XRay Instrumentation — LLVM 4 documentation

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# XRay Instrumentation

Version: | 1 as of 2016-11-08

- Introduction
- XRay in LLVM
- Using XRay
  - Instrumenting your C/C++/Objective-C Application
  - LLVM Function Attribute
  - XRay Runtime Library
  - Trace Analysis Tools
- Future Work
  - Flight Data Recorder Mode
  - Trace Analysis
  - More Platforms

## Introduction

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 the time of this writing, XRay is only available for x86_64 and arm7 32-bit (no-thumb) Linux.

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_intrument]] void always_instrumented();

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

    void alt_always_instrumented() __attribute__((xray_always_intrument));

    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.

### 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" {
      ; ...
    }

### 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.

Option | Type | Default | Description
--- | --- | --- | ---
patch_premain | `bool` | `true` | Whether to patch instrumentation points before main.
xray_naive_log | `bool` | `true` | Whether to install the naive log implementation.
xray_logfile_base | `const char*` | `xray-log.` | Filename base for the XRay logfile.

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

- `__xray_set_handler(void (*entry)(int32_t, XRayEntryType))`: Install your own logging handler for when an event is encountered. See `xray/xray_interface.h` for more details.
- `__xray_remove_handler()`: Removes whatever the installed handler is.
- `__xray_patch()`: Patch all the instrumentation points defined in the binary.
- `__xray_unpatch()`: Unpatch the instrumentation points defined in the binary.

There are some requirements on the logging handler to be installed for the thread-safety of operations to be performed by the XRay runtime library:

- The function should be thread-safe, as multiple threads may be invoking the function at the same time. If the logging function needs to do synchronisation, it must do so internally as XRay does not provide any synchronisation guarantees outside from the atomicity of updates to the pointer.
- The pointer provided to `__xray_set_handler(...)` must be live even after calls to `__xray_remove_handler()` and `__xray_unpatch()` have succeeded. XRay cannot guarantee that all threads that have ever gotten a copy of the pointer will not invoke the function.

### 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.

## Future Work

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

### Flight Data Recorder Mode

The XRay whitepaper mentions a mode for when events are kept in memory, and have the traces be dumped on demand through a triggering API. This work is currently ongoing.

### Trace Analysis

There are a few more subcommands making its way to the `llvm-xray` tool, that are currently under review:

- `convert`: Turns an XRay trace from one format to another. Currently supporting conversion from the binary XRay log to YAML.
- `account`: Do function call accounting based on data in the XRay log.

We have more subcommands and modes that we’re thinking of developing, in the following forms:

- `stack`: Reconstruct the function call stacks in a timeline.
- `convert`: Converting from one version of the XRay log to another (higher) version, and converting to other trace formats (i.e. Chrome Trace Viewer, pprof, etc.).
- `graph`: Generate a function call graph with relative timings and distributions.

### More Platforms

Since XRay is only currently available in x86_64 and arm7 32-bit (no-thumb) running Linux, we’re looking to supporting more platforms (architectures and operating systems).

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