NativeStackSampler implementation for Mac.

base/profiler/native_stack_sampler_mac.cc

+// Copyright 2017 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "base/profiler/native_stack_sampler.h"
+
+#include <dlfcn.h>
+#include <libkern/OSByteOrder.h>
+#include <libunwind.h>
+#include <mach-o/swap.h>
+#include <mach/kern_return.h>
+#include <mach/mach.h>
+#include <mach/thread_act.h>
+#include <pthread.h>
+#include <sys/syslimits.h>
+
+#include <map>
+#include <memory>
+
+#include "base/logging.h"
+#include "base/macros.h"
+#include "base/memory/ptr_util.h"
+#include "base/strings/string_number_conversions.h"
+
+namespace base {
+
+namespace {
+
+// Stack walking --------------------------------------------------------------
+
+// Copy of x86_64 thread context structure from x86_thread_state64_t type.
+// Copied struct since fields can have different names on different versions of
+// Darwin.

[Mark Mentovai, 2017/02/17 05:21:05]

?

It’s never going to not be the double-underscore versions in the 64-bit
environment. Don’t define your own, just go with x86_thread_state64_t.

[Avi (use Gerrit), 2017/02/17 17:18:12]

Done.

+struct ThreadContext {
+  uint64_t rax;
+  uint64_t rbx;
+  uint64_t rcx;
+  uint64_t rdx;
+  uint64_t rdi;
+  uint64_t rsi;
+  uint64_t rbp;
+  uint64_t rsp;
+  uint64_t r8;
+  uint64_t r9;
+  uint64_t r10;
+  uint64_t r11;
+  uint64_t r12;
+  uint64_t r13;
+  uint64_t r14;
+  uint64_t r15;
+  uint64_t rip;
+  uint64_t rflags;
+  uint64_t cs;
+  uint64_t fs;
+  uint64_t gs;
+};
+
+// Fills |state| with |target_thread|'s context.
+//
+// Note that this is called while a thread is suspended. Make very very sure
+// that no shared resources (e.g. memory allocators) are used for the duration
+// of this function.
+bool GetThreadContext(thread_act_t target_thread, ThreadContext* state) {
+  mach_msg_type_number_t count =
+      static_cast<mach_msg_type_number_t>(x86_THREAD_STATE64_COUNT);
+  return thread_get_state(target_thread, x86_THREAD_STATE64,
+                          reinterpret_cast<thread_state_t>(state),
+                          &count) == KERN_SUCCESS;
+}
+
+// If the value at |pointer| points to the original stack, rewrite it to point
+// to the corresponding location in the copied stack.
+//
+// Note that this is called while a thread is suspended. Make very very sure
+// that no shared resources (e.g. memory allocators) are used for the duration
+// of this function.
+uint64_t RewritePointerIfInOriginalStack(uint64_t* original_stack_bottom,
+                                         uint64_t* original_stack_top,
+                                         uint64_t* stack_copy_bottom,
+                                         uint64_t pointer) {
+  uint64_t original_stack_bottom_int =
+      reinterpret_cast<uint64_t>(original_stack_bottom);
+  uint64_t original_stack_top_int =
+      reinterpret_cast<uint64_t>(original_stack_top);
+  uint64_t stack_copy_bottom_int =
+      reinterpret_cast<uint64_t>(stack_copy_bottom);
+
+  if ((pointer < original_stack_bottom_int) ||
+      (pointer >= original_stack_top_int)) {
+    return pointer;
+  }
+
+  return stack_copy_bottom_int + (pointer - original_stack_bottom_int);
+}
+
+// Copy the stack to a buffer while rewriting possible pointers to locations
+// within the stack to point to the corresponding locations in the copy. This is
+// necessary to handle stack frames with dynamic stack allocation, where a
+// pointer to the beginning of the dynamic allocation area is stored on the
+// stack and/or in a non-volatile register.
+//
+// Eager rewriting of anything that looks like a pointer to the stack, as done
+// in this function, does not adversely affect the stack unwinding. The only
+// other values on the stack the unwinding depends on are return addresses,
+// which should not point within the stack memory. The rewriting is guaranteed
+// to catch all pointers because the stacks are guaranteed by the ABI to be
+// sizeof(void*) aligned.
+//
+// Note that this is called while a thread is suspended. Make very very sure
+// that no shared resources (e.g. memory allocators) are used for the duration
+// of this function.
+void CopyStackAndRewritePointers(void* dest,
+                                 void* from,
+                                 void* to,
+                                 ThreadContext* thread_context)
+    NO_SANITIZE("address") {
+  uint64_t* original_stack_bottom = static_cast<uint64_t*>(from);
+  uint64_t* original_stack_top = static_cast<uint64_t*>(to);
+  uint64_t* stack_copy_bottom = static_cast<uint64_t*>(dest);
+
+  size_t count = original_stack_top - original_stack_bottom;
+  for (size_t pos = 0; pos < count; ++pos) {
+    stack_copy_bottom[pos] = RewritePointerIfInOriginalStack(
+        original_stack_bottom, original_stack_top, stack_copy_bottom,
+        original_stack_bottom[pos]);
+  }
+
+  thread_context->rbp =
+      RewritePointerIfInOriginalStack(original_stack_bottom, original_stack_top,
+                                      stack_copy_bottom, thread_context->rbp);
+  thread_context->rsp =
+      RewritePointerIfInOriginalStack(original_stack_bottom, original_stack_top,
+                                      stack_copy_bottom, thread_context->rsp);
+}
+
+const char* LibSystemKernelName() {
+  static char path[PATH_MAX];
+  static char* name = nullptr;
+  if (name)
+    return name;
+
+  Dl_info info;
+  dladdr(reinterpret_cast<void*>(_exit), &info);
+  strncpy(path, info.dli_fname, PATH_MAX);
+  name = path;
+  DCHECK_EQ(std::string(name),
+            std::string("/usr/lib/system/libsystem_kernel.dylib"));
+  return name;
+}
+
+enum StackWalkResult : int {
+  ERROR = -1,
+  SUCCESS,
+  SYSCALL,
+};
+
+// Walks the stack represented by |unwind_context|, calling back to the provided
+// lambda for each frame.
+template <typename StackFrameCallback>
+StackWalkResult WalkStackFromContext(unw_context_t* unwind_context,
+                                     const StackFrameCallback& callback) {
+  unw_cursor_t unwind_cursor;
+  unw_init_local(&unwind_cursor, unwind_context);
+
+  int step_result;
+  unw_word_t ip;
+  size_t frames = 0;
+  do {
+    ++frames;
+    unw_get_reg(&unwind_cursor, UNW_REG_IP, &ip);
+
+    callback(static_cast<uintptr_t>(ip));
+
+    step_result = unw_step(&unwind_cursor);
+  } while (step_result > 0);
+
+  if (step_result != 0)
+    return StackWalkResult::ERROR;
+
+  Dl_info info;
+  if (frames == 1 && dladdr(reinterpret_cast<void*>(ip), &info) != 0 &&
+      strcmp(info.dli_fname, LibSystemKernelName()) == 0) {
+    return StackWalkResult::SYSCALL;
+  }
+
+  return StackWalkResult::SUCCESS;
+}
+
+// Walks the stack represented by |thread_context|, calling back to the provided
+// lambda for each frame.
+template <typename StackFrameCallback>
+void WalkStack(const ThreadContext& thread_context,
+               const StackFrameCallback& callback) {
+  // This uses libunwind to walk the stack. libunwind is designed to be used for
+  // a thread to walk its own stack. This creates two problems.
+
+  // Problem 1: There is no official way to create a unw_context other than to
+  // create it from the current state of the current thread's stack. To get
+  // around this, forge a context. A unw_context is just a copy of the register
+  // file followed by the instruction pointer. Coincidentally, the first 17

[Mark Mentovai, 2017/02/17 05:21:05]

“register file plus instruction pointer” is weird because the instruction
pointer is part of the register file. x86_thread_state64_t is actually a more
complete (but still incomplete) snapshot of the register file than
unw_context_t. Also, the ordering of fields is arbitrary in both unw_context_t
and x86_thread_state64_t (the CPU doesn’t expose any native structure for this
stuff unlike floating-point state, it’s all arbitrary in software), so this part
of the comment isn’t helpful. You should just say that the first 17 fields of
each structure are identical.

[Avi (use Gerrit), 2017/02/17 17:18:12]

Done.

+  // items of the ThreadContext type are exactly that!
+  unw_context_t unwind_context;
+  memcpy(&unwind_context, &thread_context, sizeof(uint64_t) * 17);
+  StackWalkResult result = WalkStackFromContext(&unwind_context, callback);
+
+  if (result == StackWalkResult::SYSCALL) {
+    // Problem 2: Because libunwind is designed to be triggered by user code on
+    // their own thread, if it hits a library that has no unwind info for the
+    // function that is being executed, it just stops. This isn't a problem in
+    // the normal case, but in this case, it's quite possible that the stack
+    // being walked is stopped in a function that bridges to the kernel and thus
+    // is missing the unwind info.
+    //
+    // If so, cheat by manually unwinding one stack frame and trying again.
+    unwind_context.data[7] = thread_context.rsp + 8;  // rsp++
+    unwind_context.data[16] =
+        *reinterpret_cast<uint64_t*>(thread_context.rsp);  // rip = *rsp
+    WalkStackFromContext(&unwind_context, callback);
+  }
+}
+
+// Module identifiers ---------------------------------------------------------
+
+// Helper that swaps byte order in |x| if |swap| flag is set.
+uint32_t SwapIfBig32(uint32_t x, bool swap) {

[Mark Mentovai, 2017/02/17 05:21:05]

You don’t need any of this swapping stuff.

[Avi (use Gerrit), 2017/02/17 17:18:12]

Acknowledged.

+  if (swap)
+    return OSSwapBigToHostInt32(x);
+  return x;
+}
+
+// Returns the offset in bytes where the x86_64 header is located in a binary
+// loaded at |module_addr|. Returns 0 if |module_addr| is not a valid FAT
+// Mach-O binary or has not been built for x86_64.
+off_t GetMach64HeaderOffset(const void* module_addr) {

[Mark Mentovai, 2017/02/17 05:21:05]

You don’t need any of this fat stuff either.

[Avi (use Gerrit), 2017/02/17 17:18:13]

Acknowledged.

+  const fat_header* header = reinterpret_cast<const fat_header*>(module_addr);
+  if (header->magic != FAT_MAGIC && header->magic != FAT_CIGAM)
+    return 0;
+
+  // Search all FAT architectures for x86_64.
+  const fat_arch* fat_arches = reinterpret_cast<const fat_arch*>(
+      reinterpret_cast<const uint8_t*>(module_addr) + sizeof(header));
+  uint32_t n_arches = OSSwapBigToHostInt32(header->nfat_arch);
+  for (uint32_t i = 0; i < n_arches; ++i) {
+    const fat_arch& arch = fat_arches[i];
+    if (OSSwapBigToHostInt32(arch.cputype) == CPU_TYPE_X86_64)
+      return OSSwapBigToHostInt32(arch.offset);
+  }
+  return 0;
+}
+
+// Returns true if the Mach-O binary at |module_addr| was built specifically for
+// the x86_64 CPU architecture.
+bool IsX64Header(const void* module_addr) {
+  const mach_header_64* header =
+      reinterpret_cast<const mach_header_64*>(module_addr);
+  if (header->magic != MH_MAGIC_64 && header->magic != MH_CIGAM_64)
+    return false;
+  bool swap = header->magic == MH_CIGAM_64;
+  return SwapIfBig32(header->cputype, swap) == CPU_TYPE_X86_64;
+}
+
+// Fills |id| with the UUID of the x86_64 Mach-O binary loaded at |module_addr|.
+// |offset| is the offset in bytes into |module_addr| where the x86_64 header is
+// located. |offset| is only relevant if the binary is FAT and contains multiple
+// architecture headers.  Returns false if the header is malformed or the header
+// does not specify the UUID load command.
+bool GetX64UUIDAt(const void* module_addr, unsigned char* id, off_t offset) {
+  const mach_header_64* header = reinterpret_cast<const mach_header_64*>(
+      reinterpret_cast<const uint8_t*>(module_addr) + offset);
+  if (header->magic != MH_MAGIC_64 && header->magic != MH_CIGAM_64)
+    return false;
+
+  bool swap = header->magic == MH_CIGAM_64;
+  // Search all load commands for UUID command.
+  offset += sizeof(mach_header_64);
+  for (uint32_t i = 0; i < SwapIfBig32(header->ncmds, swap); ++i) {
+    const load_command* current_cmd = reinterpret_cast<const load_command*>(

[Mark Mentovai, 2017/02/17 05:21:05]

This loop needs to be cognizant of not exceeding header->sizeofcmds. It’s not
enough to just loop up to header->ncmds.

I just reviewed a patch for Erik this week that’s yet another Mach-O load
command walker, you should take a look and perhaps share code but definitely
share ideas. We’ve got more of these than we really should have in Chrome as it
is. His was https://codereview.chromium.org/2674973004.

[Avi (use Gerrit), 2017/02/17 17:18:12]

Acknowledged.

+        reinterpret_cast<const uint8_t*>(module_addr) + offset);
+
+    if (SwapIfBig32(current_cmd->cmd, swap) == LC_UUID) {
+      const uuid_command* uuid_cmd =

[Mark Mentovai, 2017/02/17 05:21:05]

Also need to check that current_cmd->cmdsize is at least big enough for a
uuid_cmd.

[Avi (use Gerrit), 2017/02/17 17:18:12]

Done.

+          reinterpret_cast<const uuid_command*>(current_cmd);
+      static_assert(sizeof(uuid_cmd->uuid) == sizeof(uuid_t),
+                    "UUID field of UUID command should be 16 bytes.");
+      memcpy(id, &uuid_cmd->uuid, sizeof(uuid_t));

[Mark Mentovai, 2017/02/17 05:21:05]

(if you were swapping, and you’re not, this would need to be swapped too.)

[Avi (use Gerrit), 2017/02/17 17:18:12]

Acknowledged.

+      return true;
+    }
+    offset += SwapIfBig32(current_cmd->cmdsize, swap);
+  }
+  return false;
+}
+
+// Fills |id| with the Mach-O UUID retrieved from Mach-O binary loaded at
+// |module_addr|. This function returns false if the binary was not built for
+// X86_64 or if the UUID cannot be found.
+bool GetUUID(const void* module_addr, unsigned char* id) {

[Mark Mentovai, 2017/02/17 05:21:05]

(this is the only caller of the swappy and fatty functions)

[Avi (use Gerrit), 2017/02/17 17:18:13]

Acknowledged.

+  off_t offset = 0;
+  // If the module is not x86_64 exclusive, it could be a module that supports
+  // multiple architectures. In that case, the appropriate header will be at
+  // some non-zero offset.
+  if (!IsX64Header(module_addr) &&
+      !(offset = GetMach64HeaderOffset(module_addr))) {
+    return false;
+  }
+  return GetX64UUIDAt(module_addr, id, offset);
+}
+
+// Returns the hex encoding of a 16-byte ID for the binary loaded at
+// |module_addr|. Returns an empty string if the UUID cannot be found at
+// |module_addr|.
+std::string GetUniqueId(const void* module_addr) {

[Mark Mentovai, 2017/02/17 05:21:05]

(this is the only caller of the caller of the swappy and fatty functions)

[Avi (use Gerrit), 2017/02/17 17:18:12]

Acknowledged.

+  unsigned char id[sizeof(uuid_t)];
+  if (!GetUUID(module_addr, id))
+    return "";
+  return HexEncode(id, sizeof(uuid_t));
+}
+
+// Gets the index for the Module containing |instruction_pointer| in
+// |modules|, adding it if it's not already present. Returns
+// StackSamplingProfiler::Frame::kUnknownModuleIndex if no Module can be
+// determined for |module|.
+size_t GetModuleIndex(const uintptr_t instruction_pointer,
+                      std::vector<StackSamplingProfiler::Module>* modules,
+                      std::map<const void*, size_t>* profile_module_index) {
+  Dl_info inf;
+  if (!dladdr(reinterpret_cast<const void*>(instruction_pointer), &inf))
+    return StackSamplingProfiler::Frame::kUnknownModuleIndex;
+
+  auto module_index = profile_module_index->find(inf.dli_fbase);
+  if (module_index == profile_module_index->end()) {
+    StackSamplingProfiler::Module module(
+        reinterpret_cast<uintptr_t>(inf.dli_fbase), GetUniqueId(inf.dli_fbase),

[Mark Mentovai, 2017/02/17 05:21:05]

(this is the only caller of the caller of the caller of the swappy and fatty
functions).

Here’s the deal. All of the modules you’re looking at have been loaded by dyld
(or, in the case of the main executable, the kernel, but the kernel did the same
thing and told dyld about it). A process has only one bitness, and dyld will
never load a wrong-bitted module. dyld will certainly never load a wrong-endian
module. Even if the file that hosted a module was fat and contained multiple
architectures, dyld (or the kernel) will only load the single slice relevant to
the process. It maps based on the instructions in the segment load commands,
which aren’t permitted to exceed the boundaries of the containing slice. The fat
header is never mapped. No part of any slice belonging to another architecture
is ever mapped.

If it’s loaded and dyld knows about it, it’s guaranteed to be MH_MAGIC or
MH_MAGIC_64 (same-endian), and it’s guaranteed to always be one or the other for
a specific bitness, and since Chrome only supports 64-bit x86 now, it’s actually
guaranteed to always be MH_MAGIC_64. So if you want, you can check that
dli_fbase casted to a mach_header_64* has a magic field that says MH_MAGIC_64,
but you don’t need to account for swaps or other bitnesses or fat headers at
all.

[Avi (use Gerrit), 2017/02/17 17:18:12]

Done.

+        base::FilePath(inf.dli_fname));
+    modules->push_back(module);
+    module_index =
+        profile_module_index
+            ->insert(std::make_pair(inf.dli_fbase, modules->size() - 1))
+            .first;
+  }
+  return module_index->second;
+}
+
+// ScopedSuspendThread --------------------------------------------------------
+
+// Suspends a thread for the lifetime of the object.
+class ScopedSuspendThread {
+ public:
+  explicit ScopedSuspendThread(mach_port_t thread_port);
+  ~ScopedSuspendThread();
+
+  bool was_successful() const { return was_successful_; }
+
+ private:
+  mach_port_t thread_port_;
+  bool was_successful_;
+
+  DISALLOW_COPY_AND_ASSIGN(ScopedSuspendThread);
+};
+
+ScopedSuspendThread::ScopedSuspendThread(mach_port_t thread_port)
+    : thread_port_(thread_port),
+      was_successful_(thread_suspend(thread_port) == KERN_SUCCESS) {}
+
+ScopedSuspendThread::~ScopedSuspendThread() {
+  if (!was_successful_)
+    return;
+
+  kern_return_t resume_result = thread_resume(thread_port_);
+  CHECK_EQ(KERN_SUCCESS, resume_result) << "thread_resume failed";
+}
+
+// NativeStackSamplerMac ------------------------------------------------------
+
+class NativeStackSamplerMac : public NativeStackSampler {
+ public:
+  NativeStackSamplerMac(mach_port_t thread_port,
+                        AnnotateCallback annotator,
+                        NativeStackSamplerTestDelegate* test_delegate);
+  ~NativeStackSamplerMac() override;
+
+  // StackSamplingProfiler::NativeStackSampler:
+  void ProfileRecordingStarting(
+      std::vector<StackSamplingProfiler::Module>* modules) override;
+  void RecordStackSample(StackSamplingProfiler::Sample* sample) override;
+  void ProfileRecordingStopped() override;
+
+ private:
+  enum {
+    // Intended to hold the largest stack used by Chrome. The default macOS main
+    // thread stack size is 8 MB, and this allows for expansion if it occurs.
+    kStackCopyBufferSize = 12 * 1024 * 1024
+  };
+
+  // Suspends the thread with |thread_port_|, copies its stack and resumes the
+  // thread, then records the stack frames and associated modules into |sample|.
+  void SuspendThreadAndRecordStack(StackSamplingProfiler::Sample* sample);
+
+  // Weak reference: Mach port for thread being profiled.
+  mach_port_t thread_port_;
+
+  const AnnotateCallback annotator_;
+
+  NativeStackSamplerTestDelegate* const test_delegate_;
+
+  // The stack base address corresponding to |thread_handle_|.
+  const void* const thread_stack_base_address_;
+
+  // Buffer to use for copies of the stack. We use the same buffer for all the
+  // samples to avoid the overhead of multiple allocations and frees.
+  const std::unique_ptr<unsigned char[]> stack_copy_buffer_;
+
+  // Weak. Points to the modules associated with the profile being recorded
+  // between ProfileRecordingStarting() and ProfileRecordingStopped().
+  std::vector<StackSamplingProfiler::Module>* current_modules_ = nullptr;
+
+  // Maps a module's base address to the corresponding Module's index within
+  // current_modules_.
+  std::map<const void*, size_t> profile_module_index_;
+
+  DISALLOW_COPY_AND_ASSIGN(NativeStackSamplerMac);
+};
+
+NativeStackSamplerMac::NativeStackSamplerMac(
+    mach_port_t thread_port,
+    AnnotateCallback annotator,
+    NativeStackSamplerTestDelegate* test_delegate)
+    : thread_port_(thread_port),
+      annotator_(annotator),
+      test_delegate_(test_delegate),
+      thread_stack_base_address_(
+          pthread_get_stackaddr_np(pthread_from_mach_thread_np(thread_port))),
+      stack_copy_buffer_(new unsigned char[kStackCopyBufferSize]) {
+  DCHECK(annotator_);
+
+  // This class suspends threads, and those threads might be suspended in dyld.
+  // Therefore, for all the system functions that might be linked in dynamically
+  // that are used while threads are suspended, make calls to them to make sure
+  // that they are linked up.
+  ThreadContext thread_context;
+  GetThreadContext(thread_port_, &thread_context);
+}
+
+NativeStackSamplerMac::~NativeStackSamplerMac() {}
+
+void NativeStackSamplerMac::ProfileRecordingStarting(
+    std::vector<StackSamplingProfiler::Module>* modules) {
+  current_modules_ = modules;
+  profile_module_index_.clear();
+}
+
+void NativeStackSamplerMac::RecordStackSample(
+    StackSamplingProfiler::Sample* sample) {
+  DCHECK(current_modules_);
+
+  if (!stack_copy_buffer_)
+    return;
+
+  SuspendThreadAndRecordStack(sample);
+}
+
+void NativeStackSamplerMac::ProfileRecordingStopped() {
+  current_modules_ = nullptr;
+}
+
+void NativeStackSamplerMac::SuspendThreadAndRecordStack(
+    StackSamplingProfiler::Sample* sample) {
+  ThreadContext thread_context;
+
+  // Copy the stack.
+
+  {
+    // IMPORTANT NOTE: Do not do ANYTHING in this in this scope that might
+    // allocate memory, including indirectly via use of DCHECK/CHECK or other
+    // logging statements. Otherwise this code can deadlock on heap locks in the
+    // default heap acquired by the target thread before it was suspended.
+    ScopedSuspendThread suspend_thread(thread_port_);
+    if (!suspend_thread.was_successful())
+      return;
+
+    if (!GetThreadContext(thread_port_, &thread_context))
+      return;
+    uint64_t stack_top = reinterpret_cast<uint64_t>(thread_stack_base_address_);
+    uint64_t stack_bottom = thread_context.rsp;
+
+    if ((stack_top - stack_bottom) > kStackCopyBufferSize)
+      return;
+
+    (*annotator_)(sample);
+
+    CopyStackAndRewritePointers(
+        stack_copy_buffer_.get(), reinterpret_cast<void*>(stack_bottom),
+        reinterpret_cast<void*>(stack_top), &thread_context);
+  }  // ScopedSuspendThread
+
+  if (test_delegate_)
+    test_delegate_->OnPreStackWalk();
+
+  // Walk the stack and record it.
+
+  // Reserve enough memory for most stacks, to avoid repeated allocations.
+  // Approximately 99.9% of recorded stacks are 128 frames or fewer.
+  sample->frames.reserve(128);
+
+  auto current_modules = current_modules_;
+  auto profile_module_index = &profile_module_index_;
+  WalkStack(thread_context, [sample, current_modules,
+                             profile_module_index](uintptr_t frame_ip) {
+    sample->frames.push_back(StackSamplingProfiler::Frame(
+        frame_ip,
+        GetModuleIndex(frame_ip, current_modules, profile_module_index)));
+  });
+}
+
+}  // namespace
+
+std::unique_ptr<NativeStackSampler> NativeStackSampler::Create(
+    PlatformThreadId thread_id,
+    AnnotateCallback annotator,
+    NativeStackSamplerTestDelegate* test_delegate) {
+#if defined(__i386__)

[Mark Mentovai, 2017/02/17 05:21:05]

You can just make this a compile-time #error. And you can be stricter:
!defined(__x86_64__).

[Avi (use Gerrit), 2017/02/17 17:18:12]

Re compile error: the _posix version of this file returns a nullptr, and the
interface seems to specify that, so I'm going in that direction.

+  return nullptr;
+#endif
+  return base::MakeUnique<NativeStackSamplerMac>(thread_id, annotator,
+                                                 test_delegate);
+}
+
+}  // namespace base