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====================
XRay Instrumentation
====================

:Version: 1 as of 2016-11-08

.. contents::
   :local:


Introduction
============

XRay is a function call tracing system which combines compiler-inserted
instrumentation points and a runtime library that can dynamically enable and
disable the instrumentation.

More high level information about XRay can be found in the `XRay whitepaper`_.

This document describes how to use XRay as implemented in LLVM.

XRay in LLVM
============

XRay consists of three main parts:

- Compiler-inserted instrumentation points.
- A runtime library for enabling/disabling tracing at runtime.
- A suite of tools for analysing the traces.

  **NOTE:** As of February 27, 2017 , XRay is only available for the following
  architectures running Linux: x86_64, arm7 (no thumb), aarch64, powerpc64le,
  mips, mipsel, mips64, mips64el.

The compiler-inserted instrumentation points come in the form of nop-sleds in
the final generated binary, and an ELF section named ``xray_instr_map`` which
contains entries pointing to these instrumentation points. The runtime library
relies on being able to access the entries of the ``xray_instr_map``, and
overwrite the instrumentation points at runtime.

Using XRay
==========

You can use XRay in a couple of ways:

- Instrumenting your C/C++/Objective-C/Objective-C++ application.
- Generating LLVM IR with the correct function attributes.

The rest of this section covers these main ways and later on how to customise
what XRay does in an XRay-instrumented binary.

Instrumenting your C/C++/Objective-C Application
------------------------------------------------

The easiest way of getting XRay instrumentation for your application is by
enabling the ``-fxray-instrument`` flag in your clang invocation.

For example:

::

  clang -fxray-instrument ..

By default, functions that have at least 200 instructions will get XRay
instrumentation points. You can tweak that number through the
``-fxray-instruction-threshold=`` flag:

::

  clang -fxray-instrument -fxray-instruction-threshold=1 ..

You can also specifically instrument functions in your binary to either always
or never be instrumented using source-level attributes. You can do it using the
GCC-style attributes or C++11-style attributes.

.. code-block:: c++

    [[clang::xray_always_intrument]] void always_instrumented();

    [[clang::xray_never_instrument]] void never_instrumented();

    void alt_always_instrumented() __attribute__((xray_always_intrument));

    void alt_never_instrumented() __attribute__((xray_never_instrument));

When linking a binary, you can either manually link in the `XRay Runtime
Library`_ or use ``clang`` to link it in automatically with the
``-fxray-instrument`` flag. Alternatively, you can statically link-in the XRay
runtime library from compiler-rt -- those archive files will take the name of
`libclang_rt.xray-{arch}` where `{arch}` is the mnemonic supported by clang
(x86_64, arm7, etc.).

LLVM Function Attribute
-----------------------

If you're using LLVM IR directly, you can add the ``function-instrument``
string attribute to your functions, to get the similar effect that the
C/C++/Objective-C source-level attributes would get:

.. code-block:: llvm

    define i32 @always_instrument() uwtable "function-instrument"="xray-always" {
      ; ...
    }

    define i32 @never_instrument() uwtable "function-instrument"="xray-never" {
      ; ...
    }

You can also set the ``xray-instruction-threshold`` attribute and provide a
numeric string value for how many instructions should be in the function before
it gets instrumented.

.. code-block:: llvm

    define i32 @maybe_instrument() uwtable "xray-instruction-threshold"="2" {
      ; ...
    }

XRay Runtime Library
--------------------

The XRay Runtime Library is part of the compiler-rt project, which implements
the runtime components that perform the patching and unpatching of inserted
instrumentation points. When you use ``clang`` to link your binaries and the
``-fxray-instrument`` flag, it will automatically link in the XRay runtime.

The default implementation of the XRay runtime will enable XRay instrumentation
before ``main`` starts, which works for applications that have a short
lifetime. This implementation also records all function entry and exit events
which may result in a lot of records in the resulting trace.

Also by default the filename of the XRay trace is ``xray-log.XXXXXX`` where the
``XXXXXX`` part is randomly generated.

These options can be controlled through the ``XRAY_OPTIONS`` environment
variable, where we list down the options and their defaults below.

+-------------------+-----------------+---------------+------------------------+
| Option            | Type            | Default       | Description            |
+===================+=================+===============+========================+
| patch_premain     | ``bool``        | ``false``     | Whether to patch       |
|                   |                 |               | instrumentation points |
|                   |                 |               | before main.           |
+-------------------+-----------------+---------------+------------------------+
| xray_naive_log    | ``bool``        | ``true``      | Whether to install     |
|                   |                 |               | the naive log          |
|                   |                 |               | implementation.        |
+-------------------+-----------------+---------------+------------------------+
| xray_logfile_base | ``const char*`` | ``xray-log.`` | Filename base for the  |
|                   |                 |               | XRay logfile.          |
+-------------------+-----------------+---------------+------------------------+
| xray_fdr_log      | ``bool``        | ``false``     | Whether to install the |
|                   |                 |               | Flight Data Recorder   |
|                   |                 |               | (FDR) mode.            |
+-------------------+-----------------+---------------+------------------------+


If you choose to not use the default logging implementation that comes with the
XRay runtime and/or control when/how the XRay instrumentation runs, you may use
the XRay APIs directly for doing so. To do this, you'll need to include the
``xray_interface.h`` from the compiler-rt ``xray`` directory. The important API
functions we list below:

- ``__xray_set_handler(void (*entry)(int32_t, XRayEntryType))``: Install your
  own logging handler for when an event is encountered. See
  ``xray/xray_interface.h`` for more details.
- ``__xray_remove_handler()``: Removes whatever the installed handler is.
- ``__xray_patch()``: Patch all the instrumentation points defined in the
  binary.
- ``__xray_unpatch()``: Unpatch the instrumentation points defined in the
  binary.

There are some requirements on the logging handler to be installed for the
thread-safety of operations to be performed by the XRay runtime library:

- The function should be thread-safe, as multiple threads may be invoking the
  function at the same time. If the logging function needs to do
  synchronisation, it must do so internally as XRay does not provide any
  synchronisation guarantees outside from the atomicity of updates to the
  pointer.
- The pointer provided to ``__xray_set_handler(...)`` must be live even after
  calls to ``__xray_remove_handler()`` and ``__xray_unpatch()`` have succeeded.
  XRay cannot guarantee that all threads that have ever gotten a copy of the
  pointer will not invoke the function.

Flight Data Recorder Mode
-------------------------

XRay supports a logging mode which allows the application to only capture a
fixed amount of memory's worth of events. Flight Data Recorder (FDR) mode works
very much like a plane's "black box" which keeps recording data to memory in a
fixed-size circular queue of buffers, and have the data available
programmatically until the buffers are finalized and flushed. To use FDR mode
on your application, you may set the ``xray_fdr_log`` option to ``true`` in the
``XRAY_OPTIONS`` environment variable (while also optionally setting the
``xray_naive_log`` to ``false``).

When FDR mode is on, it will keep writing and recycling memory buffers until
the logging implementation is finalized -- at which point it can be flushed and
re-initialised later. To do this programmatically, we follow the workflow
provided below:

.. code-block:: c++

  // Patch the sleds, if we haven't yet.
  auto patch_status = __xray_patch();

  // Maybe handle the patch_status errors.

  // When we want to flush the log, we need to finalize it first, to give
  // threads a chance to return buffers to the queue.
  auto finalize_status = __xray_log_finalize();
  if (finalize_status != XRAY_LOG_FINALIZED) {
    // maybe retry, or bail out.
  }

  // At this point, we are sure that the log is finalized, so we may try
  // flushing the log.
  auto flush_status = __xray_log_flushLog();
  if (flush_status != XRAY_LOG_FLUSHED) {
    // maybe retry, or bail out.
  }

The default settings for the FDR mode implementation will create logs named
similarly to the naive log implementation, but will have a different log
format. All the trace analysis tools (and the trace reading library) will
support all versions of the FDR mode format as we add more functionality and
record types in the future.

  **NOTE:** We do not however promise perpetual support for when we update the
  log versions we support going forward. Deprecation of the formats will be
  announced and discussed on the developers mailing list.

XRay allows for replacing the default FDR mode logging implementation using the
following API:

- ``__xray_set_log_impl(...)``: This function takes a struct of type
  ``XRayLogImpl``, which is defined in ``xray/xray_log_interface.h``, part of
  the XRay compiler-rt installation.
- ``__xray_log_init(...)``: This function allows for initializing and
  re-initializing an installed logging implementation. See
  ``xray/xray_log_interface.h`` for details, part of the XRay compiler-rt
  installation.

Trace Analysis Tools
--------------------

We currently have the beginnings of a trace analysis tool in LLVM, which can be
found in the ``tools/llvm-xray`` directory. The ``llvm-xray`` tool currently
supports the following subcommands:

- ``extract``: Extract the instrumentation map from a binary, and return it as
  YAML.
- ``account``: Performs basic function call accounting statistics with various
  options for sorting, and output formats (supports CSV, YAML, and
  console-friendly TEXT).
- ``convert``: Converts an XRay log file from one format to another. Currently
  only converts to YAML.
- ``graph``: Generates a DOT graph of the function call relationships between
  functions found in an XRay trace.

These subcommands use various library components found as part of the XRay
libraries, distributed with the LLVM distribution. These are:

- ``llvm/XRay/Trace.h`` : A trace reading library for conveniently loading
  an XRay trace of supported forms, into a convenient in-memory representation.
  All the analysis tools that deal with traces use this implementation.
- ``llvm/XRay/Graph.h`` : A semi-generic graph type used by the graph
  subcommand to conveniently represent a function call graph with statistics
  associated with edges and vertices.
- ``llvm/XRay/InstrumentationMap.h``: A convenient tool for analyzing the
  instrumentation map in XRay-instrumented object files and binaries. The
  ``extract`` subcommand uses this particular library.

Future Work
===========

There are a number of ongoing efforts for expanding the toolset building around
the XRay instrumentation system.

Trace Analysis
--------------

We have more subcommands and modes that we're thinking of developing, in the
following forms:

- ``stack``: Reconstruct the function call stacks in a timeline.

More Platforms
--------------

We're looking forward to contributions to port XRay to more architectures and
operating systems.

.. References...

.. _`XRay whitepaper`: http://research.google.com/pubs/pub45287.html