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Testing Swift

This document describes how we test the Swift compiler, the Swift runtime, and the Swift standard library.

Testing approaches

We use multiple approaches to test the Swift toolchain.

  • LLVM lit-based testsuites for the compiler, runtime and the standard library.
  • A selection of open source projects written in Swift.

The LLVM lit-based testsuite

Purpose: primary testsuites for the Swift toolchain.

Contents: Functional and regression tests for all toolchain components.

Run by:

  • Engineers and contributors are expected to run tests from these testsuites locally before committing. (Usually on a single platform, and not necessarily all tests.)
  • Buildbots run all tests, on all supported platforms.

Running the LLVM lit-based testsuite

You can run Swift tests using the build-script, or, alternatively, using these targets in the build directory:

  • check-swift

    Runs tests from the ${SWIFT_SOURCE_ROOT}/test directory.

  • check-swift-validation

    Runs tests from the ${SWIFT_SOURCE_ROOT}/validation-test directory.

  • check-swift-all

    Runs all tests.

For day-to-day work on the Swift compiler, using check-swift should be sufficient. The buildbot runs validation tests, so if those are accidentally broken, it should not go unnoticed.

Before committing a large change to a compiler (especially a language change), or API changes to the standard library, it is recommended to run validation test suite.

For every target above, there are variants for different optimizations:

  • the target itself (e.g., check-swift) -- runs execution tests in -Onone mode;
  • the target with -optimize suffix (e.g., check-swift-optimize) -- runs execution tests in -O mode; This target will only run tests marked as executable_test.
  • the target with -optimize-unchecked suffix (e.g., check-swift-optimize-unchecked) -- runs execution tests in -Ounchecked mode. This target will only run tests marked as executable_test.

If you need to manually run certain tests, you can invoke LLVM's lit.py script directly. For example:

% ${LLVM_SOURCE_ROOT}/utils/lit/lit.py -sv ${SWIFT_BUILD_ROOT}/test-iphonesimulator-i386/Parse/

This runs the tests in the test/Parse/ directory targeting the 32-bit iOS Simulator. The -sv options give you a nice progress bar and only show you output from the tests that fail.

One downside of using this form is that you're appending relative paths from the source directory to the test directory in your build directory. (That is, there may not actually be a directory named 'Parse' in 'test-iphonesimulator-i386/'; the invocation works because there is one in the source 'test/' directory.) There is a more verbose form that specifies the testing configuration explicitly, which then allows you to test files regardless of location.

% ${LLVM_SOURCE_ROOT}/utils/lit/lit.py -sv --param swift_site_config=${SWIFT_BUILD_ROOT}/test-iphonesimulator-i386/lit.site.cfg ${SWIFT_SOURCE_ROOT}/test/Parse/

For more complicated configuration, copy the invocation from one of the build targets mentioned above and modify it as necessary. lit.py also has several useful features, like timing tests and providing a timeout. Check these features out with lit.py -h.

Writing tests

General guidelines

When adding a new testcase, try to find an existing test file focused on the same topic rather than starting a new test file. There is a fixed runtime cost for every test file. On the other hand, avoid dumping new tests in a file that is only remotely related to the purpose of the new tests.

Don't limit a test to a certain platform or hardware configuration just because this makes the test slightly easier to write. This sometimes means a little bit more work when adding the test, but the payoff from the increased testing is significant. We heavily rely on portable tests to port Swift to other platforms.

Avoid using unstable language features in tests which test something else (for example, avoid using an unstable underscored attribute when another non-underscored attribute would work).

Avoid using arbitrary implementation details of the standard library. Always prefer to define types locally in the test, if feasible.

Avoid purposefully shadowing names from the standard library, this makes the test extremely confusing (if nothing else, to understand the intent --- was the compiler bug triggered by this shadowing?) When reducing a compiler testcase from the standard library source, rename the types and APIs in the testcase to differ from the standard library APIs.

In IRGen, SILGen and SIL tests, avoid using platform-dependent implementation details of the standard library (unless doing so is point of the test). Platform-dependent details include:

  • Int (use integer types with explicit types instead).
  • Layout of String, Array, Dictionary, Set. These differ between platforms that have Objective-C interop and those that don't.

Unless testing the standard library, avoid using arbitrary standard library types and APIs, even if it is very convenient for you to do so in your tests. Using the more common APIs like Array subscript or + on IntXX is acceptable. This is important because you can't rely on the full standard library being available. The long-term plan is to introduce a mock, minimal standard library that only has a very basic set of APIs.

If you write an executable test please add REQUIRES: executable_test to the test.

Substitutions in lit tests

Substitutions that start with %target configure the compiler for building code for the target that is not the build machine:

  • %target-parse-verify-swift: parse and type check the current Swift file for the target platform and verify diagnostics, like swift -parse -verify %s.

    Use this substitution for testing semantic analysis in the compiler.

  • %target-swift-frontend: run swift -frontend for the target.

    Use this substitution (with extra arguments) for tests that don't fit any other pattern.

  • %target-swift-frontend(mock-sdk: mock sdk arguments ) other arguments: like %target-swift-frontend, but allows to specify command line parameters (typically -sdk and -I) to use a mock SDK and SDK overlay that would take precedence over the target SDK.

  • %target-build-swift: compile and link a Swift program for the target.

    Use this substitution only when you intend to run the program later in the test.

  • %target-run-simple-swift: build a one-file Swift program and run it on the target machine.

    Use this substitution for executable tests that don't require special compiler arguments.

    Add REQUIRES: executable_test to the test.

  • %target-run-stdlib-swift: like %target-run-simple-swift with -parse-stdlib -Xfrontend -disable-access-control.

    This is sometimes useful for testing the Swift standard library.

    Add REQUIRES: executable_test to the test.

  • %target-repl-run-simple-swift: run a Swift program in a REPL on the target machine.

  • %target-run: run a command on the target machine.

    Add REQUIRES: executable_test to the test.

  • %target-jit-run: run a Swift program on the target machine using a JIT compiler.

  • %target-swiftc_driver: FIXME

  • %target-sil-opt: run sil-opt for the target.

  • %target-sil-extract: run sil-extract for the target.

  • %target-swift-ide-test: run swift-ide-test for the target.

  • %target-swift-ide-test(mock-sdk: mock sdk arguments ) other arguments: like %target-swift-ide-test, but allows to specify command line parameters to use a mock SDK.

  • %target-swiftc_driver: FIXME.

  • %target-swift-autolink-extract: run swift-autolink-extract for the target to extract its autolink flags on platforms that support them (when the autolink-extract feature flag is set)

  • %target-clang: run the system's clang++ for the target.

    If you want to run the clang executable that was built alongside Swift, use %clang instead.

  • %target-ld: run ld configured with flags pointing to the standard library directory for the target.

  • %target-cc-options: the clang flags to setup the target with the right architecture and platform version.

Always use %target-* substitutions unless you have a good reason. For example, an exception would be a test that checks how the compiler handles mixing module files for incompatible platforms (that test would need to compile Swift code for two different platforms that are known to be incompatible).

When you can't use %target-* substitutions, you can use:

  • %swift_driver_plain: FIXME.
  • %swiftc_driver_plain: FIXME.
  • %swift_driver: FIXME.
  • %swiftc_driver: FIXME.
  • %sil-opt: FIXME.
  • %sil-extract: FIXME.
  • %lldb-moduleimport-test: FIXME.
  • %swift-ide-test_plain: FIXME.
  • %swift-ide-test: FIXME.
  • %llvm-opt: FIXME.
  • %swift: FIXME.
  • %clang-include-dir: FIXME.
  • %clang-importer-sdk: FIXME.

Other substitutions:

  • %leaks-runner: FIXME.
  • %clang_apinotes: FIXME.
  • %clang: FIXME.
  • %target-triple: FIXME, possible values.
  • %target-cpu: FIXME, possible values.
  • %target-os: FIXME, possible values.
  • %target-object-format: the platform's object format (elf, macho, coff).
  • %target-runtime: the platform's Swift runtime (objc, native).
  • %target-ptrsize: the pointer size of the target (32, 64).
  • %sdk: FIXME.
  • %gyb: FIXME.
  • %platform-module-dir: absolute path of the directory where the standard library module file for the target platform is stored. For example, /.../lib/swift/macosx.
  • %platform-sdk-overlay-dir: absolute path of the directory where the SDK overlay module files for the target platform are stored.
  • %target-swiftmodule-name and %target-swiftdoc-name: the basename of swiftmodule and swiftdoc files for a framework compiled for the target (for example, arm64.swiftmodule and arm64.swiftdoc).
  • %target-sdk-name: only for Apple platforms: xcrun-style SDK name (macosx, iphoneos, iphonesimulator).

When writing a test where output (or IR, SIL) depends on the bitness of the target CPU, use this pattern:

// RUN: %target-swift-frontend ... | FileCheck --check-prefix=CHECK --check-prefix=CHECK-%target-ptrsize %s

// CHECK: common line
// CHECK-32: only for 32-bit
// CHECK-64: only for 64-bit

// FileCheck does a single pass for a combined set of CHECK lines, so you can
// do this:
//
// CHECK: define @foo() {
// CHECK-32: integer_literal $Builtin.Int32, 0
// CHECK-64: integer_literal $Builtin.Int64, 0

When writing a test where output (or IR, SIL) depends on the target CPU itself, use this pattern:

// RUN: %target-swift-frontend ... | FileCheck --check-prefix=CHECK --check-prefix=CHECK-%target-cpu %s

// CHECK: common line
// CHECK-i386:   only for i386
// CHECK-x86_64: only for x86_64
// CHECK-armv7:  only for armv7
// CHECK-arm64:  only for arm64
Features for REQUIRES and XFAIL

FIXME: full list.

  • swift_ast_verifier: present if the AST verifier is enabled in this build.

When writing a test specific to x86, if possible, prefer REQUIRES: CPU=i386_or_x86_64 to REQUIRES: CPU=x86_64.

swift_test_mode_optimize[_unchecked|none] and swift_test_mode_optimize[_unchecked|none]_<CPUNAME> to specify a test mode plus cpu configuration.

optimized_stdlib_<CPUNAME>` to specify a optimized stdlib plus cpu configuration.

Feature REQUIRES: executable_test

This feature marks an executable test. The test harness makes this feature generally available. It can be used to restrict the set of tests to run.

StdlibUnittest

Tests accept command line parameters, run StdlibUnittest-based test binary with --help for more information.

Testing memory management in execution tests

In execution tests, memory management testing should be performed using local variables enclosed in a closure passed to the standard library autoreleasepool function. For example:

// A counter that's decremented by Canary's deinitializer.
var CanaryCount = 0

// A class whose instances increase a counter when they're destroyed.
class Canary {
  deinit { ++CanaryCount }
}

// Test that a local variable is correctly released before it goes out of
// scope.
CanaryCount = 0
autoreleasepool {
  let canary = Canary()
}
assert(CanaryCount == 1, "canary was not released")

Memory management tests should be performed in a local scope because Swift does not guarantee the destruction of global variables. Code that needs to interoperate with Objective-C may put references in the autorelease pool, so code that uses an if true {} or similar no-op scope instead of autoreleasepool may falsely report leaks or fail to catch overrelease bugs. If you're specifically testing the autoreleasing behavior of code, or do not expect code to interact with the Objective-C runtime, it may be OK to use if true {}, but those assumptions should be commented in the test.