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¶
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.
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" ./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.