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/* -*- 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 */
#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;
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++) {
// 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 {
return earliest_;
// Get the range index corresponding to earliestDeadlineRange().
unsigned earliestDeadlineRange() const {
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);
// 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);
// 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()) {
if (deadline == vec.back()) {
// Expected fast case: Structured control flow causes forward
// branches to be bound in reverse order.
} else {
// Slow case: Binary search + linear erase.
auto where = std::lower_bound(vec.begin(), vec.end(), deadline);
if (where == vec.end() || *where != deadline) {
if (deadline == earliest_) {
// Specialization for architectures that don't need to track short-range
// branches.
template <>
class BranchDeadlineSet<0u> {
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 {
// 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;
// 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),
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() {
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> {
// 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;
class PoolEntry {
size_t index_;
explicit PoolEntry(size_t index) : index_(index) {}
PoolEntry() : index_(-1) {}
size_t index() const { return index_; }
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_;
// 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_;
// The buffer slices are in a double linked list.
Slice* getHead() const { return this->head; }
Slice* getTail() const { return this->tail; }
AssemblerBufferWithConstantPools(unsigned guardSize, unsigned headerSize,
size_t instBufferAlign, size_t poolMaxOffset,
unsigned pcBias, uint32_t alignFillInst,
uint32_t nopFillInst, unsigned nopFill = 0)
: poolEntryCount(0),
pool_(poolMaxOffset, pcBias, this->lifoAlloc_),
#ifdef DEBUG
inhibitNops_(false) {
size_t sizeExcludingCurrentPool() const {
// Return the actual size of the buffer, excluding the current pending
// pool.
return this->nextOffset().getOffset();
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();
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++) {
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() -
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) {
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",
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) {
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.
// Get the next buffer offset where an instruction would be inserted.
// This may flush the current constant pool before returning nextOffset().
BufferOffset nextInstrOffset(int numInsts = 1) {
if (!hasSpaceForInsts(numInsts, /* numPoolEntries= */ 0)) {
"nextInstrOffset @ %d caused a constant pool spill",
return this->nextOffset();
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();
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, "_");
// 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,
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.
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) || defined(JS_CODEGEN_RISCV64)
return this->putU32Aligned(value);
return this->AssemblerBuffer<SliceSize, Inst>::putInt(value);
// 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)) {
// 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);
// 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");
// Should not be placing a pool in a no-pool region, check.
// 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()) {
// 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()) {
// 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))) {
// Reset everything to the state that it was in when we started.
void flushPool() {
if (this->oom()) {
JitSpew(JitSpew_Pools, "Requesting a pool flush");
void enterNoPool(size_t maxInst) {
if (this->oom()) {
// Don't allow re-entry.
// 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.",
finishPool(maxInst * InstSize);
if (this->oom()) {
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;
canNotPlacePool_ = true;
void leaveNoPool() {
if (this->oom()) {
canNotPlacePool_ = false;
canNotPlacePool_ = false;
// Validate the maxInst argument supplied to enterNoPool().
MOZ_ASSERT(this->nextOffset().getOffset() - canNotPlacePoolStartOffset_ <=
canNotPlacePoolMaxInst_ * InstSize);
void enterNoNops() {
inhibitNops_ = true;
void leaveNoNops() {
inhibitNops_ = false;
void assertNoPoolAndNoNops() {
MOZ_ASSERT_IF(!this->oom(), isPoolEmptyFor(InstSize) || canNotPlacePool_);
void align(unsigned alignment) { align(alignment, alignFillInst_); }
void align(unsigned alignment, uint32_t pattern) {
MOZ_ASSERT(alignment >= InstSize);
// A pool many need to be dumped at this point, so insert NOP fill here.
// Check if the code position can be aligned without dumping a pool.
unsigned requiredFill = sizeExcludingCurrentPool() & (alignment - 1);
if (requiredFill == 0) {
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());
bool prevInhibitNops = inhibitNops_;
inhibitNops_ = true;
while ((sizeExcludingCurrentPool() & (alignment - 1)) && !this->oom()) {
inhibitNops_ = prevInhibitNops;
void executableCopy(uint8_t* dest) {
if (this->oom()) {
// 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();
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:
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?");
// 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