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<div class="section" id="xray-instrumentation">
<h1>XRay Instrumentation<a class="headerlink" href="#xray-instrumentation" title="Permalink to this headline">¶</a></h1>
<table class="docutils field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field-odd field"><th class="field-name">Version:</th><td class="field-body">1 as of 2016-11-08</td>
</tr>
</tbody>
</table>
<div class="contents local topic" id="contents">
<ul class="simple">
<li><a class="reference internal" href="#introduction" id="id2">Introduction</a></li>
<li><a class="reference internal" href="#xray-in-llvm" id="id3">XRay in LLVM</a></li>
<li><a class="reference internal" href="#using-xray" id="id4">Using XRay</a><ul>
<li><a class="reference internal" href="#instrumenting-your-c-c-objective-c-application" id="id5">Instrumenting your C/C++/Objective-C Application</a></li>
<li><a class="reference internal" href="#llvm-function-attribute" id="id6">LLVM Function Attribute</a></li>
<li><a class="reference internal" href="#special-case-file" id="id7">Special Case File</a></li>
<li><a class="reference internal" href="#xray-runtime-library" id="id8">XRay Runtime Library</a></li>
<li><a class="reference internal" href="#basic-mode" id="id9">Basic Mode</a></li>
<li><a class="reference internal" href="#flight-data-recorder-mode" id="id10">Flight Data Recorder Mode</a></li>
<li><a class="reference internal" href="#trace-analysis-tools" id="id11">Trace Analysis Tools</a></li>
</ul>
</li>
<li><a class="reference internal" href="#future-work" id="id12">Future Work</a><ul>
<li><a class="reference internal" href="#id1" id="id13">Trace Analysis Tools</a></li>
<li><a class="reference internal" href="#more-platforms" id="id14">More Platforms</a></li>
</ul>
</li>
</ul>
</div>
<div class="section" id="introduction">
<h2><a class="toc-backref" href="#id2">Introduction</a><a class="headerlink" href="#introduction" title="Permalink to this headline">¶</a></h2>
<p>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.</p>
<p>More high level information about XRay can be found in the <a class="reference external" href="http://research.google.com/pubs/pub45287.html">XRay whitepaper</a>.</p>
<p>This document describes how to use XRay as implemented in LLVM.</p>
</div>
<div class="section" id="xray-in-llvm">
<h2><a class="toc-backref" href="#id3">XRay in LLVM</a><a class="headerlink" href="#xray-in-llvm" title="Permalink to this headline">¶</a></h2>
<p>XRay consists of three main parts:</p>
<ul>
<li><p class="first">Compiler-inserted instrumentation points.</p>
</li>
<li><p class="first">A runtime library for enabling/disabling tracing at runtime.</p>
</li>
<li><p class="first">A suite of tools for analysing the traces.</p>
<p><strong>NOTE:</strong> As of July 25, 2018 , XRay is only available for the following
architectures running Linux: x86_64, arm7 (no thumb), aarch64, powerpc64le,
mips, mipsel, mips64, mips64el, NetBSD: x86_64, FreeBSD: x86_64 and
OpenBSD: x86_64.</p>
</li>
</ul>
<p>The compiler-inserted instrumentation points come in the form of nop-sleds in
the final generated binary, and an ELF section named <code class="docutils literal notranslate"><span class="pre">xray_instr_map</span></code> which
contains entries pointing to these instrumentation points. The runtime library
relies on being able to access the entries of the <code class="docutils literal notranslate"><span class="pre">xray_instr_map</span></code>, and
overwrite the instrumentation points at runtime.</p>
</div>
<div class="section" id="using-xray">
<h2><a class="toc-backref" href="#id4">Using XRay</a><a class="headerlink" href="#using-xray" title="Permalink to this headline">¶</a></h2>
<p>You can use XRay in a couple of ways:</p>
<ul class="simple">
<li>Instrumenting your C/C++/Objective-C/Objective-C++ application.</li>
<li>Generating LLVM IR with the correct function attributes.</li>
</ul>
<p>The rest of this section covers these main ways and later on how to customise
what XRay does in an XRay-instrumented binary.</p>
<div class="section" id="instrumenting-your-c-c-objective-c-application">
<h3><a class="toc-backref" href="#id5">Instrumenting your C/C++/Objective-C Application</a><a class="headerlink" href="#instrumenting-your-c-c-objective-c-application" title="Permalink to this headline">¶</a></h3>
<p>The easiest way of getting XRay instrumentation for your application is by
enabling the <code class="docutils literal notranslate"><span class="pre">-fxray-instrument</span></code> flag in your clang invocation.</p>
<p>For example:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">clang</span> <span class="o">-</span><span class="n">fxray</span><span class="o">-</span><span class="n">instrument</span> <span class="o">...</span>
</pre></div>
</div>
<p>By default, functions that have at least 200 instructions will get XRay
instrumentation points. You can tweak that number through the
<code class="docutils literal notranslate"><span class="pre">-fxray-instruction-threshold=</span></code> flag:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">clang</span> <span class="o">-</span><span class="n">fxray</span><span class="o">-</span><span class="n">instrument</span> <span class="o">-</span><span class="n">fxray</span><span class="o">-</span><span class="n">instruction</span><span class="o">-</span><span class="n">threshold</span><span class="o">=</span><span class="mi">1</span> <span class="o">...</span>
</pre></div>
</div>
<p>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.</p>
<div class="highlight-c++ notranslate"><div class="highlight"><pre><span></span><span class="p">[[</span><span class="n">clang</span><span class="o">::</span><span class="n">xray_always_instrument</span><span class="p">]]</span> <span class="kt">void</span> <span class="n">always_instrumented</span><span class="p">();</span>
<span class="p">[[</span><span class="n">clang</span><span class="o">::</span><span class="n">xray_never_instrument</span><span class="p">]]</span> <span class="kt">void</span> <span class="n">never_instrumented</span><span class="p">();</span>
<span class="kt">void</span> <span class="nf">alt_always_instrumented</span><span class="p">()</span> <span class="n">__attribute__</span><span class="p">((</span><span class="n">xray_always_instrument</span><span class="p">));</span>
<span class="kt">void</span> <span class="nf">alt_never_instrumented</span><span class="p">()</span> <span class="n">__attribute__</span><span class="p">((</span><span class="n">xray_never_instrument</span><span class="p">));</span>
</pre></div>
</div>
<p>When linking a binary, you can either manually link in the <a class="reference internal" href="#xray-runtime-library">XRay Runtime
Library</a> or use <code class="docutils literal notranslate"><span class="pre">clang</span></code> to link it in automatically with the
<code class="docutils literal notranslate"><span class="pre">-fxray-instrument</span></code> flag. Alternatively, you can statically link-in the XRay
runtime library from compiler-rt – those archive files will take the name of
<cite>libclang_rt.xray-{arch}</cite> where <cite>{arch}</cite> is the mnemonic supported by clang
(x86_64, arm7, etc.).</p>
</div>
<div class="section" id="llvm-function-attribute">
<h3><a class="toc-backref" href="#id6">LLVM Function Attribute</a><a class="headerlink" href="#llvm-function-attribute" title="Permalink to this headline">¶</a></h3>
<p>If you’re using LLVM IR directly, you can add the <code class="docutils literal notranslate"><span class="pre">function-instrument</span></code>
string attribute to your functions, to get the similar effect that the
C/C++/Objective-C source-level attributes would get:</p>
<div class="highlight-llvm notranslate"><div class="highlight"><pre><span></span><span class="k">define</span> <span class="k">i32</span> <span class="vg">@always_instrument</span><span class="p">()</span> <span class="k">uwtable</span> <span class="s">"function-instrument"</span><span class="p">=</span><span class="s">"xray-always"</span> <span class="p">{</span>
<span class="c">; ...</span>
<span class="p">}</span>
<span class="k">define</span> <span class="k">i32</span> <span class="vg">@never_instrument</span><span class="p">()</span> <span class="k">uwtable</span> <span class="s">"function-instrument"</span><span class="p">=</span><span class="s">"xray-never"</span> <span class="p">{</span>
<span class="c">; ...</span>
<span class="p">}</span>
</pre></div>
</div>
<p>You can also set the <code class="docutils literal notranslate"><span class="pre">xray-instruction-threshold</span></code> attribute and provide a
numeric string value for how many instructions should be in the function before
it gets instrumented.</p>
<div class="highlight-llvm notranslate"><div class="highlight"><pre><span></span><span class="k">define</span> <span class="k">i32</span> <span class="vg">@maybe_instrument</span><span class="p">()</span> <span class="k">uwtable</span> <span class="s">"xray-instruction-threshold"</span><span class="p">=</span><span class="s">"2"</span> <span class="p">{</span>
<span class="c">; ...</span>
<span class="p">}</span>
</pre></div>
</div>
</div>
<div class="section" id="special-case-file">
<h3><a class="toc-backref" href="#id7">Special Case File</a><a class="headerlink" href="#special-case-file" title="Permalink to this headline">¶</a></h3>
<p>Attributes can be imbued through the use of special case files instead of
adding them to the original source files. You can use this to mark certain
functions and classes to be never, always, or instrumented with first-argument
logging from a file. The file’s format is described below:</p>
<div class="highlight-bash notranslate"><div class="highlight"><pre><span></span><span class="c1"># Comments are supported</span>
<span class="o">[</span>always<span class="o">]</span>
fun:always_instrument
fun:log_arg1<span class="o">=</span>arg1 <span class="c1"># Log the first argument for the function</span>
<span class="o">[</span>never<span class="o">]</span>
fun:never_instrument
</pre></div>
</div>
<p>These files can be provided through the <code class="docutils literal notranslate"><span class="pre">-fxray-attr-list=</span></code> flag to clang.
You may have multiple files loaded through multiple instances of the flag.</p>
</div>
<div class="section" id="xray-runtime-library">
<h3><a class="toc-backref" href="#id8">XRay Runtime Library</a><a class="headerlink" href="#xray-runtime-library" title="Permalink to this headline">¶</a></h3>
<p>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 <code class="docutils literal notranslate"><span class="pre">clang</span></code> to link your binaries and the
<code class="docutils literal notranslate"><span class="pre">-fxray-instrument</span></code> flag, it will automatically link in the XRay runtime.</p>
<p>The default implementation of the XRay runtime will enable XRay instrumentation
before <code class="docutils literal notranslate"><span class="pre">main</span></code> 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.</p>
<p>Also by default the filename of the XRay trace is <code class="docutils literal notranslate"><span class="pre">xray-log.XXXXXX</span></code> where the
<code class="docutils literal notranslate"><span class="pre">XXXXXX</span></code> part is randomly generated.</p>
<p>These options can be controlled through the <code class="docutils literal notranslate"><span class="pre">XRAY_OPTIONS</span></code> environment
variable, where we list down the options and their defaults below.</p>
<table border="1" class="docutils">
<colgroup>
<col width="25%" />
<col width="23%" />
<col width="20%" />
<col width="32%" />
</colgroup>
<thead valign="bottom">
<tr class="row-odd"><th class="head">Option</th>
<th class="head">Type</th>
<th class="head">Default</th>
<th class="head">Description</th>
</tr>
</thead>
<tbody valign="top">
<tr class="row-even"><td>patch_premain</td>
<td><code class="docutils literal notranslate"><span class="pre">bool</span></code></td>
<td><code class="docutils literal notranslate"><span class="pre">false</span></code></td>
<td>Whether to patch
instrumentation points
before main.</td>
</tr>
<tr class="row-odd"><td>xray_mode</td>
<td><code class="docutils literal notranslate"><span class="pre">const</span> <span class="pre">char*</span></code></td>
<td><code class="docutils literal notranslate"><span class="pre">""</span></code></td>
<td>Default mode to
install and initialize
before <code class="docutils literal notranslate"><span class="pre">main</span></code>.</td>
</tr>
<tr class="row-even"><td>xray_logfile_base</td>
<td><code class="docutils literal notranslate"><span class="pre">const</span> <span class="pre">char*</span></code></td>
<td><code class="docutils literal notranslate"><span class="pre">xray-log.</span></code></td>
<td>Filename base for the
XRay logfile.</td>
</tr>
<tr class="row-odd"><td>verbosity</td>
<td><code class="docutils literal notranslate"><span class="pre">int</span></code></td>
<td><code class="docutils literal notranslate"><span class="pre">0</span></code></td>
<td>Runtime verbosity
level.</td>
</tr>
</tbody>
</table>
<p>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
<code class="docutils literal notranslate"><span class="pre">xray_log_interface.h</span></code> from the compiler-rt <code class="docutils literal notranslate"><span class="pre">xray</span></code> directory. The important API
functions we list below:</p>
<ul class="simple">
<li><code class="docutils literal notranslate"><span class="pre">__xray_log_register_mode(...)</span></code>: Register a logging implementation against
a string Mode identifier. The implementation is an instance of
<code class="docutils literal notranslate"><span class="pre">XRayLogImpl</span></code> defined in <code class="docutils literal notranslate"><span class="pre">xray/xray_log_interface.h</span></code>.</li>
<li><code class="docutils literal notranslate"><span class="pre">__xray_log_select_mode(...)</span></code>: Select the mode to install, associated with
a string Mode identifier. Only implementations registered with
<code class="docutils literal notranslate"><span class="pre">__xray_log_register_mode(...)</span></code> can be chosen with this function.</li>
<li><code class="docutils literal notranslate"><span class="pre">__xray_log_init_mode(...)</span></code>: This function allows for initializing and
re-initializing an installed logging implementation. See
<code class="docutils literal notranslate"><span class="pre">xray/xray_log_interface.h</span></code> for details, part of the XRay compiler-rt
installation.</li>
</ul>
<p>Once a logging implementation has been initialized, it can be “stopped” by
finalizing the implementation through the <code class="docutils literal notranslate"><span class="pre">__xray_log_finalize()</span></code> function.
The finalization routine is the opposite of the initialization. When finalized,
an implementation’s data can be cleared out through the
<code class="docutils literal notranslate"><span class="pre">__xray_log_flushLog()</span></code> function. For implementations that support in-memory
processing, these should register an iterator function to provide access to the
data via the <code class="docutils literal notranslate"><span class="pre">__xray_log_set_buffer_iterator(...)</span></code> which allows code calling
the <code class="docutils literal notranslate"><span class="pre">__xray_log_process_buffers(...)</span></code> function to deal with the data in
memory.</p>
<p>All of this is better explained in the <code class="docutils literal notranslate"><span class="pre">xray/xray_log_interface.h</span></code> header.</p>
</div>
<div class="section" id="basic-mode">
<h3><a class="toc-backref" href="#id9">Basic Mode</a><a class="headerlink" href="#basic-mode" title="Permalink to this headline">¶</a></h3>
<p>XRay supports a basic logging mode which will trace the application’s
execution, and periodically append to a single log. This mode can be
installed/enabled by setting <code class="docutils literal notranslate"><span class="pre">xray_mode=xray-basic</span></code> in the <code class="docutils literal notranslate"><span class="pre">XRAY_OPTIONS</span></code>
environment variable. Combined with <code class="docutils literal notranslate"><span class="pre">patch_premain=true</span></code> this can allow for
tracing applications from start to end.</p>
<p>Like all the other modes installed through <code class="docutils literal notranslate"><span class="pre">__xray_log_select_mode(...)</span></code>, the
implementation can be configured through the <code class="docutils literal notranslate"><span class="pre">__xray_log_init_mode(...)</span></code>
function, providing the mode string and the flag options. Basic-mode specific
defaults can be provided in the <code class="docutils literal notranslate"><span class="pre">XRAY_BASIC_OPTIONS</span></code> environment variable.</p>
</div>
<div class="section" id="flight-data-recorder-mode">
<h3><a class="toc-backref" href="#id10">Flight Data Recorder Mode</a><a class="headerlink" href="#flight-data-recorder-mode" title="Permalink to this headline">¶</a></h3>
<p>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 <code class="docutils literal notranslate"><span class="pre">xray_mode</span></code> variable to <code class="docutils literal notranslate"><span class="pre">xray-fdr</span></code> in
the <code class="docutils literal notranslate"><span class="pre">XRAY_OPTIONS</span></code> environment variable. Additional options to the FDR mode
implementation can be provided in the <code class="docutils literal notranslate"><span class="pre">XRAY_FDR_OPTIONS</span></code> environment
variable. Programmatic configuration can be done by calling
<code class="docutils literal notranslate"><span class="pre">__xray_log_init_mode("xray-fdr",</span> <span class="pre"><configuration</span> <span class="pre">string>)</span></code> once it has been
selected/installed.</p>
<p>When the buffers are flushed to disk, the result is a binary trace format
described by <a class="reference external" href="XRayFDRFormat.html">XRay FDR format</a></p>
<p>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:</p>
<div class="highlight-c++ notranslate"><div class="highlight"><pre><span></span><span class="c1">// Patch the sleds, if we haven't yet.</span>
<span class="k">auto</span> <span class="n">patch_status</span> <span class="o">=</span> <span class="n">__xray_patch</span><span class="p">();</span>
<span class="c1">// Maybe handle the patch_status errors.</span>
<span class="c1">// When we want to flush the log, we need to finalize it first, to give</span>
<span class="c1">// threads a chance to return buffers to the queue.</span>
<span class="k">auto</span> <span class="n">finalize_status</span> <span class="o">=</span> <span class="n">__xray_log_finalize</span><span class="p">();</span>
<span class="k">if</span> <span class="p">(</span><span class="n">finalize_status</span> <span class="o">!=</span> <span class="n">XRAY_LOG_FINALIZED</span><span class="p">)</span> <span class="p">{</span>
<span class="c1">// maybe retry, or bail out.</span>
<span class="p">}</span>
<span class="c1">// At this point, we are sure that the log is finalized, so we may try</span>
<span class="c1">// flushing the log.</span>
<span class="k">auto</span> <span class="n">flush_status</span> <span class="o">=</span> <span class="n">__xray_log_flushLog</span><span class="p">();</span>
<span class="k">if</span> <span class="p">(</span><span class="n">flush_status</span> <span class="o">!=</span> <span class="n">XRAY_LOG_FLUSHED</span><span class="p">)</span> <span class="p">{</span>
<span class="c1">// maybe retry, or bail out.</span>
<span class="p">}</span>
</pre></div>
</div>
<p>The default settings for the FDR mode implementation will create logs named
similarly to the basic 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.</p>
<blockquote>
<div><strong>NOTE:</strong> We do not 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.</div></blockquote>
</div>
<div class="section" id="trace-analysis-tools">
<h3><a class="toc-backref" href="#id11">Trace Analysis Tools</a><a class="headerlink" href="#trace-analysis-tools" title="Permalink to this headline">¶</a></h3>
<p>We currently have the beginnings of a trace analysis tool in LLVM, which can be
found in the <code class="docutils literal notranslate"><span class="pre">tools/llvm-xray</span></code> directory. The <code class="docutils literal notranslate"><span class="pre">llvm-xray</span></code> tool currently
supports the following subcommands:</p>
<ul class="simple">
<li><code class="docutils literal notranslate"><span class="pre">extract</span></code>: Extract the instrumentation map from a binary, and return it as
YAML.</li>
<li><code class="docutils literal notranslate"><span class="pre">account</span></code>: Performs basic function call accounting statistics with various
options for sorting, and output formats (supports CSV, YAML, and
console-friendly TEXT).</li>
<li><code class="docutils literal notranslate"><span class="pre">convert</span></code>: Converts an XRay log file from one format to another. We can
convert from binary XRay traces (both basic and FDR mode) to YAML,
<a class="reference external" href="https://github.com/brendangregg/FlameGraph">flame-graph</a> friendly text
formats, as well as <cite>Chrome Trace Viewer (catapult)
<https://github.com/catapult-project/catapult></cite> formats.</li>
<li><code class="docutils literal notranslate"><span class="pre">graph</span></code>: Generates a DOT graph of the function call relationships between
functions found in an XRay trace.</li>
<li><code class="docutils literal notranslate"><span class="pre">stack</span></code>: Reconstructs function call stacks from a timeline of function
calls in an XRay trace.</li>
</ul>
<p>These subcommands use various library components found as part of the XRay
libraries, distributed with the LLVM distribution. These are:</p>
<ul class="simple">
<li><code class="docutils literal notranslate"><span class="pre">llvm/XRay/Trace.h</span></code> : 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.</li>
<li><code class="docutils literal notranslate"><span class="pre">llvm/XRay/Graph.h</span></code> : A semi-generic graph type used by the graph
subcommand to conveniently represent a function call graph with statistics
associated with edges and vertices.</li>
<li><code class="docutils literal notranslate"><span class="pre">llvm/XRay/InstrumentationMap.h</span></code>: A convenient tool for analyzing the
instrumentation map in XRay-instrumented object files and binaries. The
<code class="docutils literal notranslate"><span class="pre">extract</span></code> and <code class="docutils literal notranslate"><span class="pre">stack</span></code> subcommands uses this particular library.</li>
</ul>
</div>
</div>
<div class="section" id="future-work">
<h2><a class="toc-backref" href="#id12">Future Work</a><a class="headerlink" href="#future-work" title="Permalink to this headline">¶</a></h2>
<p>There are a number of ongoing efforts for expanding the toolset building around
the XRay instrumentation system.</p>
<div class="section" id="id1">
<h3><a class="toc-backref" href="#id13">Trace Analysis Tools</a><a class="headerlink" href="#id1" title="Permalink to this headline">¶</a></h3>
<ul class="simple">
<li>Work is in progress to integrate with or develop tools to visualize findings
from an XRay trace. Particularly, the <code class="docutils literal notranslate"><span class="pre">stack</span></code> tool is being expanded to
output formats that allow graphing and exploring the duration of time in each
call stack.</li>
<li>With a large instrumented binary, the size of generated XRay traces can
quickly become unwieldy. We are working on integrating pruning techniques and
heuristics for the analysis tools to sift through the traces and surface only
relevant information.</li>
</ul>
</div>
<div class="section" id="more-platforms">
<h3><a class="toc-backref" href="#id14">More Platforms</a><a class="headerlink" href="#more-platforms" title="Permalink to this headline">¶</a></h3>
<p>We’re looking forward to contributions to port XRay to more architectures and
operating systems.</p>
</div>
</div>
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