[XRay][Docs] Add documentation for XRay in LLVM

[dberris]

Summary

This is the initial version of the documentation for how to use XRay as
it stands in LLVM, Clang, and compiler-rt. We leave some room for later
expansion mentioining what is work in progress and what could be
expected moving forward.

We also give a high level overview of future work that's both ongoing
and planned.

[dberris]

• Add XRay documentation in index.rst

[dblaikie]

Looks like a good first start

docs/XRay.rst

19

'found /in/' rather than 'found /from/'?

44

"/in/ a couple of ways"

46

Presumably Obj C++ too? Not sure we have a tidy way to write that little matrix.

55

maybe drop the language compatibility list here ("for your application" instead) since it's mentioned previously

57

I might consider omitting this sentence or making it briefer ("For example: ").

[dberris]

• Address review comments

[dberris]

Thanks @dblaikie!

docs/XRay.rst

+====================
+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 the time of this writing, XRay is only available for x86_64
+  and arm7 32-bit (no-thumb) Linux.
+
+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.
+
+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``        | ``true``      | 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.          |
++-------------------+-----------------+---------------+------------------------+
+
+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.
+
+
+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.
+
+
+Future Work
+===========
+
+There are a number of ongoing efforts for expanding the toolset building around
+the XRay instrumentation system.
+
+Flight Data Recorder Mode
+-------------------------
+
+The `XRay whitepaper`_ mentions a mode for when events are kept in memory, and
+have the traces be dumped on demand through a triggering API. This work is
+currently ongoing.
+
+Trace Analysis
+--------------
+
+There are a few more subcommands making its way to the ``llvm-xray`` tool, that
+are currently under review:
+
+- ``convert``: Turns an XRay trace from one format to another. Currently
+  supporting conversion from the binary XRay log to YAML.
+- ``account``: Do function call accounting based on data in the XRay log.
+
+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.
+- ``convert``: Converting from one version of the XRay log to another (higher)
+  version, and converting to other trace formats (i.e. Chrome Trace Viewer,
+  pprof, etc.).
+- ``graph``: Generate a function call graph with relative timings and distributions.
+
+More Platforms
+--------------
+
+Since XRay is only currently available in x86_64 and arm7 32-bit (no-thumb)
+running Linux, we're looking to supporting more platforms (architectures and
+operating systems).
+
+.. References...
+
+.. _`XRay whitepaper`: http://research.google.com/pubs/pub45287.html
+

docs/index.rst

.. toctree::
    CoverageMappingFormat
    Statepoints
    MergeFunctions
    TypeMetadata
    FaultMaps
    MIRLangRef
    Coroutines
    GlobalISel
+   XRay
 
 :doc:`WritingAnLLVMPass`
    Information on how to write LLVM transformations and analyses.
 
 :doc:`WritingAnLLVMBackend`
    Information on how to write LLVM backends for machine targets.
 
 :doc:`CodeGenerator`
:doc:`CompileCudaWithLLVM`
   LLVM support for CUDA.
 
 :doc:`Coroutines`
   LLVM support for coroutines.
 
 :doc:`GlobalISel`
   This describes the prototype instruction selection replacement, GlobalISel.
 
+:doc:`XRay`
+  High-level documentation of how to use XRay in LLVM.
+
 Development Process Documentation
 =================================
 
 Information about LLVM's development process.
 
 .. toctree::
    :hidden: