gecko-dev/js/public/ProfilingStack.h

/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 4 -*-
 * vim: set ts=8 sts=4 et sw=4 tw=99:
 * 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 js_ProfilingStack_h
#define js_ProfilingStack_h

#include <algorithm>
#include <stdint.h>

#include "jstypes.h"

#include "js/TypeDecls.h"
#include "js/Utility.h"

#ifdef JS_BROKEN_GCC_ATTRIBUTE_WARNING
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wattributes"
#endif // JS_BROKEN_GCC_ATTRIBUTE_WARNING

class JS_PUBLIC_API(JSTracer);

#ifdef JS_BROKEN_GCC_ATTRIBUTE_WARNING
#pragma GCC diagnostic pop
#endif // JS_BROKEN_GCC_ATTRIBUTE_WARNING

class ProfilingStack;

// 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 js {

// 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.

    // WARNING WARNING WARNING
    //
    // 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.
    mozilla::Atomic<const char*, mozilla::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.
    mozilla::Atomic<const char*, mozilla::ReleaseAcquire> dynamicString_;

    // Stack pointer for non-JS stack frames, the script pointer otherwise.
    mozilla::Atomic<void*, mozilla::ReleaseAcquire> spOrScript;

    // Line number for non-JS stack frames, the bytecode offset otherwise.
    mozilla::Atomic<int32_t, mozilla::ReleaseAcquire> lineOrPcOffset;

    // Bits 0...1 hold the Kind. Bits 2...3 are unused. Bits 4...12 hold the
    // Category.
    mozilla::Atomic<uint32_t, mozilla::ReleaseAcquire> kindAndCategory_;

    static int32_t pcToOffset(JSScript* aScript, jsbytecode* aPc);

  public:
    ProfilingStackFrame() = default;
    ProfilingStackFrame& operator=(const ProfilingStackFrame& other)
    {
        label_ = other.label();
        dynamicString_ = other.dynamicString();
        void* spScript = other.spOrScript;
        spOrScript = spScript;
        int32_t offset = other.lineOrPcOffset;
        lineOrPcOffset = offset;
        uint32_t kindAndCategory = other.kindAndCategory_;
        kindAndCategory_ = kindAndCategory;
        return *this;
    }

    enum class Kind : uint32_t {
        // A regular label frame. These usually come from AutoProfilerLabel.
        LABEL = 0,

        // A special frame indicating the start of a run of JS profiling stack
        // frames. SP_MARKER 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".
        SP_MARKER = 1,

        // A normal JS frame.
        JS_NORMAL = 2,

        // An interpreter JS frame that has OSR-ed into baseline. JS_NORMAL
        // frames can be converted to JS_OSR and back. JS_OSR frames are
        // ignored.
        JS_OSR = 3,

        KIND_MASK = 0x3,
    };

    // Keep these in sync with devtools/client/performance/modules/categories.js
    enum class Category : uint32_t {
        OTHER    = 1u << 4,
        CSS      = 1u << 5,
        JS       = 1u << 6,
        GC       = 1u << 7,
        CC       = 1u << 8,
        NETWORK  = 1u << 9,
        GRAPHICS = 1u << 10,
        STORAGE  = 1u << 11,
        EVENTS   = 1u << 12,

        FIRST    = OTHER,
        LAST     = EVENTS,

        CATEGORY_MASK = ~uint32_t(Kind::KIND_MASK),
    };

    static_assert((uint32_t(Category::FIRST) & uint32_t(Kind::KIND_MASK)) == 0,
                  "Category overlaps with Kind");

    bool isLabelFrame() const
    {
        return kind() == Kind::LABEL;
    }

    bool isSpMarkerFrame() const
    {
        return kind() == Kind::SP_MARKER;
    }

    bool isJsFrame() const
    {
        Kind k = kind();
        return k == Kind::JS_NORMAL || k == Kind::JS_OSR;
    }

    void setLabel(const char* aLabel) { label_ = aLabel; }
    const char* label() const { return label_; }

    const char* dynamicString() const { return dynamicString_; }

    void initLabelFrame(const char* aLabel, const char* aDynamicString, void* sp,
                        uint32_t aLine, Category aCategory)
    {
        label_ = aLabel;
        dynamicString_ = aDynamicString;
        spOrScript = sp;
        lineOrPcOffset = static_cast<int32_t>(aLine);
        kindAndCategory_ = uint32_t(Kind::LABEL) | uint32_t(aCategory);
        MOZ_ASSERT(isLabelFrame());
    }

    void initSpMarkerFrame(void* sp)
    {
        label_ = "";
        dynamicString_ = nullptr;
        spOrScript = sp;
        lineOrPcOffset = 0;
        kindAndCategory_ = uint32_t(Kind::SP_MARKER) | uint32_t(ProfilingStackFrame::Category::OTHER);
        MOZ_ASSERT(isSpMarkerFrame());
    }

    void initJsFrame(const char* aLabel, const char* aDynamicString, JSScript* aScript,
                     jsbytecode* aPc)
    {
        label_ = aLabel;
        dynamicString_ = aDynamicString;
        spOrScript = aScript;
        lineOrPcOffset = pcToOffset(aScript, aPc);
        kindAndCategory_ = uint32_t(Kind::JS_NORMAL) | uint32_t(Category::JS);
        MOZ_ASSERT(isJsFrame());
    }

    void setKind(Kind aKind) {
        kindAndCategory_ = uint32_t(aKind) | uint32_t(category());
    }

    Kind kind() const {
        return Kind(kindAndCategory_ & uint32_t(Kind::KIND_MASK));
    }

    Category category() const {
        return Category(kindAndCategory_ & uint32_t(Category::CATEGORY_MASK));
    }

    void* stackAddress() const {
        MOZ_ASSERT(!isJsFrame());
        return spOrScript;
    }

    JS_PUBLIC_API(JSScript*) script() const;

    uint32_t line() const {
        MOZ_ASSERT(!isJsFrame());
        return static_cast<uint32_t>(lineOrPcOffset);
    }

    // Note that the pointer returned might be invalid.
    JSScript* rawScript() const {
        MOZ_ASSERT(isJsFrame());
        void* script = spOrScript;
        return static_cast<JSScript*>(script);
    }

    // We can't know the layout of JSScript, so look in vm/GeckoProfiler.cpp.
    JS_FRIEND_API(jsbytecode*) pc() const;
    void setPC(jsbytecode* pc);

    void trace(JSTracer* trc);

    // 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;
};

JS_FRIEND_API(void)
SetContextProfilingStack(JSContext* cx, ProfilingStack* profilingStack);

// GetContextProfilingStack also exists, but it's defined in RootingAPI.h.

JS_FRIEND_API(void)
EnableContextProfilingStack(JSContext* cx, bool enabled);

JS_FRIEND_API(void)
RegisterContextProfilingEventMarker(JSContext* cx, void (*fn)(const char*));

} // namespace js

// 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
{
  public:
    ProfilingStack()
      : stackPointer(0)
    {}

    ~ProfilingStack();

    void pushLabelFrame(const char* label, const char* dynamicString, void* sp,
                        uint32_t line, js::ProfilingStackFrame::Category category) {
        uint32_t oldStackPointer = stackPointer;

        if (MOZ_LIKELY(capacity > oldStackPointer) || MOZ_LIKELY(ensureCapacitySlow()))
            frames[oldStackPointer].initLabelFrame(label, dynamicString, sp, line, category);

        // 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.
        // This thread is the only one that ever changes the value of
        // stackPointer.
        stackPointer = oldStackPointer + 1;
    }

    void pushSpMarkerFrame(void* sp) {
        uint32_t oldStackPointer = stackPointer;

        if (MOZ_LIKELY(capacity > oldStackPointer) || MOZ_LIKELY(ensureCapacitySlow()))
            frames[oldStackPointer].initSpMarkerFrame(sp);

        // This must happen at the end, see the comment in pushLabelFrame.
        stackPointer = oldStackPointer + 1;
    }

    void pushJsFrame(const char* label, const char* dynamicString, JSScript* script,
                     jsbytecode* pc) {
        uint32_t oldStackPointer = stackPointer;

        if (MOZ_LIKELY(capacity > oldStackPointer) || MOZ_LIKELY(ensureCapacitySlow()))
            frames[oldStackPointer].initJsFrame(label, dynamicString, script, pc);

        // This must happen at the end, see the comment in pushLabelFrame.
        stackPointer = oldStackPointer + 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 std::min(uint32_t(stackPointer), stackCapacity()); }
    uint32_t stackCapacity() const { return capacity; }

  private:
    // Out of line path for expanding the buffer, since otherwise this would get inlined in every
    // DOM WebIDL call.
    MOZ_COLD MOZ_MUST_USE bool 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;

  public:

    // 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.
    mozilla::Atomic<js::ProfilingStackFrame*> 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).
    //
    // WARNING WARNING WARNING
    //
    // This is an atomic variable that uses ReleaseAcquire memory ordering.
    // See the "Concurrency considerations" paragraph at the top of this file
    // for more details.
    mozilla::Atomic<uint32_t, mozilla::ReleaseAcquire> stackPointer;
};

namespace js {

class AutoGeckoProfilerEntry;
class GeckoProfilerEntryMarker;
class GeckoProfilerBaselineOSRMarker;

class GeckoProfilerThread
{
    friend class AutoGeckoProfilerEntry;
    friend class GeckoProfilerEntryMarker;
    friend class GeckoProfilerBaselineOSRMarker;

    ProfilingStack*         profilingStack_;

  public:
    GeckoProfilerThread();

    uint32_t stackPointer() { MOZ_ASSERT(installed()); return profilingStack_->stackPointer; }
    ProfilingStackFrame* stack() { return profilingStack_->frames; }
    ProfilingStack* getProfilingStack() { return profilingStack_; }

    /* management of whether instrumentation is on or off */
    bool installed() { return profilingStack_ != nullptr; }

    void setProfilingStack(ProfilingStack* profilingStack);
    void trace(JSTracer* trc);

    /*
     * Functions which are the actual instrumentation to track run information
     *
     *   - enter: a function has started to execute
     *   - updatePC: updates the pc information about where a function
     *               is currently executing
     *   - exit: this function has ceased execution, and no further
     *           entries/exits will be made
     */
    bool enter(JSContext* cx, JSScript* script, JSFunction* maybeFun);
    void exit(JSScript* script, JSFunction* maybeFun);
    inline void updatePC(JSContext* cx, JSScript* script, jsbytecode* pc);
};

} // namespace js

#endif  /* js_ProfilingStack_h */