path: root/js/src/jit/shared/IonAssemblerBufferWithConstantPools.h

/* -*- 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/. */

#ifndef jit_shared_IonAssemblerBufferWithConstantPools_h
#define jit_shared_IonAssemblerBufferWithConstantPools_h

#include "mozilla/CheckedInt.h"
#include "mozilla/MathAlgorithms.h"

#include <algorithm>

#include "jit/JitSpewer.h"
#include "jit/shared/IonAssemblerBuffer.h"

// [SMDOC] JIT AssemblerBuffer constant pooling (ARM/ARM64/MIPS)
//
// This code extends the AssemblerBuffer to support the pooling of values loaded
// using program-counter relative addressing modes. This is necessary with the
// ARM instruction set because it has a fixed instruction size that can not
// encode all values as immediate arguments in instructions. Pooling the values
// allows the values to be placed in large chunks which minimizes the number of
// forced branches around them in the code. This is used for loading floating
// point constants, for loading 32 bit constants on the ARMv6, for absolute
// branch targets, and in future will be needed for large branches on the ARMv6.
//
// For simplicity of the implementation, the constant pools are always placed
// after the loads referencing them. When a new constant pool load is added to
// the assembler buffer, a corresponding pool entry is added to the current
// pending pool. The finishPool() method copies the current pending pool entries
// into the assembler buffer at the current offset and patches the pending
// constant pool load instructions.
//
// Before inserting instructions or pool entries, it is necessary to determine
// if doing so would place a pending pool entry out of reach of an instruction,
// and if so then the pool must firstly be dumped. With the allocation algorithm
// used below, the recalculation of all the distances between instructions and
// their pool entries can be avoided by noting that there will be a limiting
// instruction and pool entry pair that does not change when inserting more
// instructions. Adding more instructions makes the same increase to the
// distance, between instructions and their pool entries, for all such
// pairs. This pair is recorded as the limiter, and it is updated when new pool
// entries are added, see updateLimiter()
//
// The pools consist of: a guard instruction that branches around the pool, a
// header word that helps identify a pool in the instruction stream, and then
// the pool entries allocated in units of words. The guard instruction could be
// omitted if control does not reach the pool, and this is referred to as a
// natural guard below, but for simplicity the guard branch is always
// emitted. The pool header is an identifiable word that in combination with the
// guard uniquely identifies a pool in the instruction stream. The header also
// encodes the pool size and a flag indicating if the guard is natural. It is
// possible to iterate through the code instructions skipping or examining the
// pools. E.g. it might be necessary to skip pools when search for, or patching,
// an instruction sequence.
//
// It is often required to keep a reference to a pool entry, to patch it after
// the buffer is finished. Each pool entry is assigned a unique index, counting
// up from zero (see the poolEntryCount slot below). These can be mapped back to
// the offset of the pool entry in the finished buffer, see poolEntryOffset().
//
// The code supports no-pool regions, and for these the size of the region, in
// instructions, must be supplied. This size is used to determine if inserting
// the instructions would place a pool entry out of range, and if so then a pool
// is firstly flushed. The DEBUG code checks that the emitted code is within the
// supplied size to detect programming errors. See enterNoPool() and
// leaveNoPool().

// The only planned instruction sets that require inline constant pools are the
// ARM, ARM64, and MIPS, and these all have fixed 32-bit sized instructions so
// for simplicity the code below is specialized for fixed 32-bit sized
// instructions and makes no attempt to support variable length
// instructions. The base assembler buffer which supports variable width
// instruction is used by the x86 and x64 backends.

// The AssemblerBufferWithConstantPools template class uses static callbacks to
// the provided Asm template argument class:
//
// void Asm::InsertIndexIntoTag(uint8_t* load_, uint32_t index)
//
//   When allocEntry() is called to add a constant pool load with an associated
//   constant pool entry, this callback is called to encode the index of the
//   allocated constant pool entry into the load instruction.
//
//   After the constant pool has been placed, PatchConstantPoolLoad() is called
//   to update the load instruction with the right load offset.
//
// void Asm::WritePoolGuard(BufferOffset branch,
//                          Instruction* dest,
//                          BufferOffset afterPool)
//
//   Write out the constant pool guard branch before emitting the pool.
//
//   branch
//     Offset of the guard branch in the buffer.
//
//   dest
//     Pointer into the buffer where the guard branch should be emitted. (Same
//     as getInst(branch)). Space for guardSize_ instructions has been reserved.
//
//   afterPool
//     Offset of the first instruction after the constant pool. This includes
//     both pool entries and branch veneers added after the pool data.
//
// void Asm::WritePoolHeader(uint8_t* start, Pool* p, bool isNatural)
//
//   Write out the pool header which follows the guard branch.
//
// void Asm::PatchConstantPoolLoad(void* loadAddr, void* constPoolAddr)
//
//   Re-encode a load of a constant pool entry after the location of the
//   constant pool is known.
//
//   The load instruction at loadAddr was previously passed to
//   InsertIndexIntoTag(). The constPoolAddr is the final address of the
//   constant pool in the assembler buffer.
//
// void Asm::PatchShortRangeBranchToVeneer(AssemblerBufferWithConstantPools*,
//                                         unsigned rangeIdx,
//                                         BufferOffset deadline,
//                                         BufferOffset veneer)
//
//   Patch a short-range branch to jump through a veneer before it goes out of
//   range.
//
//   rangeIdx, deadline
//     These arguments were previously passed to registerBranchDeadline(). It is
//     assumed that PatchShortRangeBranchToVeneer() knows how to compute the
//     offset of the short-range branch from this information.
//
//   veneer
//     Space for a branch veneer, guaranteed to be <= deadline. At this
//     position, guardSize_ * InstSize bytes are allocated. They should be
//     initialized to the proper unconditional branch instruction.
//
//   Unbound branches to the same unbound label are organized as a linked list:
//
//     Label::offset -> Branch1 -> Branch2 -> Branch3 -> nil
//
//   This callback should insert a new veneer branch into the list:
//
//     Label::offset -> Branch1 -> Branch2 -> Veneer -> Branch3 -> nil
//
//   When Assembler::bind() rewrites the branches with the real label offset, it
//   probably has to bind Branch2 to target the veneer branch instead of jumping
//   straight to the label.

namespace js {
namespace jit {

// BranchDeadlineSet - Keep track of pending branch deadlines.
//
// Some architectures like arm and arm64 have branch instructions with limited
// range. When assembling a forward branch, it is not always known if the final
// target label will be in range of the branch instruction.
//
// The BranchDeadlineSet data structure is used to keep track of the set of
// pending forward branches. It supports the following fast operations:
//
// 1. Get the earliest deadline in the set.
// 2. Add a new branch deadline.
// 3. Remove a branch deadline.
//
// Architectures may have different branch encodings with different ranges. Each
// supported range is assigned a small integer starting at 0. This data
// structure does not care about the actual range of branch instructions, just
// the latest buffer offset that can be reached - the deadline offset.
//
// Branched are stored as (rangeIdx, deadline) tuples. The target-specific code
// can compute the location of the branch itself from this information. This
// data structure does not need to know.
//
template <unsigned NumRanges>
class BranchDeadlineSet {
  // Maintain a list of pending deadlines for each range separately.
  //
  // The offsets in each vector are always kept in ascending order.
  //
  // Because we have a separate vector for different ranges, as forward
  // branches are added to the assembler buffer, their deadlines will
  // always be appended to the vector corresponding to their range.
  //
  // When binding labels, we expect a more-or-less LIFO order of branch
  // resolutions. This would always hold if we had strictly structured control
  // flow.
  //
  // We allow branch deadlines to be added and removed in any order, but
  // performance is best in the expected case of near LIFO order.
  //
  typedef Vector<BufferOffset, 8, LifoAllocPolicy<Fallible>> RangeVector;

  // We really just want "RangeVector deadline_[NumRanges];", but each vector
  // needs to be initialized with a LifoAlloc, and C++ doesn't bend that way.
  //
  // Use raw aligned storage instead and explicitly construct NumRanges
  // vectors in our constructor.
  mozilla::AlignedStorage2<RangeVector[NumRanges]> deadlineStorage_;

  // Always access the range vectors through this method.
  RangeVector& vectorForRange(unsigned rangeIdx) {
    MOZ_ASSERT(rangeIdx < NumRanges, "Invalid branch range index");
    return (*deadlineStorage_.addr())[rangeIdx];
  }

  const RangeVector& vectorForRange(unsigned rangeIdx) const {
    MOZ_ASSERT(rangeIdx < NumRanges, "Invalid branch range index");
    return (*deadlineStorage_.addr())[rangeIdx];
  }

  // Maintain a precomputed earliest deadline at all times.
  // This is unassigned only when all deadline vectors are empty.
  BufferOffset earliest_;

  // The range vector owning earliest_. Uninitialized when empty.
  unsigned earliestRange_;

  // Recompute the earliest deadline after it's been invalidated.
  void recomputeEarliest() {
    earliest_ = BufferOffset();
    for (unsigned r = 0; r < NumRanges; r++) {
      auto& vec = vectorForRange(r);
      if (!vec.empty() && (!earliest_.assigned() || vec[0] < earliest_)) {
        earliest_ = vec[0];
        earliestRange_ = r;
      }
    }
  }

  // Update the earliest deadline if needed after inserting (rangeIdx,
  // deadline). Always return true for convenience:
  // return insert() && updateEarliest().
  bool updateEarliest(unsigned rangeIdx, BufferOffset deadline) {
    if (!earliest_.assigned() || deadline < earliest_) {
      earliest_ = deadline;
      earliestRange_ = rangeIdx;
    }
    return true;
  }

 public:
  explicit BranchDeadlineSet(LifoAlloc& alloc) : earliestRange_(0) {
    // Manually construct vectors in the uninitialized aligned storage.
    // This is because C++ arrays can otherwise only be constructed with
    // the default constructor.
    for (unsigned r = 0; r < NumRanges; r++) {
      new (&vectorForRange(r)) RangeVector(alloc);
    }
  }

  ~BranchDeadlineSet() {
    // Aligned storage doesn't destruct its contents automatically.
    for (unsigned r = 0; r < NumRanges; r++) {
      vectorForRange(r).~RangeVector();
    }
  }

  // Is this set completely empty?
  bool empty() const { return !earliest_.assigned(); }

  // Get the total number of deadlines in the set.
  size_t size() const {
    size_t count = 0;
    for (unsigned r = 0; r < NumRanges; r++) {
      count += vectorForRange(r).length();
    }
    return count;
  }

  // Get the number of deadlines for the range with the most elements.
  size_t maxRangeSize() const {
    size_t count = 0;
    for (unsigned r = 0; r < NumRanges; r++) {
      count = std::max(count, vectorForRange(r).length());
    }
    return count;
  }

  // Get the first deadline that is still in the set.
  BufferOffset earliestDeadline() const {
    MOZ_ASSERT(!empty());
    return earliest_;
  }

  // Get the range index corresponding to earliestDeadlineRange().
  unsigned earliestDeadlineRange() const {
    MOZ_ASSERT(!empty());
    return earliestRange_;
  }

  // Add a (rangeIdx, deadline) tuple to the set.
  //
  // It is assumed that this tuple is not already in the set.
  // This function performs best id the added deadline is later than any
  // existing deadline for the same range index.
  //
  // Return true if the tuple was added, false if the tuple could not be added
  // because of an OOM error.
  bool addDeadline(unsigned rangeIdx, BufferOffset deadline) {
    MOZ_ASSERT(deadline.assigned(), "Can only store assigned buffer offsets");
    // This is the vector where deadline should be saved.
    auto& vec = vectorForRange(rangeIdx);

    // Fast case: Simple append to the relevant array. This never affects
    // the earliest deadline.
    if (!vec.empty() && vec.back() < deadline) {
      return vec.append(deadline);
    }

    // Fast case: First entry to the vector. We need to update earliest_.
    if (vec.empty()) {
      return vec.append(deadline) && updateEarliest(rangeIdx, deadline);
    }

    return addDeadlineSlow(rangeIdx, deadline);
  }

 private:
  // General case of addDeadline. This is split into two functions such that
  // the common case in addDeadline can be inlined while this part probably
  // won't inline.
  bool addDeadlineSlow(unsigned rangeIdx, BufferOffset deadline) {
    auto& vec = vectorForRange(rangeIdx);

    // Inserting into the middle of the vector. Use a log time binary search
    // and a linear time insert().
    // Is it worthwhile special-casing the empty vector?
    auto at = std::lower_bound(vec.begin(), vec.end(), deadline);
    MOZ_ASSERT(at == vec.end() || *at != deadline,
               "Cannot insert duplicate deadlines");
    return vec.insert(at, deadline) && updateEarliest(rangeIdx, deadline);
  }

 public:
  // Remove a deadline from the set.
  // If (rangeIdx, deadline) is not in the set, nothing happens.
  void removeDeadline(unsigned rangeIdx, BufferOffset deadline) {
    auto& vec = vectorForRange(rangeIdx);

    if (vec.empty()) {
      return;
    }

    if (deadline == vec.back()) {
      // Expected fast case: Structured control flow causes forward
      // branches to be bound in reverse order.
      vec.popBack();
    } else {
      // Slow case: Binary search + linear erase.
      auto where = std::lower_bound(vec.begin(), vec.end(), deadline);
      if (where == vec.end() || *where != deadline) {
        return;
      }
      vec.erase(where);
    }
    if (deadline == earliest_) {
      recomputeEarliest();
    }
  }
};

// Specialization for architectures that don't need to track short-range
// branches.
template <>
class BranchDeadlineSet<0u> {
 public:
  explicit BranchDeadlineSet(LifoAlloc& alloc) {}
  bool empty() const { return true; }
  size_t size() const { return 0; }
  size_t maxRangeSize() const { return 0; }
  BufferOffset earliestDeadline() const { MOZ_CRASH(); }
  unsigned earliestDeadlineRange() const { MOZ_CRASH(); }
  bool addDeadline(unsigned rangeIdx, BufferOffset deadline) { MOZ_CRASH(); }
  void removeDeadline(unsigned rangeIdx, BufferOffset deadline) { MOZ_CRASH(); }
};

// The allocation unit size for pools.
typedef int32_t PoolAllocUnit;

// Hysteresis given to short-range branches.
//
// If any short-range branches will go out of range in the next N bytes,
// generate a veneer for them in the current pool. The hysteresis prevents the
// creation of many tiny constant pools for branch veneers.
const size_t ShortRangeBranchHysteresis = 128;

struct Pool {
 private:
  // The maximum program-counter relative offset below which the instruction
  // set can encode. Different classes of intructions might support different
  // ranges but for simplicity the minimum is used here, and for the ARM this
  // is constrained to 1024 by the float load instructions.
  const size_t maxOffset_;
  // An offset to apply to program-counter relative offsets. The ARM has a
  // bias of 8.
  const unsigned bias_;

  // The content of the pool entries.
  Vector<PoolAllocUnit, 8, LifoAllocPolicy<Fallible>> poolData_;

  // Flag that tracks OOM conditions. This is set after any append failed.
  bool oom_;

  // The limiting instruction and pool-entry pair. The instruction program
  // counter relative offset of this limiting instruction will go out of range
  // first as the pool position moves forward. It is more efficient to track
  // just this limiting pair than to recheck all offsets when testing if the
  // pool needs to be dumped.
  //
  // 1. The actual offset of the limiting instruction referencing the limiting
  // pool entry.
  BufferOffset limitingUser;
  // 2. The pool entry index of the limiting pool entry.
  unsigned limitingUsee;

 public:
  // A record of the code offset of instructions that reference pool
  // entries. These instructions need to be patched when the actual position
  // of the instructions and pools are known, and for the code below this
  // occurs when each pool is finished, see finishPool().
  Vector<BufferOffset, 8, LifoAllocPolicy<Fallible>> loadOffsets;

  // Create a Pool. Don't allocate anything from lifoAloc, just capture its
  // reference.
  explicit Pool(size_t maxOffset, unsigned bias, LifoAlloc& lifoAlloc)
      : maxOffset_(maxOffset),
        bias_(bias),
        poolData_(lifoAlloc),
        oom_(false),
        limitingUser(),
        limitingUsee(INT_MIN),
        loadOffsets(lifoAlloc) {}

  // If poolData() returns nullptr then oom_ will also be true.
  const PoolAllocUnit* poolData() const { return poolData_.begin(); }

  unsigned numEntries() const { return poolData_.length(); }

  size_t getPoolSize() const { return numEntries() * sizeof(PoolAllocUnit); }

  bool oom() const { return oom_; }

  // Update the instruction/pool-entry pair that limits the position of the
  // pool. The nextInst is the actual offset of the new instruction being
  // allocated.
  //
  // This is comparing the offsets, see checkFull() below for the equation,
  // but common expressions on both sides have been canceled from the ranges
  // being compared. Notably, the poolOffset cancels out, so the limiting pair
  // does not depend on where the pool is placed.
  void updateLimiter(BufferOffset nextInst) {
    ptrdiff_t oldRange =
        limitingUsee * sizeof(PoolAllocUnit) - limitingUser.getOffset();
    ptrdiff_t newRange = getPoolSize() - nextInst.getOffset();
    if (!limitingUser.assigned() || newRange > oldRange) {
      // We have a new largest range!
      limitingUser = nextInst;
      limitingUsee = numEntries();
    }
  }

  // Check if inserting a pool at the actual offset poolOffset would place
  // pool entries out of reach. This is called before inserting instructions
  // to check that doing so would not push pool entries out of reach, and if
  // so then the pool would need to be firstly dumped. The poolOffset is the
  // first word of the pool, after the guard and header and alignment fill.
  bool checkFull(size_t poolOffset) const {
    // Not full if there are no uses.
    if (!limitingUser.assigned()) {
      return false;
    }
    size_t offset = poolOffset + limitingUsee * sizeof(PoolAllocUnit) -
                    (limitingUser.getOffset() + bias_);
    return offset >= maxOffset_;
  }

  static const unsigned OOM_FAIL = unsigned(-1);

  unsigned insertEntry(unsigned num, uint8_t* data, BufferOffset off,
                       LifoAlloc& lifoAlloc) {
    if (oom_) {
      return OOM_FAIL;
    }
    unsigned ret = numEntries();
    if (!poolData_.append((PoolAllocUnit*)data, num) ||
        !loadOffsets.append(off)) {
      oom_ = true;
      return OOM_FAIL;
    }
    return ret;
  }

  void reset() {
    poolData_.clear();
    loadOffsets.clear();

    limitingUser = BufferOffset();
    limitingUsee = -1;
  }
};

// Template arguments:
//
// SliceSize
//   Number of bytes in each allocated BufferSlice. See
//   AssemblerBuffer::SliceSize.
//
// InstSize
//   Size in bytes of the fixed-size instructions. This should be equal to
//   sizeof(Inst). This is only needed here because the buffer is defined before
//   the Instruction.
//
// Inst
//   The actual type used to represent instructions. This is only really used as
//   the return type of the getInst() method.
//
// Asm
//   Class defining the needed static callback functions. See documentation of
//   the Asm::* callbacks above.
//
// NumShortBranchRanges
//   The number of short branch ranges to support. This can be 0 if no support
//   for tracking short range branches is needed. The
//   AssemblerBufferWithConstantPools class does not need to know what the range
//   of branches is - it deals in branch 'deadlines' which is the last buffer
//   position that a short-range forward branch can reach. It is assumed that
//   the Asm class is able to find the actual branch instruction given a
//   (range-index, deadline) pair.
//
//
template <size_t SliceSize, size_t InstSize, class Inst, class Asm,
          unsigned NumShortBranchRanges = 0>
struct AssemblerBufferWithConstantPools
    : public AssemblerBuffer<SliceSize, Inst> {
 private:
  // The PoolEntry index counter. Each PoolEntry is given a unique index,
  // counting up from zero, and these can be mapped back to the actual pool
  // entry offset after finishing the buffer, see poolEntryOffset().
  size_t poolEntryCount;

 public:
  class PoolEntry {
    size_t index_;

   public:
    explicit PoolEntry(size_t index) : index_(index) {}

    PoolEntry() : index_(-1) {}

    size_t index() const { return index_; }
  };

 private:
  typedef AssemblerBuffer<SliceSize, Inst> Parent;
  using typename Parent::Slice;

  // The size of a pool guard, in instructions. A branch around the pool.
  const unsigned guardSize_;
  // The size of the header that is put at the beginning of a full pool, in
  // instruction sized units.
  const unsigned headerSize_;

  // The maximum pc relative offset encoded in instructions that reference
  // pool entries. This is generally set to the maximum offset that can be
  // encoded by the instructions, but for testing can be lowered to affect the
  // pool placement and frequency of pool placement.
  const size_t poolMaxOffset_;

  // The bias on pc relative addressing mode offsets, in units of bytes. The
  // ARM has a bias of 8 bytes.
  const unsigned pcBias_;

  // The current working pool. Copied out as needed before resetting.
  Pool pool_;

  // The buffer should be aligned to this address.
  const size_t instBufferAlign_;

  struct PoolInfo {
    // The index of the first entry in this pool.
    // Pool entries are numbered uniquely across all pools, starting from 0.
    unsigned firstEntryIndex;

    // The location of this pool's first entry in the main assembler buffer.
    // Note that the pool guard and header come before this offset which
    // points directly at the data.
    BufferOffset offset;

    explicit PoolInfo(unsigned index, BufferOffset data)
        : firstEntryIndex(index), offset(data) {}
  };

  // Info for each pool that has already been dumped. This does not include
  // any entries in pool_.
  Vector<PoolInfo, 8, LifoAllocPolicy<Fallible>> poolInfo_;

  // Set of short-range forward branches that have not yet been bound.
  // We may need to insert veneers if the final label turns out to be out of
  // range.
  //
  // This set stores (rangeIdx, deadline) pairs instead of the actual branch
  // locations.
  BranchDeadlineSet<NumShortBranchRanges> branchDeadlines_;

  // When true dumping pools is inhibited.
  bool canNotPlacePool_;

#ifdef DEBUG
  // State for validating the 'maxInst' argument to enterNoPool().
  // The buffer offset when entering the no-pool region.
  size_t canNotPlacePoolStartOffset_;
  // The maximum number of word sized instructions declared for the no-pool
  // region.
  size_t canNotPlacePoolMaxInst_;
#endif

  // Instruction to use for alignment fill.
  const uint32_t alignFillInst_;

  // Insert a number of NOP instructions between each requested instruction at
  // all locations at which a pool can potentially spill. This is useful for
  // checking that instruction locations are correctly referenced and/or
  // followed.
  const uint32_t nopFillInst_;
  const unsigned nopFill_;

  // For inhibiting the insertion of fill NOPs in the dynamic context in which
  // they are being inserted.
  bool inhibitNops_;

 private:
  // The buffer slices are in a double linked list.
  Slice* getHead() const { return this->head; }
  Slice* getTail() const { return this->tail; }

 public:
  AssemblerBufferWithConstantPools(unsigned guardSize, unsigned headerSize,
                                   size_t instBufferAlign, size_t poolMaxOffset,
                                   unsigned pcBias, uint32_t alignFillInst,
                                   uint32_t nopFillInst, unsigned nopFill = 0)
      : poolEntryCount(0),
        guardSize_(guardSize),
        headerSize_(headerSize),
        poolMaxOffset_(poolMaxOffset),
        pcBias_(pcBias),
        pool_(poolMaxOffset, pcBias, this->lifoAlloc_),
        instBufferAlign_(instBufferAlign),
        poolInfo_(this->lifoAlloc_),
        branchDeadlines_(this->lifoAlloc_),
        canNotPlacePool_(false),
#ifdef DEBUG
        canNotPlacePoolStartOffset_(0),
        canNotPlacePoolMaxInst_(0),
#endif
        alignFillInst_(alignFillInst),
        nopFillInst_(nopFillInst),
        nopFill_(nopFill),
        inhibitNops_(false) {
  }

 private:
  size_t sizeExcludingCurrentPool() const {
    // Return the actual size of the buffer, excluding the current pending
    // pool.
    return this->nextOffset().getOffset();
  }

 public:
  size_t size() const {
    // Return the current actual size of the buffer. This is only accurate
    // if there are no pending pool entries to dump, check.
    MOZ_ASSERT_IF(!this->oom(), pool_.numEntries() == 0);
    return sizeExcludingCurrentPool();
  }

 private:
  void insertNopFill() {
    // Insert fill for testing.
    if (nopFill_ > 0 && !inhibitNops_ && !canNotPlacePool_) {
      inhibitNops_ = true;

      // Fill using a branch-nop rather than a NOP so this can be
      // distinguished and skipped.
      for (size_t i = 0; i < nopFill_; i++) {
        putInt(nopFillInst_);
      }

      inhibitNops_ = false;
    }
  }

  static const unsigned OOM_FAIL = unsigned(-1);
  static const unsigned DUMMY_INDEX = unsigned(-2);

  // Check if it is possible to add numInst instructions and numPoolEntries
  // constant pool entries without needing to flush the current pool.
  bool hasSpaceForInsts(unsigned numInsts, unsigned numPoolEntries) const {
    size_t nextOffset = sizeExcludingCurrentPool();
    // Earliest starting offset for the current pool after adding numInsts.
    // This is the beginning of the pool entries proper, after inserting a
    // guard branch + pool header.
    size_t poolOffset =
        nextOffset + (numInsts + guardSize_ + headerSize_) * InstSize;

    // Any constant pool loads that would go out of range?
    if (pool_.checkFull(poolOffset)) {
      return false;
    }

    // Any short-range branch that would go out of range?
    if (!branchDeadlines_.empty()) {
      size_t deadline = branchDeadlines_.earliestDeadline().getOffset();
      size_t poolEnd = poolOffset + pool_.getPoolSize() +
                       numPoolEntries * sizeof(PoolAllocUnit);

      // When NumShortBranchRanges > 1, is is possible for branch deadlines to
      // expire faster than we can insert veneers. Suppose branches are 4 bytes
      // each, we could have the following deadline set:
      //
      //   Range 0: 40, 44, 48
      //   Range 1: 44, 48
      //
      // It is not good enough to start inserting veneers at the 40 deadline; we
      // would not be able to create veneers for the second 44 deadline.
      // Instead, we need to start at 32:
      //
      //   32: veneer(40)
      //   36: veneer(44)
      //   40: veneer(44)
      //   44: veneer(48)
      //   48: veneer(48)
      //
      // This is a pretty conservative solution to the problem: If we begin at
      // the earliest deadline, we can always emit all veneers for the range
      // that currently has the most pending deadlines. That may not leave room
      // for veneers for the remaining ranges, so reserve space for those
      // secondary range veneers assuming the worst case deadlines.

      // Total pending secondary range veneer size.
      size_t secondaryVeneers = guardSize_ * (branchDeadlines_.size() -
                                              branchDeadlines_.maxRangeSize());

      if (deadline < poolEnd + secondaryVeneers) {
        return false;
      }
    }

    return true;
  }

  unsigned insertEntryForwards(unsigned numInst, unsigned numPoolEntries,
                               uint8_t* inst, uint8_t* data) {
    // If inserting pool entries then find a new limiter before we do the
    // range check.
    if (numPoolEntries) {
      pool_.updateLimiter(BufferOffset(sizeExcludingCurrentPool()));
    }

    if (!hasSpaceForInsts(numInst, numPoolEntries)) {
      if (numPoolEntries) {
        JitSpew(JitSpew_Pools, "Inserting pool entry caused a spill");
      } else {
        JitSpew(JitSpew_Pools, "Inserting instruction(%zu) caused a spill",
                sizeExcludingCurrentPool());
      }

      finishPool(numInst * InstSize);
      if (this->oom()) {
        return OOM_FAIL;
      }
      return insertEntryForwards(numInst, numPoolEntries, inst, data);
    }
    if (numPoolEntries) {
      unsigned result = pool_.insertEntry(numPoolEntries, data,
                                          this->nextOffset(), this->lifoAlloc_);
      if (result == Pool::OOM_FAIL) {
        this->fail_oom();
        return OOM_FAIL;
      }
      return result;
    }

    // The pool entry index is returned above when allocating an entry, but
    // when not allocating an entry a dummy value is returned - it is not
    // expected to be used by the caller.
    return DUMMY_INDEX;
  }

 public:
  // Get the next buffer offset where an instruction would be inserted.
  // This may flush the current constant pool before returning nextOffset().
  BufferOffset nextInstrOffset() {
    if (!hasSpaceForInsts(/* numInsts= */ 1, /* numPoolEntries= */ 0)) {
      JitSpew(JitSpew_Pools,
              "nextInstrOffset @ %d caused a constant pool spill",
              this->nextOffset().getOffset());
      finishPool(ShortRangeBranchHysteresis);
    }
    return this->nextOffset();
  }

  MOZ_NEVER_INLINE
  BufferOffset allocEntry(size_t numInst, unsigned numPoolEntries,
                          uint8_t* inst, uint8_t* data,
                          PoolEntry* pe = nullptr) {
    // The allocation of pool entries is not supported in a no-pool region,
    // check.
    MOZ_ASSERT_IF(numPoolEntries, !canNotPlacePool_);

    if (this->oom()) {
      return BufferOffset();
    }

    insertNopFill();

#ifdef JS_JITSPEW
    if (numPoolEntries && JitSpewEnabled(JitSpew_Pools)) {
      JitSpew(JitSpew_Pools, "Inserting %d entries into pool", numPoolEntries);
      JitSpewStart(JitSpew_Pools, "data is: 0x");
      size_t length = numPoolEntries * sizeof(PoolAllocUnit);
      for (unsigned idx = 0; idx < length; idx++) {
        JitSpewCont(JitSpew_Pools, "%02x", data[length - idx - 1]);
        if (((idx & 3) == 3) && (idx + 1 != length)) {
          JitSpewCont(JitSpew_Pools, "_");
        }
      }
      JitSpewFin(JitSpew_Pools);
    }
#endif

    // Insert the pool value.
    unsigned index = insertEntryForwards(numInst, numPoolEntries, inst, data);
    if (this->oom()) {
      return BufferOffset();
    }

    // Now to get an instruction to write.
    PoolEntry retPE;
    if (numPoolEntries) {
      JitSpew(JitSpew_Pools, "Entry has index %u, offset %zu", index,
              sizeExcludingCurrentPool());
      Asm::InsertIndexIntoTag(inst, index);
      // Figure out the offset within the pool entries.
      retPE = PoolEntry(poolEntryCount);
      poolEntryCount += numPoolEntries;
    }
    // Now inst is a valid thing to insert into the instruction stream.
    if (pe != nullptr) {
      *pe = retPE;
    }
    return this->putBytes(numInst * InstSize, inst);
  }

  // putInt is the workhorse for the assembler and higher-level buffer
  // abstractions: it places one instruction into the instruction stream.
  // Under normal circumstances putInt should just check that the constant
  // pool does not need to be flushed, that there is space for the single word
  // of the instruction, and write that word and update the buffer pointer.
  //
  // To do better here we need a status variable that handles both nopFill_
  // and capacity, so that we can quickly know whether to go the slow path.
  // That could be a variable that has the remaining number of simple
  // instructions that can be inserted before a more expensive check,
  // which is set to zero when nopFill_ is set.
  //
  // We assume that we don't have to check this->oom() if there is space to
  // insert a plain instruction; there will always come a later time when it
  // will be checked anyway.

  MOZ_ALWAYS_INLINE
  BufferOffset putInt(uint32_t value) {
    if (nopFill_ ||
        !hasSpaceForInsts(/* numInsts= */ 1, /* numPoolEntries= */ 0)) {
      return allocEntry(1, 0, (uint8_t*)&value, nullptr, nullptr);
    }

#if defined(JS_CODEGEN_ARM) || defined(JS_CODEGEN_ARM64) ||     \
    defined(JS_CODEGEN_MIPS32) || defined(JS_CODEGEN_MIPS64) || \
    defined(JS_CODEGEN_LOONG64)
    return this->putU32Aligned(value);
#else
    return this->AssemblerBuffer<SliceSize, Inst>::putInt(value);
#endif
  }

  // Register a short-range branch deadline.
  //
  // After inserting a short-range forward branch, call this method to
  // register the branch 'deadline' which is the last buffer offset that the
  // branch instruction can reach.
  //
  // When the branch is bound to a destination label, call
  // unregisterBranchDeadline() to stop tracking this branch,
  //
  // If the assembled code is about to exceed the registered branch deadline,
  // and unregisterBranchDeadline() has not yet been called, an
  // instruction-sized constant pool entry is allocated before the branch
  // deadline.
  //
  // rangeIdx
  //   A number < NumShortBranchRanges identifying the range of the branch.
  //
  // deadline
  //   The highest buffer offset the the short-range branch can reach
  //   directly.
  //
  void registerBranchDeadline(unsigned rangeIdx, BufferOffset deadline) {
    if (!this->oom() && !branchDeadlines_.addDeadline(rangeIdx, deadline)) {
      this->fail_oom();
    }
  }

  // Un-register a short-range branch deadline.
  //
  // When a short-range branch has been successfully bound to its destination
  // label, call this function to stop traching the branch.
  //
  // The (rangeIdx, deadline) pair must be previously registered.
  //
  void unregisterBranchDeadline(unsigned rangeIdx, BufferOffset deadline) {
    if (!this->oom()) {
      branchDeadlines_.removeDeadline(rangeIdx, deadline);
    }
  }

 private:
  // Are any short-range branches about to expire?
  bool hasExpirableShortRangeBranches(size_t reservedBytes) const {
    if (branchDeadlines_.empty()) {
      return false;
    }

    // Include branches that would expire in the next N bytes. The reservedBytes
    // argument avoids the needless creation of many tiny constant pools.
    //
    // As the reservedBytes could be of any sizes such as SIZE_MAX, in the case
    // of flushPool, we have to check for overflow when comparing the deadline
    // with our expected reserved bytes.
    size_t deadline = branchDeadlines_.earliestDeadline().getOffset();
    using CheckedSize = mozilla::CheckedInt<size_t>;
    CheckedSize current(this->nextOffset().getOffset());
    CheckedSize poolFreeSpace(reservedBytes);
    auto future = current + poolFreeSpace;
    return !future.isValid() || deadline < future.value();
  }

  bool isPoolEmptyFor(size_t bytes) const {
    return pool_.numEntries() == 0 && !hasExpirableShortRangeBranches(bytes);
  }
  void finishPool(size_t reservedBytes) {
    JitSpew(JitSpew_Pools, "Attempting to finish pool %zu with %u entries.",
            poolInfo_.length(), pool_.numEntries());

    if (reservedBytes < ShortRangeBranchHysteresis) {
      reservedBytes = ShortRangeBranchHysteresis;
    }

    if (isPoolEmptyFor(reservedBytes)) {
      // If there is no data in the pool being dumped, don't dump anything.
      JitSpew(JitSpew_Pools, "Aborting because the pool is empty");
      return;
    }

    // Should not be placing a pool in a no-pool region, check.
    MOZ_ASSERT(!canNotPlacePool_);

    // Dump the pool with a guard branch around the pool.
    BufferOffset guard = this->putBytes(guardSize_ * InstSize, nullptr);
    BufferOffset header = this->putBytes(headerSize_ * InstSize, nullptr);
    BufferOffset data = this->putBytesLarge(pool_.getPoolSize(),
                                            (const uint8_t*)pool_.poolData());
    if (this->oom()) {
      return;
    }

    // Now generate branch veneers for any short-range branches that are
    // about to expire.
    while (hasExpirableShortRangeBranches(reservedBytes)) {
      unsigned rangeIdx = branchDeadlines_.earliestDeadlineRange();
      BufferOffset deadline = branchDeadlines_.earliestDeadline();

      // Stop tracking this branch. The Asm callback below may register
      // new branches to track.
      branchDeadlines_.removeDeadline(rangeIdx, deadline);

      // Make room for the veneer. Same as a pool guard branch.
      BufferOffset veneer = this->putBytes(guardSize_ * InstSize, nullptr);
      if (this->oom()) {
        return;
      }

      // Fix the branch so it targets the veneer.
      // The Asm class knows how to find the original branch given the
      // (rangeIdx, deadline) pair.
      Asm::PatchShortRangeBranchToVeneer(this, rangeIdx, deadline, veneer);
    }

    // We only reserved space for the guard branch and pool header.
    // Fill them in.
    BufferOffset afterPool = this->nextOffset();
    Asm::WritePoolGuard(guard, this->getInst(guard), afterPool);
    Asm::WritePoolHeader((uint8_t*)this->getInst(header), &pool_, false);

    // With the pool's final position determined it is now possible to patch
    // the instructions that reference entries in this pool, and this is
    // done incrementally as each pool is finished.
    size_t poolOffset = data.getOffset();

    unsigned idx = 0;
    for (BufferOffset* iter = pool_.loadOffsets.begin();
         iter != pool_.loadOffsets.end(); ++iter, ++idx) {
      // All entries should be before the pool.
      MOZ_ASSERT(iter->getOffset() < guard.getOffset());

      // Everything here is known so we can safely do the necessary
      // substitutions.
      Inst* inst = this->getInst(*iter);
      size_t codeOffset = poolOffset - iter->getOffset();

      // That is, PatchConstantPoolLoad wants to be handed the address of
      // the pool entry that is being loaded.  We need to do a non-trivial
      // amount of math here, since the pool that we've made does not
      // actually reside there in memory.
      JitSpew(JitSpew_Pools, "Fixing entry %d offset to %zu", idx, codeOffset);
      Asm::PatchConstantPoolLoad(inst, (uint8_t*)inst + codeOffset);
    }

    // Record the pool info.
    unsigned firstEntry = poolEntryCount - pool_.numEntries();
    if (!poolInfo_.append(PoolInfo(firstEntry, data))) {
      this->fail_oom();
      return;
    }

    // Reset everything to the state that it was in when we started.
    pool_.reset();
  }

 public:
  void flushPool() {
    if (this->oom()) {
      return;
    }
    JitSpew(JitSpew_Pools, "Requesting a pool flush");
    finishPool(SIZE_MAX);
  }

  void enterNoPool(size_t maxInst) {
    if (this->oom()) {
      return;
    }
    // Don't allow re-entry.
    MOZ_ASSERT(!canNotPlacePool_);
    insertNopFill();

    // Check if the pool will spill by adding maxInst instructions, and if
    // so then finish the pool before entering the no-pool region. It is
    // assumed that no pool entries are allocated in a no-pool region and
    // this is asserted when allocating entries.
    if (!hasSpaceForInsts(maxInst, 0)) {
      JitSpew(JitSpew_Pools, "No-Pool instruction(%zu) caused a spill.",
              sizeExcludingCurrentPool());
      finishPool(maxInst * InstSize);
      if (this->oom()) {
        return;
      }
      MOZ_ASSERT(hasSpaceForInsts(maxInst, 0));
    }

#ifdef DEBUG
    // Record the buffer position to allow validating maxInst when leaving
    // the region.
    canNotPlacePoolStartOffset_ = this->nextOffset().getOffset();
    canNotPlacePoolMaxInst_ = maxInst;
#endif

    canNotPlacePool_ = true;
  }

  void leaveNoPool() {
    if (this->oom()) {
      canNotPlacePool_ = false;
      return;
    }
    MOZ_ASSERT(canNotPlacePool_);
    canNotPlacePool_ = false;

    // Validate the maxInst argument supplied to enterNoPool().
    MOZ_ASSERT(this->nextOffset().getOffset() - canNotPlacePoolStartOffset_ <=
               canNotPlacePoolMaxInst_ * InstSize);
  }

  void enterNoNops() {
    MOZ_ASSERT(!inhibitNops_);
    inhibitNops_ = true;
  }
  void leaveNoNops() {
    MOZ_ASSERT(inhibitNops_);
    inhibitNops_ = false;
  }
  void assertNoPoolAndNoNops() {
    MOZ_ASSERT(inhibitNops_);
    MOZ_ASSERT_IF(!this->oom(), isPoolEmptyFor(InstSize) || canNotPlacePool_);
  }

  void align(unsigned alignment) { align(alignment, alignFillInst_); }

  void align(unsigned alignment, uint32_t pattern) {
    MOZ_ASSERT(mozilla::IsPowerOfTwo(alignment));
    MOZ_ASSERT(alignment >= InstSize);

    // A pool many need to be dumped at this point, so insert NOP fill here.
    insertNopFill();

    // Check if the code position can be aligned without dumping a pool.
    unsigned requiredFill = sizeExcludingCurrentPool() & (alignment - 1);
    if (requiredFill == 0) {
      return;
    }
    requiredFill = alignment - requiredFill;

    // Add an InstSize because it is probably not useful for a pool to be
    // dumped at the aligned code position.
    if (!hasSpaceForInsts(requiredFill / InstSize + 1, 0)) {
      // Alignment would cause a pool dump, so dump the pool now.
      JitSpew(JitSpew_Pools, "Alignment of %d at %zu caused a spill.",
              alignment, sizeExcludingCurrentPool());
      finishPool(requiredFill);
    }

    bool prevInhibitNops = inhibitNops_;
    inhibitNops_ = true;
    while ((sizeExcludingCurrentPool() & (alignment - 1)) && !this->oom()) {
      putInt(pattern);
    }
    inhibitNops_ = prevInhibitNops;
  }

 public:
  void executableCopy(uint8_t* dest) {
    if (this->oom()) {
      return;
    }
    // The pools should have all been flushed, check.
    MOZ_ASSERT(pool_.numEntries() == 0);
    for (Slice* cur = getHead(); cur != nullptr; cur = cur->getNext()) {
      memcpy(dest, &cur->instructions[0], cur->length());
      dest += cur->length();
    }
  }

  bool appendRawCode(const uint8_t* code, size_t numBytes) {
    if (this->oom()) {
      return false;
    }
    // The pools should have all been flushed, check.
    MOZ_ASSERT(pool_.numEntries() == 0);
    while (numBytes > SliceSize) {
      this->putBytes(SliceSize, code);
      numBytes -= SliceSize;
      code += SliceSize;
    }
    this->putBytes(numBytes, code);
    return !this->oom();
  }

 public:
  size_t poolEntryOffset(PoolEntry pe) const {
    MOZ_ASSERT(pe.index() < poolEntryCount - pool_.numEntries(),
               "Invalid pool entry, or not flushed yet.");
    // Find the pool containing pe.index().
    // The array is sorted, so we can use a binary search.
    auto b = poolInfo_.begin(), e = poolInfo_.end();
    // A note on asymmetric types in the upper_bound comparator:
    // http://permalink.gmane.org/gmane.comp.compilers.clang.devel/10101
    auto i = std::upper_bound(b, e, pe.index(),
                              [](size_t value, const PoolInfo& entry) {
                                return value < entry.firstEntryIndex;
                              });
    // Since upper_bound finds the first pool greater than pe,
    // we want the previous one which is the last one less than or equal.
    MOZ_ASSERT(i != b, "PoolInfo not sorted or empty?");
    --i;
    // The i iterator now points to the pool containing pe.index.
    MOZ_ASSERT(i->firstEntryIndex <= pe.index() &&
               (i + 1 == e || (i + 1)->firstEntryIndex > pe.index()));
    // Compute the byte offset into the pool.
    unsigned relativeIndex = pe.index() - i->firstEntryIndex;
    return i->offset.getOffset() + relativeIndex * sizeof(PoolAllocUnit);
  }
};

}  // namespace jit
}  // namespace js

#endif  // jit_shared_IonAssemblerBufferWithConstantPools_h