Adding upstream version 115.7.0esr.upstream/115.7.0esrupstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 578 insertions, 0 deletions

diff --git a/js/public/ProfilingStack.h b/js/public/ProfilingStack.h
new file mode 100644
index 0000000000..b9b96c27dc
--- /dev/null
+++ b/js/public/ProfilingStack.h
@@ -0,0 +1,578 @@
+/* -*- 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 http://mozilla.org/MPL/2.0/. */
+
+#ifndef js_ProfilingStack_h
+#define js_ProfilingStack_h
+
+#include "mozilla/Atomics.h"
+
+#include <stdint.h>
+
+#include "jstypes.h"
+
+#include "js/ProfilingCategory.h"
+#include "js/TypeDecls.h"
+
+class JS_PUBLIC_API JSTracer;
+class JS_PUBLIC_API 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;
+
+  // 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.
+  mozilla::Atomic<int32_t, mozilla::ReleaseAcquire> pcOffsetIfJS_;
+
+  // Bits 0...8 hold the Flags. Bits 9...31 hold the category pair.
+  mozilla::Atomic<uint32_t, mozilla::ReleaseAcquire> flagsAndCategoryPair_;
+
+  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 offsetIfJS = other.pcOffsetIfJS_;
+    pcOffsetIfJS_ = offsetIfJS;
+    uint64_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".
+    IS_SP_MARKER_FRAME = 1 << 1,
+
+    // 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.
+    RELEVANT_FOR_JS = 1 << 7,
+
+    // If set, causes the label on this ProfilingStackFrame to be ignored
+    // and to be replaced by the subcategory's label.
+    LABEL_DETERMINED_BY_CATEGORY_PAIR = 1 << 8,
+
+    // Frame dynamic string does not contain user data.
+    NONSENSITIVE = 1 << 9,
+
+    // A JS Baseline Interpreter frame.
+    IS_BLINTERP_FRAME = 1 << 10,
+
+    FLAGS_BITCOUNT = 16,
+    FLAGS_MASK = (1 << FLAGS_BITCOUNT) - 1
+  };
+
+  static_assert(
+      uint32_t(JS::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 isNonsensitive() const {
+    return uint32_t(flagsAndCategoryPair_) & uint32_t(Flags::NONSENSITIVE);
+  }
+
+  bool isSpMarkerFrame() const {
+    return uint32_t(flagsAndCategoryPair_) &
+           uint32_t(Flags::IS_SP_MARKER_FRAME);
+  }
+
+  bool isJsFrame() const {
+    return uint32_t(flagsAndCategoryPair_) & uint32_t(Flags::IS_JS_FRAME);
+  }
+
+  bool isJsBlinterpFrame() const {
+    return uint32_t(flagsAndCategoryPair_) & uint32_t(Flags::IS_BLINTERP_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);
+    }
+  }
+
+  void setLabelCategory(JS::ProfilingCategoryPair aCategoryPair) {
+    MOZ_ASSERT(isLabelFrame());
+    flagsAndCategoryPair_ =
+        (uint32_t(aCategoryPair) << uint32_t(Flags::FLAGS_BITCOUNT)) | flags();
+  }
+
+  const char* label() const {
+    uint32_t flagsAndCategoryPair = flagsAndCategoryPair_;
+    if (flagsAndCategoryPair &
+        uint32_t(Flags::LABEL_DETERMINED_BY_CATEGORY_PAIR)) {
+      auto categoryPair = JS::ProfilingCategoryPair(
+          flagsAndCategoryPair >> uint32_t(Flags::FLAGS_BITCOUNT));
+      return JS::GetProfilingCategoryPairInfo(categoryPair).mLabel;
+    }
+    return label_;
+  }
+
+  const char* dynamicString() const { return dynamicString_; }
+
+  void initLabelFrame(const char* aLabel, const char* aDynamicString, void* sp,
+                      JS::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;
+    MOZ_ASSERT(isLabelFrame());
+  }
+
+  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(JS::ProfilingCategoryPair::OTHER)
+                             << uint32_t(Flags::FLAGS_BITCOUNT));
+    MOZ_ASSERT(isSpMarkerFrame());
+  }
+
+  template <JS::ProfilingCategoryPair Category, uint32_t ExtraFlags = 0>
+  void initJsFrame(const char* aLabel, const char* aDynamicString,
+                   JSScript* aScript, jsbytecode* aPc, uint64_t aRealmID) {
+    label_ = aLabel;
+    dynamicString_ = aDynamicString;
+    spOrScript = aScript;
+    pcOffsetIfJS_ = pcToOffset(aScript, aPc);
+    realmID_ = aRealmID;
+    flagsAndCategoryPair_ =
+        (uint32_t(Category) << uint32_t(Flags::FLAGS_BITCOUNT)) |
+        uint32_t(Flags::IS_JS_FRAME) | ExtraFlags;
+    MOZ_ASSERT(isJsFrame());
+  }
+
+  uint32_t flags() const {
+    return uint32_t(flagsAndCategoryPair_) & uint32_t(Flags::FLAGS_MASK);
+  }
+
+  JS::ProfilingCategoryPair categoryPair() const {
+    return JS::ProfilingCategoryPair(flagsAndCategoryPair_ >>
+                                     uint32_t(Flags::FLAGS_BITCOUNT));
+  }
+
+  uint64_t realmID() const { return realmID_; }
+
+  void* stackAddress() const {
+    MOZ_ASSERT(!isJsFrame());
+    return spOrScript;
+  }
+
+  JS_PUBLIC_API JSScript* script() const;
+
+  JS_PUBLIC_API JSFunction* function() const;
+
+  // 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_PUBLIC_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_PUBLIC_API void SetContextProfilingStack(JSContext* cx,
+                                            ProfilingStack* profilingStack);
+
+// GetContextProfilingStack also exists, but it's defined in RootingAPI.h.
+
+JS_PUBLIC_API void EnableContextProfilingStack(JSContext* cx, bool enabled);
+
+JS_PUBLIC_API void RegisterContextProfilingEventMarker(JSContext* cx,
+                                                       void (*fn)(const char*,
+                                                                  const char*));
+
+}  // namespace js
+
+namespace JS {
+
+typedef ProfilingStack* (*RegisterThreadCallback)(const char* threadName,
+                                                  void* stackBase);
+
+typedef void (*UnregisterThreadCallback)();
+
+// regiserThread and unregisterThread callbacks are functions which are called
+// by other threads without any locking mechanism.
+JS_PUBLIC_API void SetProfilingThreadCallbacks(
+    RegisterThreadCallback registerThread,
+    UnregisterThreadCallback unregisterThread);
+
+}  // 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 JS_PUBLIC_API ProfilingStack final {
+ public:
+  ProfilingStack() = default;
+
+  ~ProfilingStack();
+
+  void pushLabelFrame(const char* label, const char* dynamicString, void* sp,
+                      JS::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)) {
+      ensureCapacitySlow();
+    }
+    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)) {
+      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, 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)) {
+      ensureCapacitySlow();
+    }
+    frames[oldStackPointer]
+        .initJsFrame<JS::ProfilingCategoryPair::JS_Interpreter>(
+            label, dynamicString, script, pc, aRealmID);
+
+    // 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; }
+
+ private:
+  // Out of line path for expanding the buffer, since otherwise this would get
+  // inlined in every DOM WebIDL call.
+  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;
+
+ 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*, mozilla::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).
+  //
+  // 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{0};
+};
+
+namespace js {
+
+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_;
+
+ public:
+  GeckoProfilerThread();
+
+  uint32_t stackPointer() {
+    MOZ_ASSERT(infraInstalled());
+    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; }
+
+  void setProfilingStack(ProfilingStack* profilingStack, bool enabled);
+  void enable(bool enable) {
+    profilingStackIfEnabled_ = enable ? profilingStack_ : nullptr;
+  }
+  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);
+  void exit(JSContext* cx, JSScript* script);
+  inline void updatePC(JSContext* cx, JSScript* script, jsbytecode* pc);
+};
+
+}  // namespace js
+
+#endif /* js_ProfilingStack_h */