {
          reads.clear();
          writes.clear();
        }
      }
      if (sync_before_stmt) {
        insert_syncs(s.stmt);
      }
    }
    if (loop != nullptr) {
      for (size_t i = 0; i < seq.size(); ++i) {
        const StmtEntry &s = seq[i];
        if (syncs_inserted_.count(s.stmt) != 0)
          break;
        if (reads.empty() && writes.empty())
          break;
        bool sync_before_stmt = false;
        for (const AccessEntry &acc : s.access) {
          if (acc.type == kRead) {
            if (FindConflict(writes, acc, true)) {
              sync_before_stmt = true;
              break;
            }
          } else if (acc.type == kWrite) {
            if (FindConflict(reads, acc, true)) {
              sync_before_stmt = true;
              break;
            }
          } else if (acc.type == kSync) {
            reads.clear();
            writes.clear();
          }
        }
        if (sync_before_stmt) {
          insert_syncs(s.stmt);
          break;
        }
      }
    }
    // return the exposed entries, remove unecessary ones.
    int sync_count = 0;
    // head are before first sync, tail are after last sync
    std::vector<AccessEntry> head, tail;
    AccessEntry esync;
    esync.threads = this->env_threads();
    esync.type = kSync;
    esync.scope = sync_scope_;

    for (const StmtEntry &s : seq) {
      if (syncs_inserted_.count(s.stmt)) {
        if (sync_count != 0) {
          tail.clear();
        } else {
          head.push_back(esync);
        }
        ++sync_count;
      }
      for (const AccessEntry &acc : s.access) {
        if (acc.type == kSync) {
          if (sync_count != 0) {
            tail.clear();
          } else {
            head.push_back(esync);
          }
          ++sync_count;
        } else {
          if (sync_count != 0) {
            tail.push_back(acc);
          } else {
            head.push_back(acc);
          }
        }
      }
    }
    head.insert(head.end(), tail.begin(), tail.end());
    if (loop != nullptr) {
      // clear double buffer flag after a loop is finished.
      for (AccessEntry &e : head) {
        e.double_buffer_write = false;
      }
    }
    return head;
  }

private:
  // find conflicting entry in vec.
  bool FindConflict(const std::vector<AccessEntry> &prev,
                    const AccessEntry &curr, bool loop_carry) {
    for (const AccessEntry &x : prev) {
      if (FindConflict(x, curr, loop_carry)) {
        return true;
      }
    }
    return false;
  }

  bool FindConflict(const AccessEntry &prev, const AccessEntry &curr,
                    bool loop_carry) {
    // Access to different buffers does not conflict.
    if (!prev.buffer.same_as(curr.buffer)) {
      return false;
    }

    // Assumes no race between threads
    // Same index value means no conflicts
    // TODO(tqchen) more standard set based testing.
    bool has_same_index = true;
    // Even if access has the same index, those indices need to
    // depend on the innermost thread id to avoid race condition
    bool depends_on_thread_index = true;
    const VarNode *thread_index_var = nullptr;
    if (!curr.threads.empty()) {
      thread_index_var = curr.threads.back()->var.get();
    }

    for (size_t i = 0; i < prev.touched.size(); i++) {
      const auto &prev_intset = prev.touched[i];
      const auto &curr_intset = curr.touched[i];

      if (prev_intset.IsSinglePoint() && curr_intset.IsSinglePoint()) {
        PrimExpr prev_index = prev_intset.PointValue();
        PrimExpr curr_index = curr_intset.PointValue();
        has_same_index = ExprDeepEqual()(prev_index, curr_index);
        if (thread_index_var != nullptr) {
          auto f_uses_thread_index = [=](const tvm::tir::VarNode *parameter) {
            return parameter == thread_index_var;
          };
          depends_on_thread_index = depends_on_thread_index &&
                                    UsesVar(curr_index, f_uses_thread_index) &&
                                    UsesVar(prev_index, f_uses_thread_index);
        }
      } else {
        has_same_index = false;
      }

      if (!(has_same_index && depends_on_thread_index)) {
        break;
      }
    }
    if (has_same_index && depends_on_thread_index) {
      return false;
    }

    // If this is a read into a double buffer that was previously
    // swapped out, then it doesn't conflict.
    if (prev.double_buffer_write && curr.type == kRead && !loop_carry) {
      return false;
    }

    // If nothing else allows sharing the same buffer, then they are
    // in conflict.
    return true;
  }

  void VisitStmt_(const AttrStmtNode *op) final {
    if (op->attr_key == "kWarpSpecializationScope") {
      IfThenElse body = Downcast<IfThenElse>(op->body);
      auto partitions = Downcast<Array<IntImm>>(op->node);
      ICHECK(partitions.size() == 2);

      scope_.push_back(std::vector<StmtEntry>());
      num_partial_threads_ = partitions[0];
      this->VisitStmt(body->then_case);
      StmtEntry s;
      s.stmt = op;
      s.access = Summarize(std::move(scope_.back()), nullptr);
      scope_.pop_back();

      num_partial_threads_ = partitions[1];
      scope_.push_back(std::vector<StmtEntry>());
      VisitStmt(body->else_case.value());
      auto v = Summarize(std::move(scope_.back()), nullptr);
      scope_.pop_back();
      s.access.insert(s.access.end(), v.begin(), v.end());

      num_partial_threads_ = NullOpt;
    } else {
      TileLangStorageAccessVisitor::VisitStmt_(op);
    }
  }

  void insert_syncs(const Object *obj) {
    // ICHECK_EQ(condition_counter(), 0) << "Cannot insert syncs inside
    // condition";
    if (syncs_inserted_.count(obj))
      return;
    if (num_partial_threads_.defined()) {
      syncs_inserted_.insert(obj);
      partial_syncs_inserted_[obj] =
          static_cast<int>(num_partial_threads_.value()->value);
    } else {
      syncs_inserted_.insert(obj);
    }
  }

private:
  Optional<IntImm> num_partial_threads_;
  // synchronization scope
  StorageScope sync_scope_;
};

// There are cases where necessary syncthreads is not inserted by
// ThreadSyncInserter. For example, syncthreads is needed after async_wait_queue
// in the second loop below, but since ThreadSyncInserter is not aware of the
// asynchronous semantics, it cannot tell that the syncthreads is needed there.
//
// // Pipeline prologue
// for i in range(125):
//    async_commit_queue(0):
//       async_scope:
//          shared[(i + 3) % 4] = ...
// ...
//
// // Pipeline Epilogue
// for i in range(3):
//    async_wait_queue(0, 2 - i):
//       local[...] = shared[(i + 125) % 4]

// This class adds syncthreads after all async_wait_queue. That includes
// syncthreads that can be inserted by ThreadSyncInserter as well, but
// ThreadSyncInserter will not insert duplicate syncthreads if it finds an
// existing one at the synchronization point.
class ThreadSyncAfterWaitQueueInserter : public StmtExprMutator {
public:
  explicit ThreadSyncAfterWaitQueueInserter(StorageScope sync_scope)
      : sync_scope_(sync_scope) {}

  Stmt VisitStmt_(const AttrStmtNode *op) final {
    if (op->attr_key == tvm::tir::attr::async_wait_queue_scope) {
      auto sync = Evaluate(Call(DataType::Int(32), builtin::tvm_storage_sync(),
                                {StringImm(sync_scope_.to_string())}));
      auto inner = op->body.as<AttrStmtNode>();
      ICHECK(inner &&
             inner->attr_key == tvm::tir::attr::async_wait_inflight_count);
      auto zero = make_zero(DataType::Int(32));
      auto new_body = SeqStmt({sync, inner->body});
      return AttrStmt(zero, tvm::tir::attr::async_wait_queue_scope, op->value,
                      AttrStmt(zero, tvm::tir::attr::async_wait_inflight_count,
                               inner->value, new_body));
    }
    return StmtExprMutator::VisitStmt_(op);
  }

private:
  StorageScope sync_scope_;
};

class ThreadSyncInserter : public StmtExprMutator {
public:
  ThreadSyncInserter(StorageScope sync_scope,
                     const std::unordered_set<const Object *> &syncs,
                     std::unordered_map<const Object *, int> partial_syncs)
      : sync_scope_(sync_scope), syncs_(syncs), partial_syncs_(partial_syncs) {}

  Stmt VisitStmt(const Stmt &stmt) final {
    if (syncs_.size() == 0)
      return stmt;
    if (syncs_.count(stmt.get())) {
      Stmt barrier;
      if (sync_scope_.rank == StorageRank::kGlobal) {
        barrier = MakeGlobalBarrier();
      } else if (partial_syncs_.count(stmt.get())) {
        return StmtExprMutator::VisitStmt(stmt);
      } else {
        barrier = Evaluate(Call(DataType::Int(32), builtin::tvm_storage_sync(),
                                {StringImm(sync_scope_.to_string())}));
      }
      // Mutate after query, to avoid stmt change.
      auto ret = StmtExprMutator::VisitStmt(stmt);
      ret = SeqStmt({barrier, ret});
      return ret;
    } else {
      return StmtExprMutator::VisitStmt(stmt);
    }
  }
  PrimExpr VisitExpr_(const BufferLoadNode *op) final {
    if (sync_scope_.rank == StorageRank::kGlobal &&
        GetScope(op->buffer->data).rank == StorageRank::kGlobal) {
      ++rw_stats_[op->buffer->data].read_count;
    }
    return StmtExprMutator::VisitExpr_(op);
  }
  Stmt VisitStmt_(const BufferStoreNode *op) final {
    if (sync_scope_.rank == StorageRank::kGlobal &&
        GetScope(op->buffer->data).rank == StorageRank::kGlobal) {
      ++rw_stats_[op->buffer->data].write_count;
    }
    return StmtExprMutator::VisitStmt_(op);
  }
  Stmt VisitStmt_(const AttrStmtNode *op) final {
    if (op->attr_key == tvm::tir::attr::thread_extent) {
      bool temp = true;
      std::swap(temp, in_thread_env_);
      thread_extents_.push_back(op);
      Stmt ret = StmtExprMutator::VisitStmt_(op);
      thread_extents_.pop_back();
      std::swap(temp, in_thread_env_);
      // first thread scope.
      if (!in_thread_env_ && sync_scope_.rank == StorageRank::kGlobal) {
        ret = InitGlobalBarrier(ret.as<AttrStmtNode>());
        num_blocks_ = PrimExpr();
        is_lead_ = PrimExpr();
      }
      return ret;
    } else {
      return StmtExprMutator::VisitStmt_(op);
    }
  }

  PrimExpr VisitExpr_(const CallNode *op) final {
    if (op->op.same_as(builtin::tvm_access_ptr())) {
      PrimExpr expr = StmtExprMutator::VisitExpr_(op);
      op = expr.as<CallNode>();
      ICHECK_EQ(op->args.size(), 5U);
      Var buffer_var(Downcast<Var>(op->args[1]));
      const IntImmNode *flag = op->args[4].as<IntImmNode>();
      if ((flag->value & 1) && sync_scope_.rank == StorageRank::kGlobal &&
          GetScope(buffer_var).rank == StorageRank::kGlobal) {
        ++rw_stats_[buffer_var].read_count;
      }
      if (flag->value & 2 && sync_scope_.rank == StorageRank::kGlobal &&
          GetScope(buffer_var).rank == StorageRank::kGlobal) {
        ++rw_stats_[buffer_var].write_count;
      }
      return expr;
    } else if (op->op.same_as(builtin::address_of())) {
      PrimExpr expr = StmtExprMutator::VisitExpr_(op);
      op = expr.as<CallNode>();
      ICHECK_EQ(op->args.size(), 1U)
          << "address_of should only have one argument (Buffer)";

      BufferLoad load = Downcast<BufferLoad>(op->args[0]);
      Var buffer_var(Downcast<Var>(load->buffer->data));
      if (sync_scope_.rank == StorageRank::kGlobal &&
          GetScope(buffer_var).rank == StorageRank::kGlobal) {
        ++rw_stats_[buffer_var].read_count;
      }
      if (sync_scope_.rank == StorageRank::kGlobal &&
          GetScope(buffer_var).rank == StorageRank::kGlobal) {
        ++rw_stats_[buffer_var].write_count;
      }
      return expr;
    } else {
      return StmtExprMutator::VisitExpr_(op);
    }
  }

private:
  // RW statistics about data
  struct Entry {
    int read_count{0};
    int write_count{0};
  };

  // Get current storage scope.
  StorageScope GetScope(Var buffer_var) const {
    return StorageScope::Create(GetPtrStorageScope(buffer_var));
  }

  // private functions.
  Stmt InitGlobalBarrier(const AttrStmtNode *op) {
    ICHECK(op != nullptr);
    Array<PrimExpr> pargs = {
        StringImm(runtime::symbol::tvm_prepare_global_barrier)};
    Stmt prep =
        Evaluate(Call(DataType::Int(32), builtin::tvm_call_packed(), pargs));
    Stmt body = op->body;
    for (const auto &kv : rw_stats_) {
      const auto &e = kv.second;
      if (e.read_count != 0 && e.write_count != 0) {
        body = AttrStmt(kv.first, tvm::tir::attr::volatile_scope, 1, body);
      }
    }
    rw_stats_.clear();
    Stmt kinit = Evaluate(
        Call(DataType::Int(32), builtin::tvm_global_barrier_kinit(), {}));
    body = SeqStmt({kinit, body});
    body = AttrStmt(op->node, op->attr_key, op->value, body);
    return SeqStmt({prep, body});
  }
  Stmt MakeGlobalBarrier() {
    ICHECK(sync_scope_.rank == StorageRank::kGlobal);
    if (!num_blocks_.defined()) {
      ICHECK(!is_lead_.defined());
      num_work_dim_ = thread_extents_.size();
      for (const AttrStmtNode *attr : thread_extents_) {
        IterVar iv = Downcast<IterVar>(attr->node);
        runtime::ThreadScope s = runtime::ThreadScope::Create(iv->thread_tag);
        if (s.rank == 0) {
          num_blocks_ =
              (num_blocks_.defined() ? attr->value * num_blocks_ : attr->value);
        } else if (s.rank == 1) {
          PrimExpr cond = iv->var == make_zero(iv->var.dtype());
          is_lead_ = is_lead_.defined() ? (is_lead_ && cond) : cond;
        }
      }
    } else {
      ICHECK_EQ(num_work_dim_, thread_extents_.size());
    }
    return Evaluate(
        Call(DataType::Int(32), builtin::tvm_storage_sync(),
             {StringImm(sync_scope_.to_string()), is_lead_, num_blocks_}));
  }
  // data structure.
  StorageScope sync_scope_;
  const std::unordered_set<const Object *> &syncs_;
  const std::unordered_map<const Object *, int> &partial_syncs_;
  // The read write statistics of storage
  std::unordered_map<Var, Entry, ObjectPtrHash, ObjectPtrEqual> rw_stats_;
  // The statistics for global barrier
  bool in_thread_env_{false};
  // memorized results
  std::vector<const AttrStmtNode *> thread_extents_;
  size_t num_work_dim_{0};
  PrimExpr num_blocks_;
  PrimExpr is_lead_;
};

Stmt TileLangThreadSync(Stmt stmt, std::string storage_scope) {
  StorageScope sync_scope = StorageScope::Create(storage_scope);
  if (sync_scope.rank == StorageRank::kShared && sync_scope.tag == "") {
    stmt = ThreadSyncAfterWaitQueueInserter(sync_scope)(stmt);
  }
  TileLangThreadSyncPlanner planner(sync_scope);
  planner(stmt);
  return ThreadSyncInserter(sync_scope, planner.syncs_inserted_,
                            planner.partial_syncs_inserted_)(std::move(stmt));
}

using namespace tir::transform;

namespace transform {

tvm::transform::Pass ThreadSync(String storage_scope) {
  auto pass_func = [storage_scope](PrimFunc f, IRModule m, PassContext ctx) {
    auto *n = f.CopyOnWrite();
    n->body = tl::TileLangThreadSync(std::move(n->body), storage_scope);
    return f;
  };
  return CreatePrimFuncPass(pass_func, 0, "tl.ThreadSync", {});
}

TVM_REGISTER_GLOBAL("tl.transform.ThreadSync").set_body_typed(ThreadSync);

} // namespace transform
} // namespace tl
} // namespace tvm