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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;Ounchecked")
    • 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


  1. $ ./Benchmark_O --num-iters=1 --num-samples=1
  2. $ ./Benchmark_Onone --list
  3. $ ./Benchmark_Ounchecked Ackermann

Using the Harness Generator

scripts/generate_harness/ generates and replaces CMakeLists.txt and utils/main.swift from single and multiple file tests contained in the directories single-source and multi-source. It gathers information about the tests and then generates the files from templates using jinja2. The motivation for creating this script was to eliminate the need to manually add at least three lines to harness files (one to CMakeLists.txt and two to utils/main.swift) for every new benchmark added.


Since CMakeLists.txt and utils/main.swift are now generated from templates, they should not be directly modified. Work may be lost if the harness is executed after making changes to derived files. Instead, modifications should be made to the template files stored in the scripts/generate_harness directory.

Generating harness files

Start by installing jinja2 if it isn't already installed:

$ sudo easy_install -U jinja2

To generate CMakeLists.txt and utils/main.swift from test sources, run the command:

$ scripts/generate_harness/


Ensure remains in scripts/generate_harness as it modifies files relative to its location instead of the current working directory.

Modifying CMakeLists.txt or utils/main.swift

To make changes to CMakeLists.txt or utils/main.swift, modify the template files CMakeLists.txt_template and main.swift_template stored in the scripts/generate_harness directory. These are jinja2 templates, rendered by jinja2 calls in, so ensure static changes don't interfere with the template portions. Test changes by regenerating the harness (Generating harness files) and rebuilding the repository with build-script.

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. Regenerate harness files by following the directions in Generating harness files before committing changes.

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 run function (specified in the template below) must exist in the files.

  2. Regenerate harness files by following the directions in Generating harness files before committing changes.


The generator script looks for functions prefixed with run_ in order to populate utils/main.swift.

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

// YourTestNameHere benchmark
// rdar://problem/00000000
import Foundation
import TestsUtils

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

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

        # Verify work was done; break otherwise

    # Assert with CheckResults that work was done