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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
#ifndef vm_Activation_h
#define vm_Activation_h
#include "mozilla/Assertions.h" // MOZ_ASSERT
#include "mozilla/Attributes.h" // MOZ_RAII
#include <stddef.h> // size_t
#include <stdint.h> // uint8_t, uint32_t
#include "jstypes.h" // JS_PUBLIC_API
#include "jit/CalleeToken.h" // js::jit::CalleeToken
#include "js/RootingAPI.h" // JS::Handle, JS::Rooted
#include "js/TypeDecls.h" // jsbytecode
#include "js/Value.h" // JS::Value
#include "vm/SavedFrame.h" // js::SavedFrame
#include "vm/Stack.h" // js::InterpreterRegs
struct JS_PUBLIC_API JSContext;
class JSFunction;
class JSObject;
class JSScript;
namespace JS {
class CallArgs;
class JS_PUBLIC_API Compartment;
} // namespace JS
namespace js {
class InterpreterActivation;
namespace jit {
class JitActivation;
} // namespace jit
// [SMDOC] LiveSavedFrameCache: SavedFrame caching to minimize stack walking
//
// Since each SavedFrame object includes a 'parent' pointer to the SavedFrame
// for its caller, if we could easily find the right SavedFrame for a given
// stack frame, we wouldn't need to walk the rest of the stack. Traversing deep
// stacks can be expensive, and when we're profiling or instrumenting code, we
// may want to capture JavaScript stacks frequently, so such cases would benefit
// if we could avoid walking the entire stack.
//
// We could have a cache mapping frame addresses to their SavedFrame objects,
// but invalidating its entries would be a challenge. Popping a stack frame is
// extremely performance-sensitive, and SpiderMonkey stack frames can be OSR'd,
// thrown, rematerialized, and perhaps meet other fates; we would rather our
// cache not depend on handling so many tricky cases.
//
// It turns out that we can keep the cache accurate by reserving a single bit in
// the stack frame, which must be clear on any newly pushed frame. When we
// insert an entry into the cache mapping a given frame address to its
// SavedFrame, we set the bit in the frame. Then, we take care to probe the
// cache only for frames whose bit is set; the bit tells us that the frame has
// never left the stack, so its cache entry must be accurate, at least about
// which function the frame is executing (the line may have changed; more about
// that below). The code refers to this bit as the 'hasCachedSavedFrame' flag.
//
// We could manage such a cache replacing least-recently used entries, but we
// can do better than that: the cache can be a stack, of which we need examine
// only entries from the top.
//
// First, observe that stacks are walked from the youngest frame to the oldest,
// but SavedFrame chains are built from oldest to youngest, to ensure common
// tails are shared. This means that capturing a stack is necessarily a
// two-phase process: walk the stack, and then build the SavedFrames.
//
// Naturally, the first time we capture the stack, the cache is empty, and we
// must traverse the entire stack. As we build each SavedFrame, we push an entry
// associating the frame's address to its SavedFrame on the cache, and set the
// frame's bit. At the end, every frame has its bit set and an entry in the
// cache.
//
// Then the program runs some more. Some, none, or all of the frames are popped.
// Any new frames are pushed with their bit clear. Any frame with its bit set
// has never left the stack. The cache is left untouched.
//
// For the next capture, we walk the stack up to the first frame with its bit
// set, if there is one. Call it F; it must have a cache entry. We pop entries
// from the cache - all invalid, because they are above F's entry, and hence
// younger - until we find the entry matching F's address. Since F's bit is set,
// we know it never left the stack, and hence that no younger frame could have
// had a colliding address. And since the frame's bit was set when we pushed the
// cache entry, we know the entry is still valid.
//
// F's cache entry's SavedFrame covers the rest of the stack, so we don't need
// to walk the stack any further. Now we begin building SavedFrame objects for
// the new frames, pushing cache entries, and setting bits on the frames. By the
// end, the cache again covers the full stack, and every frame's bit is set.
//
// If we walk the stack to the end, and find no frame with its bit set, then the
// entire cache is invalid. At this point, it must be emptied, so that the new
// entries we are about to push are the only frames in the cache.
//
// For example, suppose we have the following stack (let 'A > B' mean "A called
// B", so the frames are listed oldest first):
//
// P > Q > R > S Initial stack, bits not set.
// P* > Q* > R* > S* Capture a SavedFrame stack, set bits.
// The cache now holds: P > Q > R > S.
// P* > Q* > R* Return from S.
// P* > Q* Return from R.
// P* > Q* > T > U Call T and U. New frames have clear bits.
//
// If we capture the stack now, the cache still holds:
//
// P > Q > R > S
//
// As we traverse the stack, we'll cross U and T, and then find Q with its bit
// set. We pop entries from the cache until we find the entry for Q; this
// removes entries R and S, which were indeed invalid. In Q's cache entry, we
// find the SavedFrame representing the stack P > Q. Now we build SavedFrames
// for the new portion of the stack, pushing an entry for T and setting the bit
// on the frame, and then doing the same for U. In the end, the call stack again
// has bits set on all its frames:
//
// P* > Q* > T* > U* All frames are now in the cache.
//
// And the cache again holds entries for the entire stack:
//
// P > Q > T > U
//
// Details:
//
// - When we find a cache entry whose frame address matches our frame F, we know
// that F has never left the stack, but it may certainly be the case that
// execution took place in that frame, and that the current source position
// within F's function has changed. This means that the entry's SavedFrame,
// which records the source line and column as well as the function, is not
// correct. To detect this case, when we push a cache entry, we record the
// frame's pc. When consulting the cache, if a frame's address matches but its
// pc does not, then we pop the cache entry, clear the frame's bit, and
// continue walking the stack. The next stack frame will definitely hit: since
// its callee frame never left the stack, the calling frame never got the
// chance to execute.
//
// - Generators, at least conceptually, have long-lived stack frames that
// disappear from the stack when the generator yields, and reappear on the
// stack when the generator's 'next' method is called. When a generator's
// frame is placed again atop the stack, its bit must be cleared - for the
// purposes of the cache, treating the frame as a new frame - to respect the
// invariants we used to justify the algorithm above. Async function
// activations usually appear atop empty stacks, since they are invoked as a
// promise callback, but the same rule applies.
//
// - SpiderMonkey has many types of stack frames, and not all have a place to
// store a bit indicating a cached SavedFrame. But as long as we don't create
// cache entries for frames we can't mark, simply omitting them from the cache
// is harmless. Uncacheable frame types include inlined Ion frames and
// non-Debug wasm frames. The LiveSavedFrameCache::FramePtr type represents
// only pointers to frames that can be cached, so if you have a FramePtr, you
// don't need to further check the frame for cachability. FramePtr provides
// access to the hasCachedSavedFrame bit.
//
// - We actually break up the cache into one cache per Activation. Popping an
// activation invalidates all its cache entries, simply by freeing the cache
// altogether.
//
// - The entire chain of SavedFrames for a given stack capture is created in the
// compartment of the code that requested the capture, *not* in that of the
// frames it represents, so in general, different compartments may have
// different SavedFrame objects representing the same actual stack frame. The
// LiveSavedFrameCache simply records whichever SavedFrames were used in the
// most recent captures. When we find a cache hit, we check the entry's
// SavedFrame's compartment against the current compartment; if they do not
// match, we clear the entire cache.
//
// This means that it is not always true that, if a frame's
// hasCachedSavedFrame bit is set, it must have an entry in the cache. The
// actual invariant is: either the cache is completely empty, or the frames'
// bits are trustworthy. This invariant holds even though capture can be
// interrupted at many places by OOM failures. Clearing the cache is a single,
// uninterruptible step. When we try to look up a frame whose bit is set and
// find an empty cache, we clear the frame's bit. And we only add the first
// frame to an empty cache once we've walked the stack all the way, so we know
// that all frames' bits are cleared by that point.
//
// - When the Debugger API evaluates an expression in some frame (the 'target
// frame'), it's SpiderMonkey's convention that the target frame be treated as
// the parent of the eval frame. In reality, of course, the eval frame is
// pushed on the top of the stack like any other frame, but stack captures
// simply jump straight over the intervening frames, so that the '.parent'
// property of a SavedFrame for the eval is the SavedFrame for the target.
// This is arranged by giving the eval frame an 'evalInFramePrev` link
// pointing to the target, which an ordinary FrameIter will notice and
// respect.
//
// If the LiveSavedFrameCache were presented with stack traversals that
// skipped frames in this way, it would cause havoc. First, with no debugger
// eval frames present, capture the stack, populating the cache. Then push a
// debugger eval frame and capture again; the skipped frames to appear to be
// absent from the stack. Now pop the debugger eval frame, and capture a third
// time: the no-longer-skipped frames seem to reappear on the stack, with
// their cached bits still set.
//
// The LiveSavedFrameCache assumes that the stack it sees is used in a
// stack-like fashion: if a frame has its bit set, it has never left the
// stack. To support this assumption, when the cache is in use, we do not skip
// the frames between a debugger eval frame an its target; we always traverse
// the entire stack, invalidating and populating the cache in the usual way.
// Instead, when we construct a SavedFrame for a debugger eval frame, we
// select the appropriate parent at that point: rather than the next-older
// frame, we find the SavedFrame for the eval's target frame. The skip appears
// in the SavedFrame chains, even as the traversal covers all the frames.
//
// - Rematerialized frames (see ../jit/RematerializedFrame.h) are always created
// with their hasCachedSavedFrame bits clear: although there may be extant
// SavedFrames built from the original IonMonkey frame, the Rematerialized
// frames will not have cache entries for them until they are traversed in a
// capture themselves.
//
// This means that, oddly, it is not always true that, once we reach a frame
// with its hasCachedSavedFrame bit set, all its parents will have the bit set
// as well. However, clear bits under younger set bits will only occur on
// Rematerialized frames.
class LiveSavedFrameCache {
public:
// The address of a live frame for which we can cache SavedFrames: it has a
// 'hasCachedSavedFrame' bit we can examine and set, and can be converted to
// a Key to index the cache.
class FramePtr {
// We use jit::CommonFrameLayout for both Baseline frames and Ion
// physical frames.
using Ptr = mozilla::Variant<InterpreterFrame*, jit::CommonFrameLayout*,
jit::RematerializedFrame*, wasm::DebugFrame*>;
Ptr ptr;
template <typename Frame>
explicit FramePtr(Frame ptr) : ptr(ptr) {}
struct HasCachedMatcher;
struct SetHasCachedMatcher;
struct ClearHasCachedMatcher;
public:
// If iter's frame is of a type that can be cached, construct a FramePtr
// for its frame. Otherwise, return Nothing.
static inline mozilla::Maybe<FramePtr> create(const FrameIter& iter);
inline bool hasCachedSavedFrame() const;
inline void setHasCachedSavedFrame();
inline void clearHasCachedSavedFrame();
// Return true if this FramePtr refers to an interpreter frame.
inline bool isInterpreterFrame() const {
return ptr.is<InterpreterFrame*>();
}
// If this FramePtr is an interpreter frame, return a pointer to it.
inline InterpreterFrame& asInterpreterFrame() const {
return *ptr.as<InterpreterFrame*>();
}
// Return true if this FramePtr refers to a rematerialized frame.
inline bool isRematerializedFrame() const {
return ptr.is<jit::RematerializedFrame*>();
}
bool operator==(const FramePtr& rhs) const { return rhs.ptr == this->ptr; }
bool operator!=(const FramePtr& rhs) const { return !(rhs == *this); }
};
private:
// A key in the cache: the address of a frame, live or dead, for which we
// can cache SavedFrames. Since the pointer may not be live, the only
// operation this type permits is comparison.
class Key {
FramePtr framePtr;
public:
MOZ_IMPLICIT Key(const FramePtr& framePtr) : framePtr(framePtr) {}
bool operator==(const Key& rhs) const {
return rhs.framePtr == this->framePtr;
}
bool operator!=(const Key& rhs) const { return !(rhs == *this); }
};
struct Entry {
const Key key;
const jsbytecode* pc;
HeapPtr<SavedFrame*> savedFrame;
Entry(const Key& key, const jsbytecode* pc, SavedFrame* savedFrame)
: key(key), pc(pc), savedFrame(savedFrame) {}
};
using EntryVector = Vector<Entry, 0, SystemAllocPolicy>;
EntryVector* frames;
LiveSavedFrameCache(const LiveSavedFrameCache&) = delete;
LiveSavedFrameCache& operator=(const LiveSavedFrameCache&) = delete;
public:
explicit LiveSavedFrameCache() : frames(nullptr) {}
LiveSavedFrameCache(LiveSavedFrameCache&& rhs) : frames(rhs.frames) {
MOZ_ASSERT(this != &rhs, "self-move disallowed");
rhs.frames = nullptr;
}
~LiveSavedFrameCache() {
if (frames) {
js_delete(frames);
frames = nullptr;
}
}
bool initialized() const { return !!frames; }
bool init(JSContext* cx) {
frames = js_new<EntryVector>();
if (!frames) {
JS_ReportOutOfMemory(cx);
return false;
}
return true;
}
void trace(JSTracer* trc);
// Set |frame| to the cached SavedFrame corresponding to |framePtr| at |pc|.
// |framePtr|'s hasCachedSavedFrame bit must be set. Remove all cache
// entries for frames younger than that one.
//
// This may set |frame| to nullptr if |pc| is different from the pc supplied
// when the cache entry was inserted. In this case, the cached SavedFrame
// (probably) has the wrong source position. Entries for younger frames are
// still removed. The next frame, if any, will be a cache hit.
//
// This may also set |frame| to nullptr if the cache was populated with
// SavedFrame objects for a different compartment than cx's current
// compartment. In this case, the entire cache is flushed.
void find(JSContext* cx, FramePtr& framePtr, const jsbytecode* pc,
MutableHandle<SavedFrame*> frame) const;
// Search the cache for a frame matching |framePtr|, without removing any
// entries. Return the matching saved frame, or nullptr if none is found.
// This is used for resolving |evalInFramePrev| links.
void findWithoutInvalidation(const FramePtr& framePtr,
MutableHandle<SavedFrame*> frame) const;
// Push a cache entry mapping |framePtr| and |pc| to |savedFrame| on the top
// of the cache's stack. You must insert entries for frames from oldest to
// youngest. They must all be younger than the frame that the |find| method
// found a hit for; or you must have cleared the entire cache with the
// |clear| method.
bool insert(JSContext* cx, FramePtr&& framePtr, const jsbytecode* pc,
Handle<SavedFrame*> savedFrame);
// Remove all entries from the cache.
void clear() {
if (frames) frames->clear();
}
};
static_assert(
sizeof(LiveSavedFrameCache) == sizeof(uintptr_t),
"Every js::Activation has a LiveSavedFrameCache, so we need to be pretty "
"careful "
"about avoiding bloat. If you're adding members to LiveSavedFrameCache, "
"maybe you "
"should consider figuring out a way to make js::Activation have a "
"LiveSavedFrameCache* instead of a Rooted<LiveSavedFrameCache>.");
class Activation {
protected:
JSContext* cx_;
JS::Compartment* compartment_;
Activation* prev_;
Activation* prevProfiling_;
// Counter incremented by JS::HideScriptedCaller and decremented by
// JS::UnhideScriptedCaller. If > 0 for the top activation,
// DescribeScriptedCaller will return null instead of querying that
// activation, which should prompt the caller to consult embedding-specific
// data structures instead.
size_t hideScriptedCallerCount_;
// The cache of SavedFrame objects we have already captured when walking
// this activation's stack.
JS::Rooted<LiveSavedFrameCache> frameCache_;
// Youngest saved frame of an async stack that will be iterated during stack
// capture in place of the actual stack of previous activations. Note that
// the stack of this activation is captured entirely before this is used.
//
// Usually this is nullptr, meaning that normal stack capture will occur.
// When this is set, the stack of any previous activation is ignored.
JS::Rooted<SavedFrame*> asyncStack_;
// Value of asyncCause to be attached to asyncStack_.
const char* asyncCause_;
// True if the async call was explicitly requested, e.g. via
// callFunctionWithAsyncStack.
bool asyncCallIsExplicit_;
enum Kind { Interpreter, Jit };
Kind kind_;
inline Activation(JSContext* cx, Kind kind);
inline ~Activation();
public:
JSContext* cx() const { return cx_; }
JS::Compartment* compartment() const { return compartment_; }
Activation* prev() const { return prev_; }
Activation* prevProfiling() const { return prevProfiling_; }
inline Activation* mostRecentProfiling();
bool isInterpreter() const { return kind_ == Interpreter; }
bool isJit() const { return kind_ == Jit; }
inline bool hasWasmExitFP() const;
inline bool isProfiling() const;
void registerProfiling();
void unregisterProfiling();
InterpreterActivation* asInterpreter() const {
MOZ_ASSERT(isInterpreter());
return (InterpreterActivation*)this;
}
jit::JitActivation* asJit() const {
MOZ_ASSERT(isJit());
return (jit::JitActivation*)this;
}
void hideScriptedCaller() { hideScriptedCallerCount_++; }
void unhideScriptedCaller() {
MOZ_ASSERT(hideScriptedCallerCount_ > 0);
hideScriptedCallerCount_--;
}
bool scriptedCallerIsHidden() const { return hideScriptedCallerCount_ > 0; }
SavedFrame* asyncStack() { return asyncStack_; }
const char* asyncCause() const { return asyncCause_; }
bool asyncCallIsExplicit() const { return asyncCallIsExplicit_; }
inline LiveSavedFrameCache* getLiveSavedFrameCache(JSContext* cx);
void clearLiveSavedFrameCache() { frameCache_.get().clear(); }
private:
Activation(const Activation& other) = delete;
void operator=(const Activation& other) = delete;
};
// This variable holds a special opcode value which is greater than all normal
// opcodes, and is chosen such that the bitwise or of this value with any
// opcode is this value.
constexpr jsbytecode EnableInterruptsPseudoOpcode = -1;
static_assert(EnableInterruptsPseudoOpcode >= JSOP_LIMIT,
"EnableInterruptsPseudoOpcode must be greater than any opcode");
static_assert(
EnableInterruptsPseudoOpcode == jsbytecode(-1),
"EnableInterruptsPseudoOpcode must be the maximum jsbytecode value");
class InterpreterFrameIterator;
class RunState;
class InterpreterActivation : public Activation {
friend class js::InterpreterFrameIterator;
InterpreterRegs regs_;
InterpreterFrame* entryFrame_;
size_t opMask_; // For debugger interrupts, see js::Interpret.
#ifdef DEBUG
size_t oldFrameCount_;
#endif
public:
inline InterpreterActivation(RunState& state, JSContext* cx,
InterpreterFrame* entryFrame);
inline ~InterpreterActivation();
inline bool pushInlineFrame(const JS::CallArgs& args,
JS::Handle<JSScript*> script,
MaybeConstruct constructing);
inline void popInlineFrame(InterpreterFrame* frame);
inline bool resumeGeneratorFrame(JS::Handle<JSFunction*> callee,
JS::Handle<JSObject*> envChain);
InterpreterFrame* current() const { return regs_.fp(); }
InterpreterRegs& regs() { return regs_; }
InterpreterFrame* entryFrame() const { return entryFrame_; }
size_t opMask() const { return opMask_; }
bool isProfiling() const { return false; }
// If this js::Interpret frame is running |script|, enable interrupts.
void enableInterruptsIfRunning(JSScript* script) {
if (regs_.fp()->script() == script) {
enableInterruptsUnconditionally();
}
}
void enableInterruptsUnconditionally() {
opMask_ = EnableInterruptsPseudoOpcode;
}
void clearInterruptsMask() { opMask_ = 0; }
};
// Iterates over a thread's activation list.
class ActivationIterator {
protected:
Activation* activation_;
public:
explicit ActivationIterator(JSContext* cx);
ActivationIterator& operator++();
Activation* operator->() const { return activation_; }
Activation* activation() const { return activation_; }
bool done() const { return activation_ == nullptr; }
};
} // namespace js
#endif // vm_Activation_h