XRay Instrumentation


1 as of 2016-11-08


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.

More high level information about XRay can be found in the XRay whitepaper.

This document describes how to use XRay as implemented in LLVM.

XRay in LLVM

XRay consists of three main parts:

  • Compiler-inserted instrumentation points.

  • A runtime library for enabling/disabling tracing at runtime.

  • A suite of tools for analysing the traces.

    NOTE: 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.

The compiler-inserted instrumentation points come in the form of nop-sleds in the final generated binary, and an ELF section named xray_instr_map which contains entries pointing to these instrumentation points. The runtime library relies on being able to access the entries of the xray_instr_map, and overwrite the instrumentation points at runtime.

Using XRay

You can use XRay in a couple of ways:

  • Instrumenting your C/C++/Objective-C/Objective-C++ application.

  • Generating LLVM IR with the correct function attributes.

The rest of this section covers these main ways and later on how to customise what XRay does in an XRay-instrumented binary.

Instrumenting your C/C++/Objective-C Application

The easiest way of getting XRay instrumentation for your application is by enabling the -fxray-instrument flag in your clang invocation.

For example:

clang -fxray-instrument ...

By default, functions that have at least 200 instructions will get XRay instrumentation points. You can tweak that number through the -fxray-instruction-threshold= flag:

clang -fxray-instrument -fxray-instruction-threshold=1 ...

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.

[[clang::xray_always_instrument]] void always_instrumented();

[[clang::xray_never_instrument]] void never_instrumented();

void alt_always_instrumented() __attribute__((xray_always_instrument));

void alt_never_instrumented() __attribute__((xray_never_instrument));

When linking a binary, you can either manually link in the XRay Runtime Library or use clang to link it in automatically with the -fxray-instrument flag. Alternatively, you can statically link-in the XRay runtime library from compiler-rt – those archive files will take the name of libclang_rt.xray-{arch} where {arch} is the mnemonic supported by clang (x86_64, arm7, etc.).

LLVM Function Attribute

If you’re using LLVM IR directly, you can add the function-instrument string attribute to your functions, to get the similar effect that the C/C++/Objective-C source-level attributes would get:

define i32 @always_instrument() uwtable "function-instrument"="xray-always" {
  ; ...

define i32 @never_instrument() uwtable "function-instrument"="xray-never" {
  ; ...

You can also set the xray-instruction-threshold attribute and provide a numeric string value for how many instructions should be in the function before it gets instrumented.

define i32 @maybe_instrument() uwtable "xray-instruction-threshold"="2" {
  ; ...

Special Case File

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:

# Comments are supported
fun:log_arg1=arg1 # Log the first argument for the function


These files can be provided through the -fxray-attr-list= flag to clang. You may have multiple files loaded through multiple instances of the flag.

XRay Runtime Library

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 clang to link your binaries and the -fxray-instrument flag, it will automatically link in the XRay runtime.

The default implementation of the XRay runtime will enable XRay instrumentation before main 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.

Also by default the filename of the XRay trace is xray-log.XXXXXX where the XXXXXX part is randomly generated.

These options can be controlled through the XRAY_OPTIONS environment variable, where we list down the options and their defaults below.








Whether to patch instrumentation points before main.


const char*


Default mode to install and initialize before main.


const char*


Filename base for the XRay logfile.




Runtime verbosity level.

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 xray_log_interface.h from the compiler-rt xray directory. The important API functions we list below:

  • __xray_log_register_mode(...): Register a logging implementation against a string Mode identifier. The implementation is an instance of XRayLogImpl defined in xray/xray_log_interface.h.

  • __xray_log_select_mode(...): Select the mode to install, associated with a string Mode identifier. Only implementations registered with __xray_log_register_mode(...) can be chosen with this function.

  • __xray_log_init_mode(...): This function allows for initializing and re-initializing an installed logging implementation. See xray/xray_log_interface.h for details, part of the XRay compiler-rt installation.

Once a logging implementation has been initialized, it can be “stopped” by finalizing the implementation through the __xray_log_finalize() function. The finalization routine is the opposite of the initialization. When finalized, an implementation’s data can be cleared out through the __xray_log_flushLog() function. For implementations that support in-memory processing, these should register an iterator function to provide access to the data via the __xray_log_set_buffer_iterator(...) which allows code calling the __xray_log_process_buffers(...) function to deal with the data in memory.

All of this is better explained in the xray/xray_log_interface.h header.

Basic Mode

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 xray_mode=xray-basic in the XRAY_OPTIONS environment variable. Combined with patch_premain=true this can allow for tracing applications from start to end.

Like all the other modes installed through __xray_log_select_mode(...), the implementation can be configured through the __xray_log_init_mode(...) function, providing the mode string and the flag options. Basic-mode specific defaults can be provided in the XRAY_BASIC_OPTIONS environment variable.

Flight Data Recorder Mode

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 xray_mode variable to xray-fdr in the XRAY_OPTIONS environment variable. Additional options to the FDR mode implementation can be provided in the XRAY_FDR_OPTIONS environment variable. Programmatic configuration can be done by calling __xray_log_init_mode("xray-fdr", <configuration string>) once it has been selected/installed.

When the buffers are flushed to disk, the result is a binary trace format described by XRay FDR format

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:

// Patch the sleds, if we haven't yet.
auto patch_status = __xray_patch();

// Maybe handle the patch_status errors.

// When we want to flush the log, we need to finalize it first, to give
// threads a chance to return buffers to the queue.
auto finalize_status = __xray_log_finalize();
if (finalize_status != XRAY_LOG_FINALIZED) {
  // maybe retry, or bail out.

// At this point, we are sure that the log is finalized, so we may try
// flushing the log.
auto flush_status = __xray_log_flushLog();
if (flush_status != XRAY_LOG_FLUSHED) {
  // maybe retry, or bail out.

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.

NOTE: 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.

Trace Analysis Tools

We currently have the beginnings of a trace analysis tool in LLVM, which can be found in the tools/llvm-xray directory. The llvm-xray tool currently supports the following subcommands:

  • extract: Extract the instrumentation map from a binary, and return it as YAML.

  • account: Performs basic function call accounting statistics with various options for sorting, and output formats (supports CSV, YAML, and console-friendly TEXT).

  • convert: Converts an XRay log file from one format to another. We can convert from binary XRay traces (both basic and FDR mode) to YAML, flame-graph friendly text formats, as well as Chrome Trace Viewer (catapult) <https://github.com/catapult-project/catapult> formats.

  • graph: Generates a DOT graph of the function call relationships between functions found in an XRay trace.

  • stack: Reconstructs function call stacks from a timeline of function calls in an XRay trace.

These subcommands use various library components found as part of the XRay libraries, distributed with the LLVM distribution. These are:

  • llvm/XRay/Trace.h : 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.

  • llvm/XRay/Graph.h : A semi-generic graph type used by the graph subcommand to conveniently represent a function call graph with statistics associated with edges and vertices.

  • llvm/XRay/InstrumentationMap.h: A convenient tool for analyzing the instrumentation map in XRay-instrumented object files and binaries. The extract and stack subcommands uses this particular library.

Future Work

There are a number of ongoing efforts for expanding the toolset building around the XRay instrumentation system.

Trace Analysis Tools

  • Work is in progress to integrate with or develop tools to visualize findings from an XRay trace. Particularly, the stack tool is being expanded to output formats that allow graphing and exploring the duration of time in each call stack.

  • 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.

More Platforms

We’re looking forward to contributions to port XRay to more architectures and operating systems.