path: root/js/src/jit/MoveResolver.cpp

/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*-
 * vim: set ts=8 sts=2 et sw=2 tw=80:
 * This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/. */

#include "jit/MoveResolver.h"

#include "mozilla/ScopeExit.h"

#include "jit/MacroAssembler.h"
#include "jit/RegisterSets.h"

using namespace js;
using namespace js::jit;

MoveOperand::MoveOperand(MacroAssembler& masm, const ABIArg& arg) : disp_(0) {
  switch (arg.kind()) {
    case ABIArg::GPR:
      kind_ = REG;
      code_ = arg.gpr().code();
      break;
#ifdef JS_CODEGEN_REGISTER_PAIR
    case ABIArg::GPR_PAIR:
      kind_ = REG_PAIR;
      code_ = arg.evenGpr().code();
      MOZ_ASSERT(code_ % 2 == 0);
      MOZ_ASSERT(code_ + 1 == arg.oddGpr().code());
      break;
#endif
    case ABIArg::FPU:
      kind_ = FLOAT_REG;
      code_ = arg.fpu().code();
      break;
    case ABIArg::Stack:
      kind_ = MEMORY;
      if (IsHiddenSP(masm.getStackPointer())) {
        MOZ_CRASH(
            "Hidden SP cannot be represented as register code on this "
            "platform");
      } else {
        code_ = AsRegister(masm.getStackPointer()).code();
      }
      disp_ = arg.offsetFromArgBase();
      break;
    case ABIArg::Uninitialized:
      MOZ_CRASH("Uninitialized ABIArg kind");
  }
}

MoveResolver::MoveResolver() : numCycles_(0), curCycles_(0) {}

void MoveResolver::resetState() {
  numCycles_ = 0;
  curCycles_ = 0;
}

bool MoveResolver::addMove(const MoveOperand& from, const MoveOperand& to,
                           MoveOp::Type type) {
  // Assert that we're not doing no-op moves.
  MOZ_ASSERT(!(from == to));
  PendingMove* pm = movePool_.allocate(from, to, type);
  if (!pm) {
    return false;
  }
  pending_.pushBack(pm);
  return true;
}

// Given move (A -> B), this function attempts to find any move (B -> *) in the
// pending move list, and returns the first one.
MoveResolver::PendingMove* MoveResolver::findBlockingMove(
    const PendingMove* last) {
  for (PendingMoveIterator iter = pending_.begin(); iter != pending_.end();
       iter++) {
    PendingMove* other = *iter;

    if (other->from().aliases(last->to())) {
      // We now have pairs in the form (A -> X) (X -> y). The second pair
      // blocks the move in the first pair, so return it.
      return other;
    }
  }

  // No blocking moves found.
  return nullptr;
}

// Given move (A -> B), this function attempts to find any move (B -> *) in the
// move list iterator, and returns the first one.
// N.B. It is unclear if a single move can complete more than one cycle, so to
// be conservative, this function operates on iterators, so the caller can
// process all instructions that start a cycle.
MoveResolver::PendingMove* MoveResolver::findCycledMove(
    PendingMoveIterator* iter, PendingMoveIterator end,
    const PendingMove* last) {
  for (; *iter != end; (*iter)++) {
    PendingMove* other = **iter;
    if (other->from().aliases(last->to())) {
      // We now have pairs in the form (A -> X) (X -> y). The second pair
      // blocks the move in the first pair, so return it.
      (*iter)++;
      return other;
    }
  }
  // No blocking moves found.
  return nullptr;
}

#ifdef JS_CODEGEN_ARM
static inline bool MoveIsDouble(const MoveOperand& move) {
  if (!move.isFloatReg()) {
    return false;
  }
  return move.floatReg().isDouble();
}
#endif

#ifdef JS_CODEGEN_ARM
static inline bool MoveIsSingle(const MoveOperand& move) {
  if (!move.isFloatReg()) {
    return false;
  }
  return move.floatReg().isSingle();
}
#endif

#ifdef JS_CODEGEN_ARM
bool MoveResolver::isDoubleAliasedAsSingle(const MoveOperand& move) {
  if (!MoveIsDouble(move)) {
    return false;
  }

  for (auto iter = pending_.begin(); iter != pending_.end(); ++iter) {
    PendingMove* other = *iter;
    if (other->from().aliases(move) && MoveIsSingle(other->from())) {
      return true;
    }
    if (other->to().aliases(move) && MoveIsSingle(other->to())) {
      return true;
    }
  }
  return false;
}
#endif

#ifdef JS_CODEGEN_ARM
static MoveOperand SplitIntoLowerHalf(const MoveOperand& move) {
  if (MoveIsDouble(move)) {
    FloatRegister lowerSingle = move.floatReg().asSingle();
    return MoveOperand(lowerSingle);
  }

  MOZ_ASSERT(move.isMemoryOrEffectiveAddress());
  return move;
}
#endif

#ifdef JS_CODEGEN_ARM
static MoveOperand SplitIntoUpperHalf(const MoveOperand& move) {
  if (MoveIsDouble(move)) {
    FloatRegister lowerSingle = move.floatReg().asSingle();
    FloatRegister upperSingle =
        VFPRegister(lowerSingle.code() + 1, VFPRegister::Single);
    return MoveOperand(upperSingle);
  }

  MOZ_ASSERT(move.isMemoryOrEffectiveAddress());
  return MoveOperand(move.base(), move.disp() + sizeof(float));
}
#endif

// Resolves the pending_ list to a list in orderedMoves_.
bool MoveResolver::resolve() {
  resetState();
  orderedMoves_.clear();

  // Upon return from this function, the pending_ list must be cleared.
  auto clearPending = mozilla::MakeScopeExit([this]() { pending_.clear(); });

#ifdef JS_CODEGEN_ARM
  // Some of ARM's double registers alias two of its single registers,
  // but the algorithm below assumes that every register can participate
  // in at most one cycle. To satisfy the algorithm, any double registers
  // that may conflict are split into their single-register halves.
  //
  // This logic is only applicable because ARM only uses registers d0-d15,
  // all of which alias s0-s31. Double registers d16-d31 are unused.
  // Therefore there is never a double move that cannot be split.
  // If this changes in the future, the algorithm will have to be fixed.

  bool splitDoubles = false;
  for (auto iter = pending_.begin(); iter != pending_.end(); ++iter) {
    PendingMove* pm = *iter;

    if (isDoubleAliasedAsSingle(pm->from()) ||
        isDoubleAliasedAsSingle(pm->to())) {
      splitDoubles = true;
      break;
    }
  }

  if (splitDoubles) {
    for (auto iter = pending_.begin(); iter != pending_.end(); ++iter) {
      PendingMove* pm = *iter;

      if (!MoveIsDouble(pm->from()) && !MoveIsDouble(pm->to())) {
        continue;
      }

      MoveOperand fromLower = SplitIntoLowerHalf(pm->from());
      MoveOperand toLower = SplitIntoLowerHalf(pm->to());

      PendingMove* lower =
          movePool_.allocate(fromLower, toLower, MoveOp::FLOAT32);
      if (!lower) {
        return false;
      }

      // Insert the new node before the current position to not affect
      // iteration.
      pending_.insertBefore(pm, lower);

      // Overwrite pm in place for the upper move. Iteration proceeds as normal.
      MoveOperand fromUpper = SplitIntoUpperHalf(pm->from());
      MoveOperand toUpper = SplitIntoUpperHalf(pm->to());
      pm->overwrite(fromUpper, toUpper, MoveOp::FLOAT32);
    }
  }
#endif

  InlineList<PendingMove> stack;

  // This is a depth-first-search without recursion, which tries to find
  // cycles in a list of moves.
  //
  // Algorithm.
  //
  // S = Traversal stack.
  // P = Pending move list.
  // O = Ordered list of moves.
  //
  // As long as there are pending moves in P:
  //      Let |root| be any pending move removed from P
  //      Add |root| to the traversal stack.
  //      As long as S is not empty:
  //          Let |L| be the most recent move added to S.
  //
  //          Find any pending move M whose source is L's destination, thus
  //          preventing L's move until M has completed.
  //
  //          If a move M was found,
  //              Remove M from the pending list.
  //              If M's destination is |root|,
  //                  Annotate M and |root| as cycles.
  //                  Add M to S.
  //                  do not Add M to O, since M may have other conflictors in P
  //                  that have not yet been processed.
  //              Otherwise,
  //                  Add M to S.
  //         Otherwise,
  //              Remove L from S.
  //              Add L to O.
  //
  while (!pending_.empty()) {
    PendingMove* pm = pending_.popBack();

    // Add this pending move to the cycle detection stack.
    stack.pushBack(pm);

    while (!stack.empty()) {
      PendingMove* blocking = findBlockingMove(stack.peekBack());

      if (blocking) {
        PendingMoveIterator stackiter = stack.begin();
        PendingMove* cycled = findCycledMove(&stackiter, stack.end(), blocking);
        if (cycled) {
          // Find the cycle's start.
          // We annotate cycles at each move in the cycle, and
          // assert that we do not find two cycles in one move chain
          // traversal (which would indicate two moves to the same
          // destination).
          // Since there can be more than one cycle, find them all.
          do {
            cycled->setCycleEnd(curCycles_);
            cycled = findCycledMove(&stackiter, stack.end(), blocking);
          } while (cycled);

          blocking->setCycleBegin(pm->type(), curCycles_);
          curCycles_++;
          pending_.remove(blocking);
          stack.pushBack(blocking);
        } else {
          // This is a new link in the move chain, so keep
          // searching for a cycle.
          pending_.remove(blocking);
          stack.pushBack(blocking);
        }
      } else {
        // Otherwise, pop the last move on the search stack because it's
        // complete and not participating in a cycle. The resulting
        // move can safely be added to the ordered move list.
        PendingMove* done = stack.popBack();
        if (!addOrderedMove(*done)) {
          return false;
        }
        movePool_.free(done);
      }
    }
    // If the current queue is empty, it is certain that there are
    // all previous cycles cannot conflict with future cycles,
    // so re-set the counter of pending cycles, while keeping a high-water mark.
    if (numCycles_ < curCycles_) {
      numCycles_ = curCycles_;
    }
    curCycles_ = 0;
  }

  return true;
}

bool MoveResolver::addOrderedMove(const MoveOp& move) {
  // Sometimes the register allocator generates move groups where multiple
  // moves have the same source. Try to optimize these cases when the source
  // is in memory and the target of one of the moves is in a register.
  MOZ_ASSERT(!move.from().aliases(move.to()));

  if (!move.from().isMemory() || move.isCycleBegin() || move.isCycleEnd()) {
    return orderedMoves_.append(move);
  }

  // Look for an earlier move with the same source, where no intervening move
  // touches either the source or destination of the new move.
  for (int i = orderedMoves_.length() - 1; i >= 0; i--) {
    const MoveOp& existing = orderedMoves_[i];

    if (existing.from() == move.from() && !existing.to().aliases(move.to()) &&
        existing.type() == move.type() && !existing.isCycleBegin() &&
        !existing.isCycleEnd()) {
      MoveOp* after = orderedMoves_.begin() + i + 1;
      if (existing.to().isGeneralReg() || existing.to().isFloatReg()) {
        MoveOp nmove(existing.to(), move.to(), move.type());
        return orderedMoves_.insert(after, nmove);
      } else if (move.to().isGeneralReg() || move.to().isFloatReg()) {
        MoveOp nmove(move.to(), existing.to(), move.type());
        orderedMoves_[i] = move;
        return orderedMoves_.insert(after, nmove);
      }
    }

    if (existing.aliases(move)) {
      break;
    }
  }

  return orderedMoves_.append(move);
}

void MoveResolver::reorderMove(size_t from, size_t to) {
  MOZ_ASSERT(from != to);

  MoveOp op = orderedMoves_[from];
  if (from < to) {
    for (size_t i = from; i < to; i++) {
      orderedMoves_[i] = orderedMoves_[i + 1];
    }
  } else {
    for (size_t i = from; i > to; i--) {
      orderedMoves_[i] = orderedMoves_[i - 1];
    }
  }
  orderedMoves_[to] = op;
}

void MoveResolver::sortMemoryToMemoryMoves() {
  // Try to reorder memory->memory moves so that they are executed right
  // before a move that clobbers some register. This will allow the move
  // emitter to use that clobbered register as a scratch register for the
  // memory->memory move, if necessary.
  for (size_t i = 0; i < orderedMoves_.length(); i++) {
    const MoveOp& base = orderedMoves_[i];
    if (!base.from().isMemory() || !base.to().isMemory()) {
      continue;
    }
    if (base.type() != MoveOp::GENERAL && base.type() != MoveOp::INT32) {
      continue;
    }

    // Look for an earlier move clobbering a register.
    bool found = false;
    for (int j = i - 1; j >= 0; j--) {
      const MoveOp& previous = orderedMoves_[j];
      if (previous.aliases(base) || previous.isCycleBegin() ||
          previous.isCycleEnd()) {
        break;
      }

      if (previous.to().isGeneralReg()) {
        reorderMove(i, j);
        found = true;
        break;
      }
    }
    if (found) {
      continue;
    }

    // Look for a later move clobbering a register.
    if (i + 1 < orderedMoves_.length()) {
      bool found = false, skippedRegisterUse = false;
      for (size_t j = i + 1; j < orderedMoves_.length(); j++) {
        const MoveOp& later = orderedMoves_[j];
        if (later.aliases(base) || later.isCycleBegin() || later.isCycleEnd()) {
          break;
        }

        if (later.to().isGeneralReg()) {
          if (skippedRegisterUse) {
            reorderMove(i, j);
            found = true;
          } else {
            // There is no move that uses a register between the
            // original memory->memory move and this move that
            // clobbers a register. The move should already be able
            // to use a scratch register, so don't shift anything
            // around.
          }
          break;
        }

        if (later.from().isGeneralReg()) {
          skippedRegisterUse = true;
        }
      }

      if (found) {
        // Redo the search for memory->memory moves at the current
        // index, so we don't skip the move just shifted back.
        i--;
      }
    }
  }
}