llvm/docs/XRayExample.rst

===================
Debugging with XRay
===================

This document shows an example of how you would go about analyzing applications
built with XRay instrumentation. Here we will attempt to debug ``llc``
compiling some sample LLVM IR generated by Clang.

.. contents::
  :local:

Building with XRay
------------------

To debug an application with XRay instrumentation, we need to build it with a
Clang that supports the ``-fxray-instrument`` option. See `XRay
<http://llvm.org/docs/XRay.html` for more technical details of how XRay works
for background information.

In our example, we need to add ``-fxray-instrument`` to the list of flags
passed to Clang when building a binary. Note that we need to link with Clang as
well to get the XRay runtime linked in appropriately. For building ``llc`` with
XRay, we do something similar below for our LLVM build:

::

  $ mkdir -p llvm-build && cd llvm-build
  # Assume that the LLVM sources are at ../llvm
  $ cmake -GNinja ../llvm -DCMAKE_BUILD_TYPE=Release \
      -DCMAKE_C_FLAGS_RELEASE="-fxray-instrument" -DCMAKE_CXX_FLAGS="-fxray-instrument" \
  # Once this finishes, we should build llc
  $ ninja llc


To verify that we have an XRay instrumented binary, we can use ``objdump`` to
look for the ``xray_instr_map`` section.

::

  $ objdump -h -j xray_instr_map ./bin/llc
  ./bin/llc:     file format elf64-x86-64
  
  Sections:
  Idx Name          Size      VMA               LMA               File off  Algn
   14 xray_instr_map 00002fc0  00000000041516c6  00000000041516c6  03d516c6  2**0
                    CONTENTS, ALLOC, LOAD, READONLY, DATA

Getting Traces
--------------

By default, XRay does not write out the trace files or patch the application
before main starts. If we just run ``llc`` it should just work like a normally
built binary. However, if we want to get a full trace of the application's
operations (of the functions we do end up instrumenting with XRay) then we need
to enable XRay at application start. To do this, XRay checks the
``XRAY_OPTIONS`` environment variable.

::

  # The following doesn't create an XRay trace by default.
  $ ./bin/llc input.ll

  # We need to set the XRAY_OPTIONS to enable some features.
  $ XRAY_OPTIONS="patch_premain=true" ./bin/llc input.ll
  ==69819==XRay: Log file in 'xray-log.llc.m35qPB'

At this point we now have an XRay trace we can start analysing.

The ``llvm-xray`` Tool
----------------------

Having a trace then allows us to do basic accounting of the functions that were
instrumented, and how much time we're spending in parts of the code. To make
sense of this data, we use the ``llvm-xray`` tool which has a few subcommands
to help us understand our trace.

One of the simplest things we can do is to get an accounting of the functions
that have been instrumented. We can see an example accounting with ``llvm-xray
account``:

::

  $ llvm-xray account xray-log.llc.m35qPB -top=10 -sort=sum -sortorder=dsc -instr_map ./bin/llc
  Functions with latencies: 29
     funcid      count [      min,       med,       90p,       99p,       max]       sum  function
        187        360 [ 0.000000,  0.000001,  0.000014,  0.000032,  0.000075]  0.001596  LLLexer.cpp:446:0: llvm::LLLexer::LexIdentifier()
         85        130 [ 0.000000,  0.000000,  0.000018,  0.000023,  0.000156]  0.000799  X86ISelDAGToDAG.cpp:1984:0: (anonymous namespace)::X86DAGToDAGISel::Select(llvm::SDNode*)
        138        130 [ 0.000000,  0.000000,  0.000017,  0.000155,  0.000155]  0.000774  SelectionDAGISel.cpp:2963:0: llvm::SelectionDAGISel::SelectCodeCommon(llvm::SDNode*, unsigned char const*, unsigned int)
        188        103 [ 0.000000,  0.000000,  0.000003,  0.000123,  0.000214]  0.000737  LLParser.cpp:2692:0: llvm::LLParser::ParseValID(llvm::ValID&, llvm::LLParser::PerFunctionState*)
         88          1 [ 0.000562,  0.000562,  0.000562,  0.000562,  0.000562]  0.000562  X86ISelLowering.cpp:83:0: llvm::X86TargetLowering::X86TargetLowering(llvm::X86TargetMachine const&, llvm::X86Subtarget const&)
        125        102 [ 0.000001,  0.000003,  0.000010,  0.000017,  0.000049]  0.000471  Verifier.cpp:3714:0: (anonymous namespace)::Verifier::visitInstruction(llvm::Instruction&)
         90          8 [ 0.000023,  0.000035,  0.000106,  0.000106,  0.000106]  0.000342  X86ISelLowering.cpp:3363:0: llvm::X86TargetLowering::LowerCall(llvm::TargetLowering::CallLoweringInfo&, llvm::SmallVectorImpl<llvm::SDValue>&) const
        124         32 [ 0.000003,  0.000007,  0.000016,  0.000041,  0.000041]  0.000310  Verifier.cpp:1967:0: (anonymous namespace)::Verifier::visitFunction(llvm::Function const&)
        123          1 [ 0.000302,  0.000302,  0.000302,  0.000302,  0.000302]  0.000302  LLVMContextImpl.cpp:54:0: llvm::LLVMContextImpl::~LLVMContextImpl()
        139         46 [ 0.000000,  0.000002,  0.000006,  0.000008,  0.000019]  0.000138  TargetLowering.cpp:506:0: llvm::TargetLowering::SimplifyDemandedBits(llvm::SDValue, llvm::APInt const&, llvm::APInt&, llvm::APInt&, llvm::TargetLowering::TargetLoweringOpt&, unsigned int, bool) const

This shows us that for our input file, ``llc`` spent the most cumulative time
in the lexer (a total of 1 millisecond). If we wanted for example to work with
this data in a spreadsheet, we can output the results as CSV using the
``-format=csv`` option to the command for further analysis.

If we want to get a textual representation of the raw trace we can use the
``llvm-xray convert`` tool to get YAML output. The first few lines of that
ouput for an example trace would look like the following:

::

  $ llvm-xray convert -f yaml -symbolize -instr_map=./bin/llc xray-log.llc.m35qPB
  ---
  header:          
    version:         1
    type:            0
    constant-tsc:    true
    nonstop-tsc:     true
    cycle-frequency: 2601000000
  records:         
    - { type: 0, func-id: 110, function: __cxx_global_var_init.8, cpu: 37, thread: 69819, kind: function-enter, tsc: 5434426023268520 }
    - { type: 0, func-id: 110, function: __cxx_global_var_init.8, cpu: 37, thread: 69819, kind: function-exit, tsc: 5434426023523052 }
    - { type: 0, func-id: 164, function: __cxx_global_var_init, cpu: 37, thread: 69819, kind: function-enter, tsc: 5434426029925386 }
    - { type: 0, func-id: 164, function: __cxx_global_var_init, cpu: 37, thread: 69819, kind: function-exit, tsc: 5434426030031128 }
    - { type: 0, func-id: 142, function: '(anonymous namespace)::CommandLineParser::ParseCommandLineOptions(int, char const* const*, llvm::StringRef, llvm::raw_ostream*)', cpu: 37, thread: 69819, kind: function-enter, tsc: 5434426046951388 }
    - { type: 0, func-id: 142, function: '(anonymous namespace)::CommandLineParser::ParseCommandLineOptions(int, char const* const*, llvm::StringRef, llvm::raw_ostream*)', cpu: 37, thread: 69819, kind: function-exit, tsc: 5434426047282020 }
    - { type: 0, func-id: 187, function: 'llvm::LLLexer::LexIdentifier()', cpu: 37, thread: 69819, kind: function-enter, tsc: 5434426047857332 }
    - { type: 0, func-id: 187, function: 'llvm::LLLexer::LexIdentifier()', cpu: 37, thread: 69819, kind: function-exit, tsc: 5434426047984152 }
    - { type: 0, func-id: 187, function: 'llvm::LLLexer::LexIdentifier()', cpu: 37, thread: 69819, kind: function-enter, tsc: 5434426048036584 }
    - { type: 0, func-id: 187, function: 'llvm::LLLexer::LexIdentifier()', cpu: 37, thread: 69819, kind: function-exit, tsc: 5434426048042292 }
    - { type: 0, func-id: 187, function: 'llvm::LLLexer::LexIdentifier()', cpu: 37, thread: 69819, kind: function-enter, tsc: 5434426048055056 }
    - { type: 0, func-id: 187, function: 'llvm::LLLexer::LexIdentifier()', cpu: 37, thread: 69819, kind: function-exit, tsc: 5434426048067316 }

Controlling Fidelity
--------------------

So far in our examples, we haven't been getting full coverage of the functions
we have in the binary. To get that, we need to modify the compiler flags so
that we can instrument more (if not all) the functions we have in the binary.
We have two options for doing that, and we explore both of these below.

Instruction Threshold
`````````````````````

The first "blunt" way of doing this is by setting the minimum threshold for
function bodies to 1. We can do that with the
``-fxray-instruction-threshold=N`` flag when building our binary. We rebuild
``llc`` with this option and observe the results:

::

  $ rm CMakeCache.txt
  $ cmake -GNinja ../llvm -DCMAKE_BUILD_TYPE=Release \
      -DCMAKE_C_FLAGS_RELEASE="-fxray-instrument -fxray-instruction-threshold=1" \
      -DCMAKE_CXX_FLAGS="-fxray-instrument -fxray-instruction-threshold=1"
  $ ninja llc
  $ XRAY_OPTIONS="patch_premain=true" ./bin/llc input.ll
  ==69819==XRay: Log file in 'xray-log.llc.5rqxkU'

  $ llvm-xray account xray-log.llc.5rqxkU -top=10 -sort=sum -sortorder=dsc -instr_map ./bin/llc
  Functions with latencies: 36652
   funcid      count [      min,       med,       90p,       99p,       max]       sum  function    
       75          1 [ 0.672368,  0.672368,  0.672368,  0.672368,  0.672368]  0.672368  llc.cpp:271:0: main
       78          1 [ 0.626455,  0.626455,  0.626455,  0.626455,  0.626455]  0.626455  llc.cpp:381:0: compileModule(char**, llvm::LLVMContext&)
   139617          1 [ 0.472618,  0.472618,  0.472618,  0.472618,  0.472618]  0.472618  LegacyPassManager.cpp:1723:0: llvm::legacy::PassManager::run(llvm::Module&)
   139610          1 [ 0.472618,  0.472618,  0.472618,  0.472618,  0.472618]  0.472618  LegacyPassManager.cpp:1681:0: llvm::legacy::PassManagerImpl::run(llvm::Module&)
   139612          1 [ 0.470948,  0.470948,  0.470948,  0.470948,  0.470948]  0.470948  LegacyPassManager.cpp:1564:0: (anonymous namespace)::MPPassManager::runOnModule(llvm::Module&)
   139607          2 [ 0.147345,  0.315994,  0.315994,  0.315994,  0.315994]  0.463340  LegacyPassManager.cpp:1530:0: llvm::FPPassManager::runOnModule(llvm::Module&)
   139605         21 [ 0.000002,  0.000002,  0.102593,  0.213336,  0.213336]  0.463331  LegacyPassManager.cpp:1491:0: llvm::FPPassManager::runOnFunction(llvm::Function&)
   139563      26096 [ 0.000002,  0.000002,  0.000037,  0.000063,  0.000215]  0.225708  LegacyPassManager.cpp:1083:0: llvm::PMDataManager::findAnalysisPass(void const*, bool)
   108055        188 [ 0.000002,  0.000120,  0.001375,  0.004523,  0.062624]  0.159279  MachineFunctionPass.cpp:38:0: llvm::MachineFunctionPass::runOnFunction(llvm::Function&)
    62635         22 [ 0.000041,  0.000046,  0.000050,  0.126744,  0.126744]  0.127715  X86TargetMachine.cpp:242:0: llvm::X86TargetMachine::getSubtargetImpl(llvm::Function const&) const


Instrumentation Attributes
``````````````````````````

The other way is to use configuration files for selecting which functions
should always be instrumented by the compiler. This gives us a way of ensuring
that certain functions are either always or never instrumented by not having to
add the attribute to the source.

To use this feature, you can define one file for the functions to always
instrument, and another for functions to never instrument. The format of these
files are exactly the same as the SanitizerLists files that control similar
things for the sanitizer implementations. For example, we can have two
different files like below:

::

  # always-instrument.txt
  # always instrument functions that match the following filters:
  fun:main

  # never-instrument.txt
  # never instrument functions that match the following filters:
  fun:__cxx_*

Given the above two files we can re-build by providing those two files as
arguments to clang as ``-fxray-always-instrument=always-instrument.txt`` or
``-fxray-never-instrument=never-instrument.txt``.

Further Exploration
-------------------

The ``llvm-xray`` tool has a few other subcommands that are in various stages
of being developed. One interesting subcommand that can highlight a few
interesting things is the ``graph`` subcommand. Given for example the following
toy program that we build with XRay instrumentation, we can see how the
generated graph may be a helpful indicator of where time is being spent for the
application.

.. code-block:: c++

  // sample.cc
  #include <iostream>
  #include <thread>

  [[clang::xray_always_intrument]] void f() {
    std::cerr << '.';
  }

  [[clang::xray_always_intrument]] void g() {
    for (int i = 0; i < 1 << 10; ++i) {
      std::cerr << '-';
    }
  }

  int main(int argc, char* argv[]) {
    std::thread t1([] {
      for (int i = 0; i < 1 << 10; ++i)
        f();
    });
    std::thread t2([] {
      g();
    });
    t1.join();
    t2.join();
    std::cerr << '\n';
  }

We then build the above with XRay instrumentation:

::

  $ clang++ -o sample -O3 sample.cc -std=c++11 -fxray-instrument -fxray-instruction-threshold=1
  $ XRAY_OPTIONS="patch_premain=true" ./sample

We can then explore the graph rendering of the trace generated by this sample
application. We assume you have the graphviz toosl available in your system,
including both ``unflatten`` and ``dot``. If you prefer rendering or exploring
the graph using another tool, then that should be feasible as well. ``llvm-xray
graph`` will create DOT format graphs which should be usable in most graph
rendering applications. One example invocation of the ``llvm-xray graph``
command should yield some interesting insights to the workings of C++
applications:

::

  $ llvm-xray graph xray-log.sample.* -m sample -color-edges=sum -edge-label=sum \
      | unflatten -f -l10 | dot -Tsvg -o sample.svg

Next Steps
----------

If you have some interesting analyses you'd like to implement as part of the
llvm-xray tool, please feel free to propose them on the llvm-dev@ mailing list.
The following are some ideas to inspire you in getting involved and potentially
making things better.

  - Implement a query/filtering library that allows for finding patterns in the
    XRay traces.
  - A conversion from the XRay trace onto something that can be visualised
    better by other tools (like the Chrome trace viewer for example).
  - Collecting function call stacks and how often they're encountered in the
    XRay trace.