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Swift Benchmark Suite

This directory contains the Swift Benchmark Suite.

Running Swift Benchmarks

To run Swift benchmarks, pass the --benchmark flag to build-script. The current benchmark results will be compared to the previous run's results if available. Results for each benchmark run are logged for future comparison.

For branch based development, take a baseline benchmark on the Swift master branch, switch to a development branch containing potentially performance impacting changes, and run the benchmarks again. Upon benchmark completion, the benchmark results for the development branch will be compared to the most recent benchmark results for master.

Building with build-script

By default, Swift benchmarks for OS X are compiled during the Swift build process. To build Swift benchmarks for additional platforms, pass the following flags:

$ swift/utils/build-script --ios --watchos --tvos

OS X benchmark driver binaries are placed in bin alongside swiftc. Additional platform binaries are placed in the benchmark/bin build directory.

Building Independently

To build the Swift benchmarks using only an Xcode installation: install an Xcode version with Swift support, install cmake 2.8.12, and ensure Xcode is selected with xcode-select.

The following build options are available:

    • An absolute path to the Swift driver (swiftc) to use to compile the benchmarks (default: Xcode's swiftc)
    • An absolute path to the Swift standard library to use during compilation (default: swiftc_directory/../lib/swift)
    • A list of platforms to build the benchmarks for (default: "macosx;iphoneos;appletvos;watchos")
    • A list of Swift optimization levels to build against (default: "O;Onone;Osize")
    • A boolean value indicating whether .sib files should be generated alongside .o files (default: FALSE)

The following build targets are available:

  1. swift-benchmark-macosx-x86_64
  2. swift-benchmark-iphoneos-arm64
  3. swift-benchmark-iphoneos-armv7
  4. swift-benchmark-appletvos-arm64
  5. swift-benchmark-watchos-armv7k

Build steps (with example options):

  1. $ cd benchmark
  2. $ mkdir build
  3. $ cd build
  4. $ cmake ..
  5. $ make -j8 swift-benchmark-macosx-x86_64

Benchmark driver binaries are placed in build/bin and the required Swift standard library dylibs are placed in build/lib. The drivers dynamically link Swift standard library dylibs from a path relative to their location (../lib/swift) so the standard library should be distributed alongside them.

Using the Benchmark Driver


./Driver [ test_name [ test_name ] ] [ option [ option ] ]

  • --num-iters
    • Control the number of loop iterations in each test sample
  • --num-samples
    • Control the number of samples to take for each test
  • --list
    • Print a list of available tests matching specified criteria
  • --tags
    • Run tests that are labeled with specified tags (comma separated list); multiple tags are interpreted as logical AND, i.e. run only test that are labeled with all the supplied tags
  • --skip-tags
    • Don't run tests that are labeled with any of the specified tags (comma separated list); default value: skip,unstable; to get complete list of tests, specify empty --skip-tags=


  • $ ./Benchmark_O --num-iters=1 --num-samples=1
  • $ ./Benchmark_Onone --list
  • $ ./Benchmark_Osize Ackermann
  • $ ./Benchmark_O --tags=Dictionary
  • $ ./Benchmark_O --skip-tags=unstable,skip,validation


As a shortcut, you can also refer to benchmarks by their ordinal numbers. These are printed out together with benchmark names and tags using the --list parameter. For a complete list of all available performance tests run

  • $ ./Benchmark_O --list --skip-tags=

You can use test numbers instead of test names like this:

  • $ ./Benchmark_O 1 42
  • $ ./Benchmark_Driver run 1 42

Test numbers are not stable in the long run, adding and removing tests from the benchmark suite will reorder them, but they are stable for a given build.

Using the Harness Generator

scripts/generate_harness/generate_harness.py runs gyb to automate generation of some benchmarks.

** FIXME ** gyb should be invoked automatically during the build so that manually invoking generate_harness.py is not required.

Adding New Benchmarks

The harness generator supports both single and multiple file tests.

To add a new single file test:

  1. Add a new Swift file (YourTestNameHere.swift), built according to the template below, to the single-source directory.
  2. Add the filename of the new Swift file to CMakeLists.txt
  3. Edit main.swift. Import and register your new Swift module.

To add a new multiple file test:

  1. Add a new directory and files under the multi-source directory as specified below:

    +-- multi-source
    |   +-- YourTestName
    |   |   +-- TestFile1.swift
    |   |   +-- TestFile2.swift
    |   |   +-- TestFile3.swift

    At least one file must define a public YourTestName variable, initialized to an instance of BenchmarkInfo (specified in the template below).

  2. In CMakeLists.txt add the new directory name to SWIFT_MULTISOURCE_SWIFT3_BENCHES, and set YourTestName_sources to the list of source file paths.

  3. Edit main.swift. Import and register your new Swift module.


The benchmark harness will execute the routine referenced by BenchmarkInfo.runFunction.

The benchmark driver will measure the time taken for N = 1 and automatically calculate the necessary number of iterations N to run each benchmark in approximately one second, so the test should ideally run in a few milliseconds for N = 1. If the test contains any setup code before the loop, ensure the time spent on setup is insignificant compared to the time spent inside the loop (for N = 1) -- otherwise the automatic calculation of N might be significantly off and any performance gains/regressions will be masked by the fixed setup time. If needed you can multiply N by a fixed amount (e.g. 1...100*N) to achieve this.

Performance Test Template

// YourTestName benchmark
// rdar://problem/00000000
import TestsUtils

public let YourTestName = BenchmarkInfo(
  name: "YourTestName",
  runFunction: run_YourTestName,
  tags: [.regression])

public func run_YourTestName(N: Int) {
    # Declare variables

    for i in 1...N {
        # Perform work

        # Verify work was done; break otherwise

    # Assert with CheckResults that work was done

The current set of tags are defined by the BenchmarkCategory enum in TestsUtils.swift .

Testing the Benchmark Drivers

When working on tests, after the initial build

swift-source$ ./swift/utils/build-script -R -B

you can rebuild just the benchmarks:

swift-source$ export SWIFT_BUILD_DIR=`pwd`/build/Ninja-ReleaseAssert/swift-macosx-x86_64
swift-source$ ninja -C ${SWIFT_BUILD_DIR} swift-benchmark-macosx-x86_64

When modifying the testing infrastructure, you should verify that your changes pass all the tests:

swift-source$ ./llvm/utils/lit/lit.py -sv ${SWIFT_BUILD_DIR}/test-macosx-x86_64/benchmark