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/* -*- 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 */
#ifndef vm_SPSProfiler_h
#define vm_SPSProfiler_h
#include "mozilla/DebugOnly.h"
#include "mozilla/GuardObjects.h"
#include <stddef.h>
#include "jslock.h"
#include "jsscript.h"
#include "js/ProfilingStack.h"
* SPS Profiler integration with the JS Engine
* The SPS profiler (found in tools/profiler) is an implementation of a profiler
* which has the ability to walk the C++ stack as well as use instrumentation to
* gather information. When dealing with JS, however, SPS needs integration
* with the engine because otherwise it is very difficult to figure out what
* javascript is executing.
* The current method of integration with SPS is a form of instrumentation:
* every time a JS function is entered, a bit of information is pushed onto a
* stack that SPS owns and maintains. This information is then popped at the end
* of the JS function. SPS informs the JS engine of this stack at runtime, and
* it can by turned on/off dynamically.
* The SPS stack has three parameters: a base pointer, a size, and a maximum
* size. The stack is the ProfileEntry stack which will have information written
* to it. The size location is a pointer to an integer which represents the
* current size of the stack (number of valid frames). This size will be
* modified when JS functions are called. The maximum specified is the maximum
* capacity of the ProfileEntry stack.
* Throughout execution, the size of the stack recorded in memory may exceed the
* maximum. The JS engine will not write any information past the maximum limit,
* but it will still maintain the size of the stack. SPS code is aware of this
* and iterates the stack accordingly.
* There is some information pushed on the SPS stack for every JS function that
* is entered. First is a char* pointer of a description of what function was
* entered. Currently this string is of the form "function (file:line)" if
* there's a function name, or just "file:line" if there's no function name
* available. The other bit of information is the relevant C++ (native) stack
* pointer. This stack pointer is what enables the interleaving of the C++ and
* the JS stack. Finally, throughout execution of the function, some extra
* information may be updated on the ProfileEntry structure.
* = Profile Strings
* The profile strings' allocations and deallocation must be carefully
* maintained, and ideally at a very low overhead cost. For this reason, the JS
* engine maintains a mapping of all known profile strings. These strings are
* keyed in lookup by a JSScript*, but are serialized with a JSFunction*,
* JSScript* pair. A JSScript will destroy its corresponding profile string when
* the script is finalized.
* For this reason, a char* pointer pushed on the SPS stack is valid only while
* it is on the SPS stack. SPS uses sampling to read off information from this
* instrumented stack, and it therefore copies the string byte for byte when a
* JS function is encountered during sampling.
* = Native Stack Pointer
* The actual value pushed as the native pointer is nullptr for most JS
* functions. The reason for this is that there's actually very little
* correlation between the JS stack and the C++ stack because many JS functions
* all run in the same C++ frame, or can even go backwards in C++ when going
* from the JIT back to the interpreter.
* To alleviate this problem, all JS functions push nullptr as their "native
* stack pointer" to indicate that it's a JS function call. The function
* RunScript(), however, pushes an actual C++ stack pointer onto the SPS stack.
* This way when interleaving C++ and JS, if SPS sees a nullptr native stack
* pointer on the SPS stack, it looks backwards for the first non-nullptr
* pointer and uses that for all subsequent nullptr native stack pointers.
* = Line Numbers
* One goal of sampling is to get both a backtrace of the JS stack, but also
* know where within each function on the stack execution currently is. For
* this, each ProfileEntry has a 'pc' field to tell where its execution
* currently is. This field is updated whenever a call is made to another JS
* function, and for the JIT it is also updated whenever the JIT is left.
* This field is in a union with a uint32_t 'line' so that C++ can make use of
* the field as well. It was observed that tracking 'line' via PCToLineNumber in
* JS was far too expensive, so that is why the pc instead of the translated
* line number is stored.
* As an invariant, if the pc is nullptr, then the JIT is currently executing
* generated code. Otherwise execution is in another JS function or in C++. With
* this in place, only the top entry of the stack can ever have nullptr as its
* pc. Additionally with this invariant, it is possible to maintain mappings of
* JIT code to pc which can be accessed safely because they will only be
* accessed from a signal handler when the JIT code is executing.
namespace js {
typedef HashMap<JSScript*, const char*, DefaultHasher<JSScript*>, SystemAllocPolicy>
class AutoSPSEntry;
class SPSEntryMarker;
class SPSBaselineOSRMarker;
class SPSProfiler
friend class AutoSPSEntry;
friend class SPSEntryMarker;
friend class SPSBaselineOSRMarker;
JSRuntime* rt;
ProfileStringMap strings;
ProfileEntry* stack_;
uint32_t* size_;
uint32_t max_;
bool slowAssertions;
uint32_t enabled_;
PRLock* lock_;
void (*eventMarker_)(const char*);
const char* allocProfileString(JSScript* script, JSFunction* function);
void push(const char* string, void* sp, JSScript* script, jsbytecode* pc, bool copy,
ProfileEntry::Category category = ProfileEntry::Category::JS);
void pop();
explicit SPSProfiler(JSRuntime* rt);
bool init();
uint32_t** addressOfSizePointer() {
return &size_;
uint32_t* addressOfMaxSize() {
return &max_;
ProfileEntry** addressOfStack() {
return &stack_;
uint32_t* sizePointer() { return size_; }
uint32_t maxSize() { return max_; }
uint32_t size() { MOZ_ASSERT(installed()); return *size_; }
ProfileEntry* stack() { return stack_; }
/* management of whether instrumentation is on or off */
bool enabled() { MOZ_ASSERT_IF(enabled_, installed()); return enabled_; }
bool installed() { return stack_ != nullptr && size_ != nullptr; }
void enable(bool enabled);
void enableSlowAssertions(bool enabled) { slowAssertions = enabled; }
bool slowAssertionsEnabled() { return slowAssertions; }
* 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);
void updatePC(JSScript* script, jsbytecode* pc) {
if (enabled() && *size_ - 1 < max_) {
MOZ_ASSERT(*size_ > 0);
MOZ_ASSERT(stack_[*size_ - 1].script() == script);
stack_[*size_ - 1].setPC(pc);
/* Enter asm.js code */
void beginPseudoJS(const char* string, void* sp);
void endPseudoJS() { pop(); }
jsbytecode* ipToPC(JSScript* script, size_t ip) { return nullptr; }
void setProfilingStack(ProfileEntry* stack, uint32_t* size, uint32_t max);
void setEventMarker(void (*fn)(const char*));
const char* profileString(JSScript* script, JSFunction* maybeFun);
void onScriptFinalized(JSScript* script);
void markEvent(const char* event);
/* meant to be used for testing, not recommended to call in normal code */
size_t stringsCount();
void stringsReset();
uint32_t* addressOfEnabled() {
return &enabled_;
* This class is used to make sure the strings table
* is only accessed on one thread at a time.
class AutoSPSLock
explicit AutoSPSLock(PRLock* lock)
MOZ_ASSERT(lock, "Parameter should not be null!");
lock_ = lock;
~AutoSPSLock() { PR_Unlock(lock_); }
PRLock* lock_;
* This class is used to suppress profiler sampling during
* critical sections where stack state is not valid.
class MOZ_RAII AutoSuppressProfilerSampling
explicit AutoSuppressProfilerSampling(JSContext* cx MOZ_GUARD_OBJECT_NOTIFIER_PARAM);
explicit AutoSuppressProfilerSampling(JSRuntime* rt MOZ_GUARD_OBJECT_NOTIFIER_PARAM);
JSRuntime* rt_;
bool previouslyEnabled_;
inline size_t
AutoSPSLock lock(lock_);
return strings.count();
inline void
AutoSPSLock lock(lock_);
* This class is used in RunScript() to push the marker onto the sampling stack
* that we're about to enter JS function calls. This is the only time in which a
* valid stack pointer is pushed to the sampling stack.
class MOZ_RAII SPSEntryMarker
explicit SPSEntryMarker(JSRuntime* rt,
JSScript* script
SPSProfiler* profiler;
mozilla::DebugOnly<uint32_t> size_before;
* RAII class to automatically add SPS psuedo frame entries.
* NB: The `label` string must be statically allocated.
explicit AutoSPSEntry(JSRuntime* rt, const char* label,
ProfileEntry::Category category = ProfileEntry::Category::JS
SPSProfiler* profiler_;
mozilla::DebugOnly<uint32_t> sizeBefore_;
* This class is used in the interpreter to bound regions where the baseline JIT
* being entered via OSR. It marks the current top pseudostack entry as
* OSR-ed
class MOZ_RAII SPSBaselineOSRMarker
explicit SPSBaselineOSRMarker(JSRuntime* rt, bool hasSPSFrame
SPSProfiler* profiler;
mozilla::DebugOnly<uint32_t> size_before;
* SPS is the profiling backend used by the JS engine to enable time profiling.
* More information can be found in vm/SPSProfiler.{h,cpp}. This class manages
* the instrumentation portion of the profiling for JIT code.
* The instrumentation tracks entry into functions, leaving those functions via
* a function call, reentering the functions from a function call, and exiting
* the functions from returning. This class also handles inline frames and
* manages the instrumentation which needs to be attached to them as well.
* The basic methods which emit instrumentation are at the end of this class,
* and the management functions are all described in the middle.
template<class Assembler, class Register>
class SPSInstrumentation
SPSProfiler* profiler_; // Instrumentation location management
* Creates instrumentation which writes information out the the specified
* profiler's stack and constituent fields.
explicit SPSInstrumentation(SPSProfiler* profiler) : profiler_(profiler) {}
/* Small proxies around SPSProfiler */
bool enabled() { return profiler_ && profiler_->enabled(); }
SPSProfiler* profiler() { MOZ_ASSERT(enabled()); return profiler_; }
void disable() { profiler_ = nullptr; }
/* Get a pointer to the top-most profiling frame, given the exit frame pointer. */
void* GetTopProfilingJitFrame(uint8_t* exitFramePtr);
} /* namespace js */
#endif /* vm_SPSProfiler_h */