Debugging with XRay — LLVM 12 documentation

# 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.

- Building with XRay

- Getting Traces

- The `llvm-xray` Tool

- Controlling Fidelity

  - Instruction Threshold

  - Instrumentation Attributes

- The XRay stack tool

- Flame Graph Generation

- Chrome Trace Viewer Visualization

- Further Exploration

- Next Steps

## 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 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 run `llc` it should work like a normally built binary. 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 xray_mode=xray-basic verbosity=1" ./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 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 output 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:

    # xray-attr-list.txt
    # always instrument functions that match the following filters:
    [always]
    fun:main

    # never instrument functions that match the following filters:
    [never]
    fun:__cxx_*

Given the file above we can re-build by providing it to the `-fxray-attr-list=` flag to clang. You can have multiple files, each defining different sets of attribute sets, to be combined into a single list by clang.

## The XRay stack tool

Given a trace, and optionally an instrumentation map, the `llvm-xray stack` command can be used to analyze a call stack graph constructed from the function call timeline.

The way to use the command is to output the top stacks by call count and time spent.

    $ llvm-xray stack xray-log.llc.5rqxkU -instr_map ./bin/llc

    Unique Stacks: 3069
    Top 10 Stacks by leaf sum:

    Sum: 9633790
    lvl   function                                                            count              sum
    #0    main                                                                    1         58421550
    #1    compileModule(char**, llvm::LLVMContext&)                               1         51440360
    #2    llvm::legacy::PassManagerImpl::run(llvm::Module&)                       1         40535375
    #3    llvm::FPPassManager::runOnModule(llvm::Module&)                         2         39337525
    #4    llvm::FPPassManager::runOnFunction(llvm::Function&)                     6         39331465
    #5    llvm::PMDataManager::verifyPreservedAnalysis(llvm::Pass*)             399         16628590
    #6    llvm::PMTopLevelManager::findAnalysisPass(void const*)               4584         15155600
    #7    llvm::PMDataManager::findAnalysisPass(void const*, bool)            32088          9633790

    ..etc..

In the default mode, identical stacks on different threads are independently aggregated. In a multithreaded program, you may end up having identical call stacks fill your list of top calls.

To address this, you may specify the `-aggregate-threads` or `-per-thread-stacks` flags. `-per-thread-stacks` treats the thread id as an implicit root in each call stack tree, while `-aggregate-threads` combines identical stacks from all threads.

## Flame Graph Generation

The `llvm-xray stack` tool may also be used to generate flamegraphs for visualizing your instrumented invocations. The tool does not generate the graphs themselves, but instead generates a format that can be used with Brendan Gregg’s FlameGraph tool, currently available on github.

To generate output for a flamegraph, a few more options are necessary.

- `-all-stacks` - Emits all of the stacks.

- `-stack-format` - Choose the flamegraph output format ‘flame’.

- `-aggregation-type` - Choose the metric to graph.

You may pipe the command output directly to the flamegraph tool to obtain an svg file.

    $llvm-xray stack xray-log.llc.5rqxkU -instr_map ./bin/llc -stack-format=flame -aggregation-type=time -all-stacks | \
    /path/to/FlameGraph/flamegraph.pl > flamegraph.svg

If you open the svg in a browser, mouse events allow exploring the call stacks.

## Chrome Trace Viewer Visualization

We can also generate a trace which can be loaded by the Chrome Trace Viewer from the same generated trace:

    $ llvm-xray convert -symbolize -instr_map=./bin/llc \
      -output-format=trace_event xray-log.llc.5rqxkU \
        | gzip > llc-trace.txt.gz

From a Chrome browser, navigating to `chrome:///tracing` allows us to load the `sample-trace.txt.gz` file to visualize the execution trace.

## 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.

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

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

    [[clang::xray_always_instrument]] 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 xray_mode=xray-basic" ./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.

> - Collecting function call stacks and how often they’re encountered in the XRay trace.