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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 BaseProfilingStack_h
#define BaseProfilingStack_h
# error Do not #include this header when MOZ_GECKO_PROFILER is not #defined.
#include "BaseProfilingCategory.h"
#include "mozilla/Assertions.h"
#include "mozilla/Atomics.h"
#include <stdint.h>
// This file defines the classes ProfilingStack and ProfilingStackFrame.
// The ProfilingStack manages an array of ProfilingStackFrames.
// It keeps track of the "label stack" and the JS interpreter stack.
// The two stack types are interleaved.
// Usage:
// ProfilingStack* profilingStack = ...;
// // For label frames:
// profilingStack->pushLabelFrame(...);
// // Execute some code. When finished, pop the frame:
// profilingStack->pop();
// // For JS stack frames:
// profilingStack->pushJSFrame(...);
// // Execute some code. When finished, pop the frame:
// profilingStack->pop();
// Concurrency considerations
// A thread's profiling stack (and the frames inside it) is only modified by
// that thread. However, the profiling stack can be *read* by a different
// thread, the sampler thread: Whenever the profiler wants to sample a given
// thread A, the following happens:
// (1) Thread A is suspended.
// (2) The sampler thread (thread S) reads the ProfilingStack of thread A,
// including all ProfilingStackFrames that are currently in that stack
// (profilingStack->frames[0..profilingStack->stackSize()]).
// (3) Thread A is resumed.
// Thread suspension is achieved using platform-specific APIs; refer to each
// platform's Sampler::SuspendAndSampleAndResumeThread implementation in
// platform-*.cpp for details.
// When the thread is suspended, the values in profilingStack->stackPointer and
// in the stack frame range
// profilingStack->frames[0..profilingStack->stackPointer] need to be in a
// consistent state, so that thread S does not read partially- constructed stack
// frames. More specifically, we have two requirements:
// (1) When adding a new frame at the top of the stack, its ProfilingStackFrame
// data needs to be put in place *before* the stackPointer is incremented,
// and the compiler + CPU need to know that this order matters.
// (2) When popping an frame from the stack and then preparing the
// ProfilingStackFrame data for the next frame that is about to be pushed,
// the decrement of the stackPointer in pop() needs to happen *before* the
// ProfilingStackFrame for the new frame is being popuplated, and the
// compiler + CPU need to know that this order matters.
// We can express the relevance of these orderings in multiple ways.
// Option A is to make stackPointer an atomic with SequentiallyConsistent
// memory ordering. This would ensure that no writes in thread A would be
// reordered across any writes to stackPointer, which satisfies requirements
// (1) and (2) at the same time. Option A is the simplest.
// Option B is to use ReleaseAcquire memory ordering both for writes to
// stackPointer *and* for writes to ProfilingStackFrame fields. Release-stores
// ensure that all writes that happened *before this write in program order* are
// not reordered to happen after this write. ReleaseAcquire ordering places no
// requirements on the ordering of writes that happen *after* this write in
// program order.
// Using release-stores for writes to stackPointer expresses requirement (1),
// and using release-stores for writes to the ProfilingStackFrame fields
// expresses requirement (2).
// Option B is more complicated than option A, but has much better performance
// on x86/64: In a microbenchmark run on a Macbook Pro from 2017, switching
// from option A to option B reduced the overhead of pushing+popping a
// ProfilingStackFrame by 10 nanoseconds.
// On x86/64, release-stores require no explicit hardware barriers or lock
// instructions.
// On ARM/64, option B may be slower than option A, because the compiler will
// generate hardware barriers for every single release-store instead of just
// for the writes to stackPointer. However, the actual performance impact of
// this has not yet been measured on ARM, so we're currently using option B
// everywhere. This is something that we may want to change in the future once
// we've done measurements.
namespace mozilla {
namespace baseprofiler {
// A call stack can be specified to the JS engine such that all JS entry/exits
// to functions push/pop a stack frame to/from the specified stack.
// For more detailed information, see vm/GeckoProfiler.h.
class ProfilingStackFrame {
// A ProfilingStackFrame represents either a label frame or a JS frame.
// All the fields below are Atomic<...,ReleaseAcquire>. This is needed so
// that writes to these fields are release-writes, which ensures that
// earlier writes in this thread don't get reordered after the writes to
// these fields. In particular, the decrement of the stack pointer in
// ProfilingStack::pop() is a write that *must* happen before the values in
// this ProfilingStackFrame are changed. Otherwise, the sampler thread might
// see an inconsistent state where the stack pointer still points to a
// ProfilingStackFrame which has already been popped off the stack and whose
// fields have now been partially repopulated with new values.
// See the "Concurrency considerations" paragraph at the top of this file
// for more details.
// Descriptive label for this stack frame. Must be a static string! Can be
// an empty string, but not a null pointer.
Atomic<const char*, ReleaseAcquire> label_;
// An additional descriptive string of this frame which is combined with
// |label_| in profiler output. Need not be (and usually isn't) static. Can
// be null.
Atomic<const char*, ReleaseAcquire> dynamicString_;
// Stack pointer for non-JS stack frames, the script pointer otherwise.
Atomic<void*, ReleaseAcquire> spOrScript;
// ID of the JS Realm for JS stack frames.
// Must not be used on non-JS frames; it'll contain either the default 0,
// or a leftover value from a previous JS stack frame that was using this
// ProfilingStackFrame object.
mozilla::Atomic<uint64_t, mozilla::ReleaseAcquire> realmID_;
// The bytecode offset for JS stack frames.
// Must not be used on non-JS frames; it'll contain either the default 0,
// or a leftover value from a previous JS stack frame that was using this
// ProfilingStackFrame object.
Atomic<int32_t, ReleaseAcquire> pcOffsetIfJS_;
// Bits 0...8 hold the Flags. Bits 9...31 hold the category pair.
Atomic<uint32_t, ReleaseAcquire> flagsAndCategoryPair_;
ProfilingStackFrame() = default;
ProfilingStackFrame& operator=(const ProfilingStackFrame& other) {
label_ = other.label();
dynamicString_ = other.dynamicString();
void* spScript = other.spOrScript;
spOrScript = spScript;
int32_t offsetIfJS = other.pcOffsetIfJS_;
pcOffsetIfJS_ = offsetIfJS;
int64_t realmID = other.realmID_;
realmID_ = realmID;
uint32_t flagsAndCategory = other.flagsAndCategoryPair_;
flagsAndCategoryPair_ = flagsAndCategory;
return *this;
// Reserve up to 16 bits for flags, and 16 for category pair.
enum class Flags : uint32_t {
// The first three flags describe the kind of the frame and are
// mutually exclusive. (We still give them individual bits for
// simplicity.)
// A regular label frame. These usually come from AutoProfilerLabel.
IS_LABEL_FRAME = 1 << 0,
// A special frame indicating the start of a run of JS profiling stack
// frames. IS_SP_MARKER_FRAME frames are ignored, except for the sp
// field. These frames are needed to get correct ordering between JS
// and LABEL frames because JS frames don't carry sp information.
// SP is short for "stack pointer".
// A JS frame.
IS_JS_FRAME = 1 << 2,
// An interpreter JS frame that has OSR-ed into baseline. IS_JS_FRAME
// frames can have this flag set and unset during their lifetime.
// JS_OSR frames are ignored.
JS_OSR = 1 << 3,
// The next three are mutually exclusive.
// By default, for profiling stack frames that have both a label and a
// dynamic string, the two strings are combined into one string of the
// form "<label> <dynamicString>" during JSON serialization. The
// following flags can be used to change this preset.
STRING_TEMPLATE_METHOD = 1 << 4, // "<label>.<dynamicString>"
STRING_TEMPLATE_GETTER = 1 << 5, // "get <label>.<dynamicString>"
STRING_TEMPLATE_SETTER = 1 << 6, // "set <label>.<dynamicString>"
// If set, causes this stack frame to be marked as "relevantForJS" in
// the profile JSON, which will make it show up in the "JS only" call
// tree view.
// If set, causes the label on this ProfilingStackFrame to be ignored
// and to be replaced by the subcategory's label.
// Frame dynamic string does not contain user data.
// A JS Baseline Interpreter frame.
uint32_t(ProfilingCategoryPair::LAST) <=
(UINT32_MAX >> uint32_t(Flags::FLAGS_BITCOUNT)),
"Too many category pairs to fit into u32 with together with the "
"reserved bits for the flags");
bool isLabelFrame() const {
return uint32_t(flagsAndCategoryPair_) & uint32_t(Flags::IS_LABEL_FRAME);
bool isSpMarkerFrame() const {
return uint32_t(flagsAndCategoryPair_) &
bool isJsFrame() const {
return uint32_t(flagsAndCategoryPair_) & uint32_t(Flags::IS_JS_FRAME);
bool isOSRFrame() const {
return uint32_t(flagsAndCategoryPair_) & uint32_t(Flags::JS_OSR);
void setIsOSRFrame(bool isOSR) {
if (isOSR) {
flagsAndCategoryPair_ =
uint32_t(flagsAndCategoryPair_) | uint32_t(Flags::JS_OSR);
} else {
flagsAndCategoryPair_ =
uint32_t(flagsAndCategoryPair_) & ~uint32_t(Flags::JS_OSR);
const char* label() const {
uint32_t flagsAndCategoryPair = flagsAndCategoryPair_;
if (flagsAndCategoryPair &
auto categoryPair = ProfilingCategoryPair(
flagsAndCategoryPair >> uint32_t(Flags::FLAGS_BITCOUNT));
return GetProfilingCategoryPairInfo(categoryPair).mLabel;
return label_;
const char* dynamicString() const { return dynamicString_; }
void initLabelFrame(const char* aLabel, const char* aDynamicString, void* sp,
ProfilingCategoryPair aCategoryPair, uint32_t aFlags) {
label_ = aLabel;
dynamicString_ = aDynamicString;
spOrScript = sp;
// pcOffsetIfJS_ is not set and must not be used on label frames.
flagsAndCategoryPair_ =
uint32_t(Flags::IS_LABEL_FRAME) |
(uint32_t(aCategoryPair) << uint32_t(Flags::FLAGS_BITCOUNT)) | aFlags;
void initSpMarkerFrame(void* sp) {
label_ = "";
dynamicString_ = nullptr;
spOrScript = sp;
// pcOffsetIfJS_ is not set and must not be used on sp marker frames.
flagsAndCategoryPair_ = uint32_t(Flags::IS_SP_MARKER_FRAME) |
<< uint32_t(Flags::FLAGS_BITCOUNT));
void initJsFrame(const char* aLabel, const char* aDynamicString,
void* /* JSScript* */ aScript, int32_t aOffset,
uint64_t aRealmID) {
label_ = aLabel;
dynamicString_ = aDynamicString;
spOrScript = aScript;
pcOffsetIfJS_ = aOffset;
realmID_ = aRealmID;
flagsAndCategoryPair_ =
uint32_t(Flags::IS_JS_FRAME) | (uint32_t(ProfilingCategoryPair::JS)
<< uint32_t(Flags::FLAGS_BITCOUNT));
uint32_t flags() const {
return uint32_t(flagsAndCategoryPair_) & uint32_t(Flags::FLAGS_MASK);
ProfilingCategoryPair categoryPair() const {
return ProfilingCategoryPair(flagsAndCategoryPair_ >>
uint64_t realmID() const { return realmID_; }
void* stackAddress() const {
return spOrScript;
// Note that the pointer returned might be invalid.
void* rawScript() const {
return spOrScript;
void setRawScript(void* aScript) {
spOrScript = aScript;
int32_t pcOffset() const {
return pcOffsetIfJS_;
void setPCOffset(int32_t aOffset) {
pcOffsetIfJS_ = aOffset;
// The offset of a pc into a script's code can actually be 0, so to
// signify a nullptr pc, use a -1 index. This is checked against in
// pc() and setPC() to set/get the right pc.
static const int32_t NullPCOffset = -1;
// Each thread has its own ProfilingStack. That thread modifies the
// ProfilingStack, pushing and popping elements as necessary.
// The ProfilingStack is also read periodically by the profiler's sampler
// thread. This happens only when the thread that owns the ProfilingStack is
// suspended. So there are no genuine parallel accesses.
// However, it is possible for pushing/popping to be interrupted by a periodic
// sample. Because of this, we need pushing/popping to be effectively atomic.
// - When pushing a new frame, we increment the stack pointer -- making the new
// frame visible to the sampler thread -- only after the new frame has been
// fully written. The stack pointer is Atomic<uint32_t,ReleaseAcquire>, so
// the increment is a release-store, which ensures that this store is not
// reordered before the writes of the frame.
// - When popping an old frame, the only operation is the decrementing of the
// stack pointer, which is obviously atomic.
class ProfilingStack final {
ProfilingStack() = default;
MFBT_API ~ProfilingStack();
void pushLabelFrame(const char* label, const char* dynamicString, void* sp,
ProfilingCategoryPair categoryPair, uint32_t flags = 0) {
// This thread is the only one that ever changes the value of
// stackPointer.
// Store the value of the atomic in a non-atomic local variable so that
// the compiler won't generate two separate loads from the atomic for
// the size check and the frames[] array indexing operation.
uint32_t stackPointerVal = stackPointer;
if (MOZ_UNLIKELY(stackPointerVal >= capacity)) {
frames[stackPointerVal].initLabelFrame(label, dynamicString, sp,
categoryPair, flags);
// This must happen at the end! The compiler will not reorder this
// update because stackPointer is Atomic<..., ReleaseAcquire>, so any
// the writes above will not be reordered below the stackPointer store.
// Do the read and the write as two separate statements, in order to
// make it clear that we don't need an atomic increment, which would be
// more expensive on x86 than the separate operations done here.
// However, don't use stackPointerVal here; instead, allow the compiler
// to turn this store into a non-atomic increment instruction which
// takes up less code size.
stackPointer = stackPointer + 1;
void pushSpMarkerFrame(void* sp) {
uint32_t oldStackPointer = stackPointer;
if (MOZ_UNLIKELY(oldStackPointer >= capacity)) {
// This must happen at the end, see the comment in pushLabelFrame.
stackPointer = oldStackPointer + 1;
void pushJsOffsetFrame(const char* label, const char* dynamicString,
void* script, int32_t offset, uint64_t aRealmID) {
// This thread is the only one that ever changes the value of
// stackPointer. Only load the atomic once.
uint32_t oldStackPointer = stackPointer;
if (MOZ_UNLIKELY(oldStackPointer >= capacity)) {
frames[oldStackPointer].initJsFrame(label, dynamicString, script, offset,
// This must happen at the end, see the comment in pushLabelFrame.
stackPointer = stackPointer + 1;
void pop() {
MOZ_ASSERT(stackPointer > 0);
// Do the read and the write as two separate statements, in order to
// make it clear that we don't need an atomic decrement, which would be
// more expensive on x86 than the separate operations done here.
// This thread is the only one that ever changes the value of
// stackPointer.
uint32_t oldStackPointer = stackPointer;
stackPointer = oldStackPointer - 1;
uint32_t stackSize() const { return stackPointer; }
uint32_t stackCapacity() const { return capacity; }
// Out of line path for expanding the buffer, since otherwise this would get
// inlined in every DOM WebIDL call.
MFBT_API MOZ_COLD void ensureCapacitySlow();
// No copying.
ProfilingStack(const ProfilingStack&) = delete;
void operator=(const ProfilingStack&) = delete;
// No moving either.
ProfilingStack(ProfilingStack&&) = delete;
void operator=(ProfilingStack&&) = delete;
uint32_t capacity = 0;
// The pointer to the stack frames, this is read from the profiler thread and
// written from the current thread.
// This is effectively a unique pointer.
Atomic<ProfilingStackFrame*, SequentiallyConsistent> frames{nullptr};
// This may exceed the capacity, so instead use the stackSize() method to
// determine the number of valid frames in stackFrames. When this is less
// than stackCapacity(), it refers to the first free stackframe past the top
// of the in-use stack (i.e. frames[stackPointer - 1] is the top stack
// frame).
// This is an atomic variable that uses ReleaseAcquire memory ordering.
// See the "Concurrency considerations" paragraph at the top of this file
// for more details.
Atomic<uint32_t, ReleaseAcquire> stackPointer{0};
class AutoGeckoProfilerEntry;
class GeckoProfilerEntryMarker;
class GeckoProfilerBaselineOSRMarker;
class GeckoProfilerThread {
friend class AutoGeckoProfilerEntry;
friend class GeckoProfilerEntryMarker;
friend class GeckoProfilerBaselineOSRMarker;
ProfilingStack* profilingStack_;
// Same as profilingStack_ if the profiler is currently active, otherwise
// null.
ProfilingStack* profilingStackIfEnabled_;
MFBT_API GeckoProfilerThread();
uint32_t stackPointer() {
return profilingStack_->stackPointer;
ProfilingStackFrame* stack() { return profilingStack_->frames; }
ProfilingStack* getProfilingStack() { return profilingStack_; }
ProfilingStack* getProfilingStackIfEnabled() {
return profilingStackIfEnabled_;
* True if the profiler infrastructure is setup. Should be true in builds
* that include profiler support except during early startup or late
* shutdown. Unrelated to the presence of the Gecko Profiler addon.
bool infraInstalled() { return profilingStack_ != nullptr; }
MFBT_API void setProfilingStack(ProfilingStack* profilingStack, bool enabled);
void enable(bool enable) {
profilingStackIfEnabled_ = enable ? profilingStack_ : nullptr;
} // namespace baseprofiler
} // namespace mozilla
#endif /* BaseProfilingStack_h */