mirror of
https://github.com/RPCS3/llvm-mirror.git
synced 2024-11-23 19:23:23 +01:00
3153e6c6d9
Summary: This change consolidates the always/never lists that may be provided to clang to externally control which functions should be XRay instrumented by imbuing attributes. The files follow the same format as defined in https://clang.llvm.org/docs/SanitizerSpecialCaseList.html for the sanitizer blacklist. We also deprecate the existing `-fxray-instrument-always=` and `-fxray-instrument-never=` flags, in favour of `-fxray-attr-list=`. This fixes http://llvm.org/PR34721. Reviewers: echristo, vlad.tsyrklevich, eugenis Reviewed By: vlad.tsyrklevich Subscribers: llvm-commits, cfe-commits Differential Revision: https://reviews.llvm.org/D45357 llvm-svn: 329543
338 lines
16 KiB
ReStructuredText
338 lines
16 KiB
ReStructuredText
===================
|
|
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 <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 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 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
|
|
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 simplest way to use the command is simply 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
|
|
<https://github.com/brendangregg/FlameGraph>`_.
|
|
|
|
To generate output for a flamegraph, a few more options are necessary.
|
|
|
|
- ``-all-stacks`` - Emits all of the stacks instead of just the top 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.
|
|
|
|
.. code-block:: c++
|
|
|
|
// 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.
|
|
|
|
|