LLVM Testing Infrastructure Guide


This document is the reference manual for the LLVM testing infrastructure. It documents the structure of the LLVM testing infrastructure, the tools needed to use it, and how to add and run tests.


In order to use the LLVM testing infrastructure, you will need all of the software required to build LLVM, as well as Python 3.6 or later.

LLVM Testing Infrastructure Organization

The LLVM testing infrastructure contains three major categories of tests: unit tests, regression tests and whole programs. The unit tests and regression tests are contained inside the LLVM repository itself under llvm/unittests and llvm/test respectively and are expected to always pass – they should be run before every commit.

The whole programs tests are referred to as the “LLVM test suite” (or “test-suite”) and are in the test-suite repository on GitHub. For historical reasons, these tests are also referred to as the “nightly tests” in places, which is less ambiguous than “test-suite” and remains in use although we run them much more often than nightly.

Unit tests

Unit tests are written using Google Test and Google Mock and are located in the llvm/unittests directory. In general unit tests are reserved for targeting the support library and other generic data structure, we prefer relying on regression tests for testing transformations and analysis on the IR.

Regression tests

The regression tests are small pieces of code that test a specific feature of LLVM or trigger a specific bug in LLVM. The language they are written in depends on the part of LLVM being tested. These tests are driven by the Lit testing tool (which is part of LLVM), and are located in the llvm/test directory.

Typically when a bug is found in LLVM, a regression test containing just enough code to reproduce the problem should be written and placed somewhere underneath this directory. For example, it can be a small piece of LLVM IR distilled from an actual application or benchmark.

Testing Analysis

An analysis is a pass that infer properties on some part of the IR and not transforming it. They are tested in general using the same infrastructure as the regression tests, by creating a separate “Printer” pass to consume the analysis result and print it on the standard output in a textual format suitable for FileCheck. See llvm/test/Analysis/BranchProbabilityInfo/loop.ll for an example of such test.


The test suite contains whole programs, which are pieces of code which can be compiled and linked into a stand-alone program that can be executed. These programs are generally written in high level languages such as C or C++.

These programs are compiled using a user specified compiler and set of flags, and then executed to capture the program output and timing information. The output of these programs is compared to a reference output to ensure that the program is being compiled correctly.

In addition to compiling and executing programs, whole program tests serve as a way of benchmarking LLVM performance, both in terms of the efficiency of the programs generated as well as the speed with which LLVM compiles, optimizes, and generates code.

The test-suite is located in the test-suite repository on GitHub.

See the test-suite Guide for details.

Debugging Information tests

The test suite contains tests to check quality of debugging information. The test are written in C based languages or in LLVM assembly language.

These tests are compiled and run under a debugger. The debugger output is checked to validate of debugging information. See README.txt in the test suite for more information. This test suite is located in the cross-project-tests/debuginfo-tests directory.

Quick start

The tests are located in two separate repositories. The unit and regression tests are in the main “llvm”/ directory under the directories llvm/unittests and llvm/test (so you get these tests for free with the main LLVM tree). Use make check-all to run the unit and regression tests after building LLVM.

The test-suite module contains more comprehensive tests including whole C and C++ programs. See the test-suite Guide for details.

Unit and Regression tests

To run all of the LLVM unit tests use the check-llvm-unit target:

% make check-llvm-unit

To run all of the LLVM regression tests use the check-llvm target:

% make check-llvm

In order to get reasonable testing performance, build LLVM and subprojects in release mode, i.e.


If you have Clang checked out and built, you can run the LLVM and Clang tests simultaneously using:

% make check-all

To run the tests with Valgrind (Memcheck by default), use the LIT_ARGS make variable to pass the required options to lit. For example, you can use:

% make check LIT_ARGS="-v --vg --vg-leak"

to enable testing with valgrind and with leak checking enabled.

To run individual tests or subsets of tests, you can use the llvm-lit script which is built as part of LLVM. For example, to run the Integer/BitPacked.ll test by itself you can run:

% llvm-lit ~/llvm/test/Integer/BitPacked.ll

or to run all of the ARM CodeGen tests:

% llvm-lit ~/llvm/test/CodeGen/ARM

The regression tests will use the Python psutil module only if installed in a non-user location. Under Linux, install with sudo or within a virtual environment. Under Windows, install Python for all users and then run pip install psutil in an elevated command prompt.

For more information on using the lit tool, see llvm-lit --help or the lit man page.

Debugging Information tests

To run debugging information tests simply add the cross-project-tests project to your LLVM_ENABLE_PROJECTS define on the cmake command-line.

Regression test structure

The LLVM regression tests are driven by lit and are located in the llvm/test directory.

This directory contains a large array of small tests that exercise various features of LLVM and to ensure that regressions do not occur. The directory is broken into several sub-directories, each focused on a particular area of LLVM.

Writing new regression tests

The regression test structure is very simple, but does require some information to be set. This information is gathered via cmake and is written to a file, test/lit.site.cfg.py in the build directory. The llvm/test Makefile does this work for you.

In order for the regression tests to work, each directory of tests must have a lit.local.cfg file. lit looks for this file to determine how to run the tests. This file is just Python code and thus is very flexible, but we’ve standardized it for the LLVM regression tests. If you’re adding a directory of tests, just copy lit.local.cfg from another directory to get running. The standard lit.local.cfg simply specifies which files to look in for tests. Any directory that contains only directories does not need the lit.local.cfg file. Read the Lit documentation for more information.

Each test file must contain lines starting with “RUN:” that tell lit how to run it. If there are no RUN lines, lit will issue an error while running a test.

RUN lines are specified in the comments of the test program using the keyword RUN followed by a colon, and lastly the command (pipeline) to execute. Together, these lines form the “script” that lit executes to run the test case. The syntax of the RUN lines is similar to a shell’s syntax for pipelines including I/O redirection and variable substitution. However, even though these lines may look like a shell script, they are not. RUN lines are interpreted by lit. Consequently, the syntax differs from shell in a few ways. You can specify as many RUN lines as needed.

lit performs substitution on each RUN line to replace LLVM tool names with the full paths to the executable built for each tool (in $(LLVM_OBJ_ROOT)/bin). This ensures that lit does not invoke any stray LLVM tools in the user’s path during testing.

Each RUN line is executed on its own, distinct from other lines unless its last character is \. This continuation character causes the RUN line to be concatenated with the next one. In this way you can build up long pipelines of commands without making huge line lengths. The lines ending in \ are concatenated until a RUN line that doesn’t end in \ is found. This concatenated set of RUN lines then constitutes one execution. lit will substitute variables and arrange for the pipeline to be executed. If any process in the pipeline fails, the entire line (and test case) fails too.

Below is an example of legal RUN lines in a .ll file:

; RUN: llvm-as < %s | llvm-dis > %t1
; RUN: llvm-dis < %s.bc-13 > %t2
; RUN: diff %t1 %t2

As with a Unix shell, the RUN lines permit pipelines and I/O redirection to be used.

There are some quoting rules that you must pay attention to when writing your RUN lines. In general nothing needs to be quoted. lit won’t strip off any quote characters so they will get passed to the invoked program. To avoid this use curly braces to tell lit that it should treat everything enclosed as one value.

In general, you should strive to keep your RUN lines as simple as possible, using them only to run tools that generate textual output you can then examine. The recommended way to examine output to figure out if the test passes is using the FileCheck tool. [The usage of grep in RUN lines is deprecated - please do not send or commit patches that use it.]

Put related tests into a single file rather than having a separate file per test. Check if there are files already covering your feature and consider adding your code there instead of creating a new file.

Generating assertions in regression tests

Some regression test cases are very large and complex to write/update by hand. In that case to reduce the human work we can use the scripts available in llvm/utils/ to generate the assertions.

For example to generate assertions in an llc-based test, after adding one or more RUN lines use:

% llvm/utils/update_llc_test_checks.py --llc-binary build/bin/llc test.ll

This will generate FileCheck assertions, and insert a NOTE: line at the top to indicate that assertions were automatically generated.

If you want to update assertions in an existing test case, pass the -u option which first checks the NOTE: line exists and matches the script name.

Sometimes a test absolutely depends on hand-written assertions and should not have assertions automatically generated. In that case, add the text NOTE: Do not autogenerate to the first line, and the scripts will skip that test. It is a good idea to explain why generated assertions will not work for the test so future developers will understand what is going on.

These are the most common scripts and their purposes/applications in generating assertions:

opt -passes='print<cost-model>'

C/C++, or clang/clang++ (IR checks)

llc (assembly checks)


llc (MIR checks)


Precommit workflow for tests

If the test does not crash, assert, or infinite loop, commit the test with baseline check-lines first. That is, the test will show a miscompile or missing optimization. Add a “TODO” or “FIXME” comment to indicate that something is expected to change in a test.

A follow-up patch with code changes to the compiler will then show check-line differences to the tests, so it is easier to see the effect of the patch. Remove TODO/FIXME comments added in the previous step if a problem is solved.

Baseline tests (no-functional-change or NFC patch) may be pushed to main without pre-commit review if you have commit access.

Best practices for regression tests

  • Use auto-generated check lines (produced by the scripts mentioned above) whenever feasible.

  • Include comments about what is tested/expected in a particular test. If there are relevant issues in the bug tracker, add references to those bug reports (for example, “See PR999 for more details”).

  • Avoid undefined behavior and poison/undef values unless necessary. For example, do not use patterns like br i1 undef, which are likely to break as a result of future optimizations.

  • Minimize tests by removing unnecessary instructions, metadata, attributes, etc. Tools like llvm-reduce can help automate this.

  • Outside PhaseOrdering tests, only run a minimal set of passes. For example, prefer opt -S -passes=instcombine over opt -S -O3.

  • Avoid unnamed instructions/blocks (such as %0 or 1:), because they may require renumbering on future test modifications. These can be removed by running the test through opt -S -passes=instnamer.

  • Try to give values (including variables, blocks and functions) meaningful names, and avoid retaining complex names generated by the optimization pipeline (such as %foo.

Extra files

If your test requires extra files besides the file containing the RUN: lines and the extra files are small, consider specifying them in the same file and using split-file to extract them. For example,

; RUN: split-file %s %t
; RUN: llvm-link -S %t/a.ll %t/b.ll | FileCheck %s

; CHECK: ...

;--- a.ll
;--- b.ll

The parts are separated by the regex ^(.|//)--- <part>.

If you want to test relative line numbers like [[#@LINE+1]], specify --leading-lines to add leading empty lines to preserve line numbers.

If the extra files are large, the idiomatic place to put them is in a subdirectory Inputs. You can then refer to the extra files as %S/Inputs/foo.bar.

For example, consider test/Linker/ident.ll. The directory structure is as follows:


For convenience, these are the contents:

;;;;; ident.ll:

; RUN: llvm-link %S/Inputs/ident.a.ll %S/Inputs/ident.b.ll -S | FileCheck %s

; Verify that multiple input llvm.ident metadata are linked together.

; CHECK-DAG: !llvm.ident = !{!0, !1, !2}
; CHECK-DAG: "Compiler V1"
; CHECK-DAG: "Compiler V2"
; CHECK-DAG: "Compiler V3"

;;;;; Inputs/ident.a.ll:

!llvm.ident = !{!0, !1}
!0 = metadata !{metadata !"Compiler V1"}
!1 = metadata !{metadata !"Compiler V2"}

;;;;; Inputs/ident.b.ll:

!llvm.ident = !{!0}
!0 = metadata !{metadata !"Compiler V3"}

For symmetry reasons, ident.ll is just a dummy file that doesn’t actually participate in the test besides holding the RUN: lines.


Some existing tests use RUN: true in extra files instead of just putting the extra files in an Inputs/ directory. This pattern is deprecated.

Elaborated tests

Generally, IR and assembly test files benefit from being cleaned to remove unnecessary details. However, for tests requiring elaborate IR or assembly files where cleanup is less practical (e.g., large amount of debug information output from Clang), you can include generation instructions within split-file part called gen. Then, run llvm/utils/update_test_body.py on the test file to generate the needed content.

; RUN: rm -rf %t && split-file %s %t && cd %t
; RUN: opt -S a.ll ... | FileCheck %s

; CHECK: hello

;--- a.cc
int va;
;--- gen
clang --target=x86_64-linux -S -emit-llvm -g a.cc -o -

;--- a.ll
# content generated by the script 'gen'
PATH=/path/to/clang_build/bin:$PATH llvm/utils/update_test_body.py path/to/test.ll

The script will prepare extra files with split-file, invoke gen, and then rewrite the part after gen with its stdout.

For convenience, if the test needs one single assembly file, you can also wrap gen and its required files with .ifdef and .endif. Then you can skip split-file in RUN lines.

# RUN: llvm-mc -filetype=obj -triple=x86_64 %s -o a.o
# RUN: ... | FileCheck %s

# CHECK: hello

.ifdef GEN
#--- a.cc
int va;
#--- gen
clang --target=x86_64-linux -S -g a.cc -o -
# content generated by the script 'gen'


Consider specifying an explicit target triple to avoid differences when regeneration is needed on another machine.

gen is invoked with PWD set to /proc/self/cwd. Clang commands don’t need -fdebug-compilation-dir= since its default value is PWD.

Check prefixes should be placed before .endif since the part after .endif is replaced.

If the test body contains multiple files, you can print --- separators and utilize split-file in RUN lines.

# RUN: rm -rf %t && split-file %s %t && cd %t

#--- a.cc
int va;
#--- b.cc
int vb;
#--- gen
clang --target=x86_64-linux -S -O1 -g a.cc -o -
echo '#--- b.s'
clang --target=x86_64-linux -S -O1 -g b.cc -o -
#--- a.s

Fragile tests

It is easy to write a fragile test that would fail spuriously if the tool being tested outputs a full path to the input file. For example, opt by default outputs a ModuleID:

$ cat example.ll
define i32 @main() nounwind {
    ret i32 0

$ opt -S /path/to/example.ll
; ModuleID = '/path/to/example.ll'

define i32 @main() nounwind {
    ret i32 0

ModuleID can unexpectedly match against CHECK lines. For example:

; RUN: opt -S %s | FileCheck

define i32 @main() nounwind {
    ; CHECK-NOT: load
    ret i32 0

This test will fail if placed into a download directory.

To make your tests robust, always use opt ... < %s in the RUN line. opt does not output a ModuleID when input comes from stdin.

Platform-Specific Tests

Whenever adding tests that require the knowledge of a specific platform, either related to code generated, specific output or back-end features, you must make sure to isolate the features, so that buildbots that run on different architectures (and don’t even compile all back-ends), don’t fail.

The first problem is to check for target-specific output, for example sizes of structures, paths and architecture names, for example:

  • Tests containing Windows paths will fail on Linux and vice-versa.

  • Tests that check for x86_64 somewhere in the text will fail anywhere else.

  • Tests where the debug information calculates the size of types and structures.

Also, if the test rely on any behaviour that is coded in any back-end, it must go in its own directory. So, for instance, code generator tests for ARM go into test/CodeGen/ARM and so on. Those directories contain a special lit configuration file that ensure all tests in that directory will only run if a specific back-end is compiled and available.

For instance, on test/CodeGen/ARM, the lit.local.cfg is:

config.suffixes = ['.ll', '.c', '.cpp', '.test']
if not 'ARM' in config.root.targets:
  config.unsupported = True

Other platform-specific tests are those that depend on a specific feature of a specific sub-architecture, for example only to Intel chips that support AVX2.

For instance, test/CodeGen/X86/psubus.ll tests three sub-architecture variants:

; RUN: llc -mcpu=core2 < %s | FileCheck %s -check-prefix=SSE2
; RUN: llc -mcpu=corei7-avx < %s | FileCheck %s -check-prefix=AVX1
; RUN: llc -mcpu=core-avx2 < %s | FileCheck %s -check-prefix=AVX2

And the checks are different:

; SSE2: @test1
; SSE2: psubusw LCPI0_0(%rip), %xmm0
; AVX1: @test1
; AVX1: vpsubusw LCPI0_0(%rip), %xmm0, %xmm0
; AVX2: @test1
; AVX2: vpsubusw LCPI0_0(%rip), %xmm0, %xmm0

So, if you’re testing for a behaviour that you know is platform-specific or depends on special features of sub-architectures, you must add the specific triple, test with the specific FileCheck and put it into the specific directory that will filter out all other architectures.

Constraining test execution

Some tests can be run only in specific configurations, such as with debug builds or on particular platforms. Use REQUIRES and UNSUPPORTED to control when the test is enabled.

Some tests are expected to fail. For example, there may be a known bug that the test detect. Use XFAIL to mark a test as an expected failure. An XFAIL test will be successful if its execution fails, and will be a failure if its execution succeeds.

; This test will be only enabled in the build with asserts.
; REQUIRES: asserts
; This test is disabled when running on Linux.
; UNSUPPORTED: system-linux
; This test is expected to fail when targeting PowerPC.
; XFAIL: target=powerpc{{.*}}

REQUIRES and UNSUPPORTED and XFAIL all accept a comma-separated list of boolean expressions. The values in each expression may be:

  • Features added to config.available_features by configuration files such as lit.cfg. String comparison of features is case-sensitive. Furthermore, a boolean expression can contain any Python regular expression enclosed in {{ }}, in which case the boolean expression is satisfied if any feature matches the regular expression. Regular expressions can appear inside an identifier, so for example he{{l+}}o would match helo, hello, helllo, and so on.

  • The default target triple, preceded by the string target= (for example, target=x86_64-pc-windows-msvc). Typically regular expressions are used to match parts of the triple (for example, target={{.*}}-windows{{.*}} to match any Windows target triple).

REQUIRES enables the test if all expressions are true.
UNSUPPORTED disables the test if any expression is true.
XFAIL expects the test to fail if any expression is true.

Use, XFAIL: * if the test is expected to fail everywhere. Similarly, use UNSUPPORTED: target={{.*}} to disable the test everywhere.

; This test is disabled when running on Windows,
; and is disabled when targeting Linux, except for Android Linux.
; UNSUPPORTED: system-windows, target={{.*linux.*}} && !target={{.*android.*}}
; This test is expected to fail when targeting PowerPC or running on Darwin.
; XFAIL: target=powerpc{{.*}}, system-darwin

Tips for writing constraints


These are logical inverses. In principle, UNSUPPORTED isn’t absolutely necessary (the logical negation could be used with REQUIRES to get exactly the same effect), but it can make these clauses easier to read and understand. Generally, people use REQUIRES to state things that the test depends on to operate correctly, and UNSUPPORTED to exclude cases where the test is expected never to work.


Both of these indicate that the test isn’t expected to work; however, they have different effects. UNSUPPORTED causes the test to be skipped; this saves execution time, but then you’ll never know whether the test actually would start working. Conversely, XFAIL actually runs the test but expects a failure output, taking extra execution time but alerting you if/when the test begins to behave correctly (an XPASS test result). You need to decide which is more appropriate in each case.

Using ``target=…``

Checking the target triple can be tricky; it’s easy to mis-specify. For example, target=mips{{.*}} will match not only mips, but also mipsel, mips64, and mips64el. target={{.*}}-linux-gnu will match x86_64-unknown-linux-gnu, but not armv8l-unknown-linux-gnueabihf. Prefer to use hyphens to delimit triple components (target=mips-{{.*}}) and it’s generally a good idea to use a trailing wildcard to allow for unexpected suffixes.

Also, it’s generally better to write regular expressions that use entire triple components, than to do something clever to shorten them. For example, to match both freebsd and netbsd in an expression, you could write target={{.*(free|net)bsd.*}} and that would work. However, it would prevent a grep freebsd from finding this test. Better to use: target={{.+-freebsd.*}} || target={{.+-netbsd.*}}


Besides replacing LLVM tool names the following substitutions are performed in RUN lines:


Replaced by a single %. This allows escaping other substitutions.


File path to the test case’s source. This is suitable for passing on the command line as the input to an LLVM tool.

Example: /home/user/llvm/test/MC/ELF/foo_test.s


Directory path to the test case’s source.

Example: /home/user/llvm/test/MC/ELF


File path to a temporary file name that could be used for this test case. The file name won’t conflict with other test cases. You can append to it if you need multiple temporaries. This is useful as the destination of some redirected output.

Example: /home/user/llvm.build/test/MC/ELF/Output/foo_test.s.tmp


Directory of %t. Deprecated. Shouldn’t be used, because it can be easily misused and cause race conditions between tests.

Use rm -rf %t && mkdir %t instead if a temporary directory is necessary.

Example: /home/user/llvm.build/test/MC/ELF/Output


Expands to the path separator, i.e. : (or ; on Windows).


Expands to the root component of file system paths for the source directory, i.e. / on Unix systems or C:\ (or another drive) on Windows.


Expands to the root component of file system paths for the test’s temporary directory, i.e. / on Unix systems or C:\ (or another drive) on Windows.


Expands to the file system separator, i.e. / or \ on Windows.

%/s, %/S, %/t, %/T

Act like the corresponding substitution above but replace any \ character with a /. This is useful to normalize path separators.

Example: %s:  C:\Desktop Files/foo_test.s.tmp

Example: %/s: C:/Desktop Files/foo_test.s.tmp

%{s:real}, %{S:real}, %{t:real}, %{T:real} %{/s:real}, %{/S:real}, %{/t:real}, %{/T:real}

Act like the corresponding substitution, including with /, but use the real path by expanding all symbolic links and substitute drives.

Example: %s:  S:\foo_test.s.tmp

Example: %{/s:real}: C:/SDrive/foo_test.s.tmp

%:s, %:S, %:t, %:T

Act like the corresponding substitution above but remove colons at the beginning of Windows paths. This is useful to allow concatenation of absolute paths on Windows to produce a legal path.

Example: %s:  C:\Desktop Files\foo_test.s.tmp

Example: %:s: C\Desktop Files\foo_test.s.tmp


Some error messages may be substituted to allow different spellings based on the host platform.

The following error codes are currently supported: ENOENT, EISDIR, EINVAL, EACCES.

Example: Linux %errc_ENOENT: No such file or directory

Example: Windows %errc_ENOENT: no such file or directory

%if feature %{<if branch>%} %else %{<else branch>%}

Conditional substitution: if feature is available it expands to <if branch>, otherwise it expands to <else branch>. %else %{<else branch>%} is optional and treated like %else %{%} if not present.

%(line), %(line+<number>), %(line-<number>)

The number of the line where this substitution is used, with an optional integer offset. These expand only if they appear immediately in RUN:, DEFINE:, and REDEFINE: directives. Occurrences in substitutions defined elsewhere are never expanded. For example, this can be used in tests with multiple RUN lines, which reference the test file’s line numbers.

LLVM-specific substitutions:


The suffix for the host platforms shared library files. This includes the period as the first character.

Example: .so (Linux), .dylib (macOS), .dll (Windows)


The suffix for the host platforms executable files. This includes the period as the first character.

Example: .exe (Windows), empty on Linux.

Clang-specific substitutions:


Invokes the Clang driver.


Invokes the Clang driver for C++.


Invokes the CL-compatible Clang driver.


Invokes the G++-compatible Clang driver.


Invokes the Clang frontend.

%itanium_abi_triple, %ms_abi_triple

These substitutions can be used to get the current target triple adjusted to the desired ABI. For example, if the test suite is running with the i686-pc-win32 target, %itanium_abi_triple will expand to i686-pc-mingw32. This allows a test to run with a specific ABI without constraining it to a specific triple.

FileCheck-specific substitutions:


This should precede a FileCheck call if and only if the call’s textual output affects test results. It’s usually easy to tell: just look for redirection or piping of the FileCheck call’s stdout or stderr.

Test-specific substitutions:

Additional substitutions can be defined as follows:

  • Lit configuration files (e.g., lit.cfg or lit.local.cfg) can define substitutions for all tests in a test directory. They do so by extending the substitution list, config.substitutions. Each item in the list is a tuple consisting of a pattern and its replacement, which lit applies using python’s re.sub function.

  • To define substitutions within a single test file, lit supports the DEFINE: and REDEFINE: directives, described in detail below. So that they have no effect on other test files, these directives modify a copy of the substitution list that is produced by lit configuration files.

For example, the following directives can be inserted into a test file to define %{cflags} and %{fcflags} substitutions with empty initial values, which serve as the parameters of another newly defined %{check} substitution:

; DEFINE: %{cflags} =
; DEFINE: %{fcflags} =

; DEFINE: %{check} =                                                  \
; DEFINE:   %clang_cc1 -verify -fopenmp -fopenmp-version=51 %{cflags} \
; DEFINE:              -emit-llvm -o - %s |                           \
; DEFINE:     FileCheck %{fcflags} %s

Alternatively, the above substitutions can be defined in a lit configuration file to be shared with other test files. Either way, the test file can then specify directives like the following to redefine the parameter substitutions as desired before each use of %{check} in a RUN: line:

; REDEFINE: %{cflags} = -triple x86_64-apple-darwin10.6.0 -fopenmp-simd
; REDEFINE: %{fcflags} = -check-prefix=SIMD
; RUN: %{check}

; REDEFINE: %{cflags} = -triple x86_64-unknown-linux-gnu -fopenmp-simd
; REDEFINE: %{fcflags} = -check-prefix=SIMD
; RUN: %{check}

; REDEFINE: %{cflags} = -triple x86_64-apple-darwin10.6.0
; REDEFINE: %{fcflags} = -check-prefix=NO-SIMD
; RUN: %{check}

; REDEFINE: %{cflags} = -triple x86_64-unknown-linux-gnu
; REDEFINE: %{fcflags} = -check-prefix=NO-SIMD
; RUN: %{check}

Besides providing initial values, the initial DEFINE: directives for the parameter substitutions in the above example serve a second purpose: they establish the substitution order so that both %{check} and its parameters expand as desired. There’s a simple way to remember the required definition order in a test file: define a substitution before any substitution that might refer to it.

In general, substitution expansion behaves as follows:

  • Upon arriving at each RUN: line, lit expands all substitutions in that RUN: line using their current values from the substitution list. No substitution expansion is performed immediately at DEFINE: and REDEFINE: directives except %(line), %(line+<number>), and %(line-<number>).

  • When expanding substitutions in a RUN: line, lit makes only one pass through the substitution list by default. In this case, a substitution must have been inserted earlier in the substitution list than any substitution appearing in its value in order for the latter to expand. (For greater flexibility, you can enable multiple passes through the substitution list by setting recursiveExpansionLimit in a lit configuration file.)

  • While lit configuration files can insert anywhere in the substitution list, the insertion behavior of the DEFINE: and REDEFINE: directives is specified below and is designed specifically for the use case presented in the example above.

  • Defining a substitution in terms of itself, whether directly or via other substitutions, should be avoided. It usually produces an infinitely recursive definition that cannot be fully expanded. It does not define the substitution in terms of its previous value, even when using REDEFINE:.

The relationship between the DEFINE: and REDEFINE: directive is analogous to the relationship between a variable declaration and variable assignment in many programming languages:

  • DEFINE: %{name} = value

    This directive assigns the specified value to a new substitution whose pattern is %{name}, or it reports an error if there is already a substitution whose pattern contains %{name} because that could produce confusing expansions (e.g., a lit configuration file might define a substitution with the pattern %{name}\[0\]). The new substitution is inserted at the start of the substitution list so that it will expand first. Thus, its value can contain any substitution previously defined, whether in the same test file or in a lit configuration file, and both will expand.

  • REDEFINE: %{name} = value

    This directive assigns the specified value to an existing substitution whose pattern is %{name}, or it reports an error if there are no substitutions with that pattern or if there are multiple substitutions whose patterns contain %{name}. The substitution’s current position in the substitution list does not change so that expansion order relative to other existing substitutions is preserved.

The following properties apply to both the DEFINE: and REDEFINE: directives:

  • Substitution name: In the directive, whitespace immediately before or after %{name} is optional and discarded. %{name} must start with %{, it must end with }, and the rest must start with a letter or underscore and contain only alphanumeric characters, hyphens, underscores, and colons. This syntax has a few advantages:

    • It is impossible for %{name} to contain sequences that are special in python’s re.sub patterns. Otherwise, attempting to specify %{name} as a substitution pattern in a lit configuration file could produce confusing expansions.

    • The braces help avoid the possibility that another substitution’s pattern will match part of %{name} or vice-versa, producing confusing expansions. However, the patterns of substitutions defined by lit configuration files and by lit itself are not restricted to this form, so overlaps are still theoretically possible.

  • Substitution value: The value includes all text from the first non-whitespace character after = to the last non-whitespace character. If there is no non-whitespace character after =, the value is the empty string. Escape sequences that can appear in python re.sub replacement strings are treated as plain text in the value.

  • Line continuations: If the last non-whitespace character on the line after : is \, then the next directive must use the same directive keyword (e.g., DEFINE:) , and it is an error if there is no additional directive. That directive serves as a continuation. That is, before following the rules above to parse the text after : in either directive, lit joins that text together to form a single directive, replaces the \ with a single space, and removes any other whitespace that is now adjacent to that space. A continuation can be continued in the same manner. A continuation containing only whitespace after its : is an error.


As described in the previous section, when expanding substitutions in a RUN: line, lit makes only one pass through the substitution list by default. Thus, if substitutions are not defined in the proper order, some will remain in the RUN: line unexpanded. For example, the following directives refer to %{inner} within %{outer} but do not define %{inner} until after %{outer}:

; By default, this definition order does not enable full expansion.

; DEFINE: %{outer} = %{inner}
; DEFINE: %{inner} = expanded

; RUN: echo '%{outer}'

DEFINE: inserts substitutions at the start of the substitution list, so %{inner} expands first but has no effect because the original RUN: line does not contain %{inner}. Next, %{outer} expands, and the output of the echo command becomes:


Of course, one way to fix this simple case is to reverse the definitions of %{outer} and %{inner}. However, if a test has a complex set of substitutions that can all reference each other, there might not exist a sufficient substitution order.

To address such use cases, lit configuration files support config.recursiveExpansionLimit, which can be set to a non-negative integer to specify the maximum number of passes through the substitution list. Thus, in the above example, setting the limit to 2 would cause lit to make a second pass that expands %{inner} in the RUN: line, and the output from the echo command when then be:


To improve performance, lit will stop making passes when it notices the RUN: line has stopped changing. In the above example, setting the limit higher than 2 is thus harmless.

To facilitate debugging, after reaching the limit, lit will make one extra pass and report an error if the RUN: line changes again. In the above example, setting the limit to 1 will thus cause lit to report an error instead of producing incorrect output.


The llvm lit configuration allows to customize some things with user options:

llc, opt, …

Substitute the respective llvm tool name with a custom command line. This allows to specify custom paths and default arguments for these tools. Example:

% llvm-lit “-Dllc=llc -verify-machineinstrs”


Enable the execution of long running tests.


Load the specified lit configuration instead of the default one.

Other Features

To make RUN line writing easier, there are several helper programs. These helpers are in the PATH when running tests, so you can just call them using their name. For example:


This program runs its arguments and then inverts the result code from it. Zero result codes become 1. Non-zero result codes become 0.

To make the output more useful, lit will scan the lines of the test case for ones that contain a pattern that matches PR[0-9]+. This is the syntax for specifying a PR (Problem Report) number that is related to the test case. The number after “PR” specifies the LLVM Bugzilla number. When a PR number is specified, it will be used in the pass/fail reporting. This is useful to quickly get some context when a test fails.

Finally, any line that contains “END.” will cause the special interpretation of lines to terminate. This is generally done right after the last RUN: line. This has two side effects:

  1. it prevents special interpretation of lines that are part of the test program, not the instructions to the test case, and

  2. it speeds things up for really big test cases by avoiding interpretation of the remainder of the file.