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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_JitHints_h
#define jit_JitHints_h
#include "mozilla/BloomFilter.h"
#include "mozilla/HashTable.h"
#include "mozilla/LinkedList.h"
#include "jit/JitOptions.h"
#include "vm/BytecodeLocation.h"
#include "vm/JSScript.h"
namespace js::jit {
* [SMDOC] JitHintsMap
* The Jit hints map is an in process cache used to collect Baseline and Ion
* JIT hints to try and skip as much of the warmup as possible and jump
* straight into those tiers. Whenever a script enters one of these tiers
* a hint is recorded in this cache using the script's filename+sourceStart
* value, and if we ever encounter this script again later, e.g. during a
* navigation, then we try to eagerly compile it into baseline and ion
* based on its previous execution history.
class JitHintsMap {
// ScriptKey is a hash on the filename+sourceStart.
using ScriptKey = HashNumber;
ScriptKey getScriptKey(JSScript* script) const;
/* Ion Hints
* -------------------------------------------------------------------------
* This implementation uses a combination of a HashMap and PriorityQueue
* to store a threshold value for each script that has been Ion compiled.
* The PriorityQueue is used to track the least recently used entries so
* that the cache does not exceed |IonHintMaxEntries| entries.
* After a script has entered Ion the first time, an eager threshold hint
* value is set using the warmup counter of when the last IC stub was
* attached, if available. This minimizes the risk that the script will
* bailout. If that script is bailout invalidated, the threshold value
* is incremented by |InvalidationThresholdIncrement| up to a maximum value of
* |JitOptions.normalIonWarmUpThreshold|.
* Each IonHint object also contains a list of bytecode offsets for locations
* of monomorphic inline calls that is used as a hint for future compilations.
class IonHint : public mozilla::LinkedListElement<IonHint> {
ScriptKey key_ = 0;
// We use a value of 0 to indicate that the script has not entered Ion
// yet, but has been monomorphically inlined and Ion compiled into
// another script and contains bytecode offsets of a nested call.
uint32_t threshold_ = 0;
// List of bytecode offsets that have been successfully inlined with
// a state of monomorphic inline.
Vector<uint32_t, 0, SystemAllocPolicy> monomorphicInlineOffsets;
explicit IonHint(ScriptKey key) { key_ = key; }
void initThreshold(uint32_t lastStubCounter) {
threshold_ = IonHintEagerThresholdValue(lastStubCounter);
uint32_t threshold() { return threshold_; }
void incThreshold(uint32_t inc) {
uint32_t newThreshold = threshold() + inc;
threshold_ = (newThreshold > JitOptions.normalIonWarmUpThreshold)
? JitOptions.normalIonWarmUpThreshold
: newThreshold;
bool hasSpaceForMonomorphicInlineEntry() {
return monomorphicInlineOffsets.length() < MonomorphicInlineMaxEntries;
bool hasMonomorphicInlineOffset(uint32_t offset) {
for (uint32_t iterOffset : monomorphicInlineOffsets) {
if (iterOffset == offset) {
return true;
return false;
bool addMonomorphicInlineOffset(uint32_t newOffset) {
if (hasMonomorphicInlineOffset(newOffset)) {
return true;
return monomorphicInlineOffsets.append(newOffset);
ScriptKey key() {
MOZ_ASSERT(key_ != 0, "Should have valid key.");
return key_;
using ScriptToHintMap =
HashMap<ScriptKey, IonHint*, js::DefaultHasher<ScriptKey>,
using IonHintPriorityQueue = mozilla::LinkedList<IonHint>;
static constexpr uint32_t InvalidationThresholdIncrement = 500;
static constexpr uint32_t IonHintMaxEntries = 5000;
static constexpr uint32_t MonomorphicInlineMaxEntries = 16;
static uint32_t IonHintEagerThresholdValue(uint32_t lastStubCounter) {
// Use the counter when the last IC stub was attached but add 10
// for some wiggle room and to safeguard against cases where the
// lastStubCounter is 0.
uint32_t eagerThreshold = lastStubCounter + 10;
// Do not exceed the default Ion threshold value set in the options.
return std::min(eagerThreshold, JitOptions.normalIonWarmUpThreshold);
ScriptToHintMap ionHintMap_;
IonHintPriorityQueue ionHintQueue_;
/* Baseline Hints
* --------------------------------------------------------------------------
* This implementation uses a BitBloomFilter to track whether or not a script
* has been baseline compiled before in the same process. This can occur
* frequently during navigations.
* The bloom filter allows us to have very efficient storage and lookup costs,
* at the expense of occasional false positives. Using a bloom filter also
* allows us to have many more entries at minimal memory and allocation cost.
* The number of entries added to the bloom filter is monitored in order to
* try and keep the false positivity rate below 1%. If the entry count
* exceeds MaxEntries_, which indicates the false positivity rate may exceed
* 1.5%, then the filter is completely cleared to reset the cache.
static constexpr uint32_t EagerBaselineCacheSize_ = 16;
mozilla::BitBloomFilter<EagerBaselineCacheSize_, ScriptKey> baselineHintMap_;
* MaxEntries_ is the approximate entry count for which the
* false positivity rate will exceed p=0.015 using k=2 and m=2**CacheSize.
* Formula is as follows:
* MaxEntries_ = floor(m / (-k / ln(1-exp(ln(p) / k))))
static constexpr uint32_t MaxEntries_ = 4281;
static_assert(EagerBaselineCacheSize_ == 16 && MaxEntries_ == 4281,
"MaxEntries should be recalculated for given CacheSize.");
uint32_t baselineEntryCount_ = 0;
void incrementBaselineEntryCount();
void updateAsRecentlyUsed(IonHint* hint);
IonHint* addIonHint(ScriptKey key, ScriptToHintMap::AddPtr& p);
void setEagerBaselineHint(JSScript* script);
bool mightHaveEagerBaselineHint(JSScript* script) const;
bool recordIonCompilation(JSScript* script);
bool getIonThresholdHint(JSScript* script, uint32_t& thresholdOut);
bool addMonomorphicInlineLocation(JSScript* script, BytecodeLocation loc);
bool hasMonomorphicInlineHintAtOffset(JSScript* script, uint32_t offset);
void recordInvalidation(JSScript* script);
} // namespace js::jit
#endif /* jit_JitHints_h */