Testing Cranelift

Cranelift is tested at multiple levels of abstraction and integration. When possible, Rust unit tests are used to verify single functions and types. When testing the interaction between compiler passes, file-level tests are appropriate.

The top-level shell script test-all.sh runs all of the tests in the Cranelift repository.

Rust tests

Rust and Cargo have good support for testing. Cranelift uses unit tests, doc tests, and integration tests where appropriate.

Unit tests

Unit test live in a tests sub-module of the code they are testing:

pub fn add(x: u32, y: u32) -> u32 {
    x + y

mod tests {
    use super::add;

    check_add() {
        assert_eq!(add(2, 2), 4);

Since sub-modules have access to non-public items in a Rust module, unit tests can be used to test module-internal functions and types too.

Doc tests

Documentation comments can contain code snippets which are also compiled and tested:

//! The `Flags` struct is immutable once it has been created. A `Builder` instance is used to
//! create it.
//! # Example
//! ```
//! use cranelift_codegen::settings::{self, Configurable};
//! let mut b = settings::builder();
//! b.set("opt_level", "fastest");
//! let f = settings::Flags::new(&b);
//! assert_eq!(f.opt_level(), settings::OptLevel::Fastest);
//! ```

These tests are useful for demonstrating how to use an API, and running them regularly makes sure that they stay up to date. Documentation tests are not appropriate for lots of assertions; use unit tests for that.

Integration tests

Integration tests are Rust source files that are compiled and linked individually. They are used to exercise the external API of the crates under test.

These tests are usually found in the tests top-level directory where they have access to all the crates in the Cranelift repository. The cranelift-codegen and cranelift-reader crates have no external dependencies, which can make testing tedious. Integration tests that don’t need to depend on other crates can be placed in cranelift-codegen/tests and cranelift-reader/tests.

File tests

Compilers work with large data structures representing programs, and it quickly gets unwieldy to generate test data programmatically. File-level tests make it easier to provide substantial input functions for the compiler tests.

File tests are *.clif files in the filetests/ directory hierarchy. Each file has a header describing what to test followed by a number of input functions in the Cranelift textual intermediate representation:

test_file     ::=  test_header function_list
test_header   ::=  test_commands (isa_specs | settings)
test_commands ::=  test_command { test_command }
test_command  ::=  "test" test_name { option } "\n"

The available test commands are described below.

Many test commands only make sense in the context of a target instruction set architecture. These tests require one or more ISA specifications in the test header:

isa_specs ::=  { [settings] isa_spec }
isa_spec  ::=  "isa" isa_name { option } "\n"

The options given on the isa line modify the ISA-specific settings defined in cranelift-codegen/meta-python/isa/*/settings.py.

All types of tests allow shared Cranelift settings to be modified:

settings ::=  { setting }
setting  ::=  "set" { option } "\n"
option   ::=  flag | setting "=" value

The shared settings available for all target ISAs are defined in cranelift-codegen/meta-python/base/settings.py.

The set lines apply settings cumulatively:

test legalizer
set opt_level=best
set is_pic=1
isa riscv64
set is_pic=0
isa supports_m=false

function %foo() {}

This example will run the legalizer test twice. Both runs will have opt_level=best, but they will have different is_pic settings. The 32-bit run will also have the RISC-V specific flag supports_m disabled.

The filetests are run automatically as part of cargo test, and they can also be run manually with the clif-util test command.


Many of the test commands described below use filecheck to verify their output. Filecheck is a Rust implementation of the LLVM tool of the same name. See the documentation for details of its syntax.

Comments in .clif files are associated with the entity they follow. This typically means an instruction or the whole function. Those tests that use filecheck will extract comments associated with each function (or its entities) and scan them for filecheck directives. The test output for each function is then matched against the filecheck directives for that function.

Comments appearing before the first function in a file apply to every function. This is useful for defining common regular expression variables with the regex: directive, for example.

Note that LLVM’s file tests don’t separate filecheck directives by their associated function. It verifies the concatenated output against all filecheck directives in the test file. LLVM’s FileCheck command has a CHECK-LABEL: directive to help separate the output from different functions. Cranelift’s tests don’t need this.

test cat

This is one of the simplest file tests, used for testing the conversion to and from textual IR. The test cat command simply parses each function and converts it back to text again. The text of each function is then matched against the associated filecheck directives.


function %r1() -> i32, f32 {
    v10 = iconst.i32 3
    v20 = f32const 0.0
    return v10, v20
; sameln: function %r1() -> i32, f32 {
; nextln: ebb0:
; nextln:     v10 = iconst.i32 3
; nextln:     v20 = f32const 0.0
; nextln:     return v10, v20
; nextln: }

test verifier

Run each function through the IR verifier and check that it produces the expected error messages.

Expected error messages are indicated with an error: directive on the instruction that produces the verifier error. Both the error message and reported location of the error is verified:

test verifier

function %test(i32) {
    ebb0(v0: i32):
        jump ebb1       ; error: terminator

This example test passes if the verifier fails with an error message containing the sub-string "terminator" and the error is reported for the jump instruction.

If a function contains no error: annotations, the test passes if the function verifies correctly.

test print-cfg

Print the control flow graph of each function as a Graphviz graph, and run filecheck over the result. See also the clif-util print-cfg command:

; For testing cfg generation. This code is nonsense.
test print-cfg
test verifier

function %nonsense(i32, i32) -> f32 {
; check: digraph %nonsense {
; regex: I=\binst\d+\b
; check: label="{ebb0 | <$(BRZ=$I)>brz ebb2 | <$(JUMP=$I)>jump ebb1}"]

ebb0(v0: i32, v1: i32):
    brz v1, ebb2            ; unordered: ebb0:$BRZ -> ebb2
    v2 = iconst.i32 0
    jump ebb1(v2)           ; unordered: ebb0:$JUMP -> ebb1

ebb1(v5: i32):
    return v0

    v100 = f32const 0.0
    return v100

test domtree

Compute the dominator tree of each function and validate it against the dominates: annotations:

test domtree

function %test(i32) {
    ebb0(v0: i32):
        jump ebb1     ; dominates: ebb1
        brz v0, ebb3  ; dominates: ebb3
        jump ebb2     ; dominates: ebb2
        jump ebb3

Every reachable extended basic block except for the entry block has an immediate dominator which is a jump or branch instruction. This test passes if the dominates: annotations on the immediate dominator instructions are both correct and complete.

This test also sends the computed CFG post-order through filecheck.

test legalizer

Legalize each function for the specified target ISA and run the resulting function through filecheck. This test command can be used to validate the encodings selected for legal instructions as well as the instruction transformations performed by the legalizer.

test regalloc

Test the register allocator.

First, each function is legalized for the specified target ISA. This is required for register allocation since the instruction encodings provide register class constraints to the register allocator.

Second, the register allocator is run on the function, inserting spill code and assigning registers and stack slots to all values.

The resulting function is then run through filecheck.

test binemit

Test the emission of binary machine code.

The functions must contains instructions that are annotated with both encodings and value locations (registers or stack slots). For instructions that are annotated with a bin: directive, the emitted hexadecimal machine code for that instruction is compared to the directive:

test binemit
isa riscv

function %int32() {
    [-,%x5]             v0 = iconst.i32 1
    [-,%x6]             v1 = iconst.i32 2
    [R#0c,%x7]          v10 = iadd v0, v1       ; bin: 006283b3
    [R#200c,%x8]        v11 = isub v0, v1       ; bin: 40628433

If any instructions are unencoded (indicated with a [-] encoding field), they will be encoded using the same mechanism as the legalizer uses. However, illegal instructions for the ISA won’t be expanded into other instruction sequences. Instead the test will fail.

Value locations must be present if they are required to compute the binary bits. Missing value locations will cause the test to crash.

test simple-gvn

Test the simple GVN pass.

The simple GVN pass is run on each function, and then results are run through filecheck.

test licm

Test the LICM pass.

The LICM pass is run on each function, and then results are run through filecheck.

test dce

Test the DCE pass.

The DCE pass is run on each function, and then results are run through filecheck.

test shrink

Test the instruction shrinking pass.

The shrink pass is run on each function, and then results are run through filecheck.

test preopt

Test the preopt pass.

The preopt pass is run on each function, and then results are run through filecheck.

test postopt

Test the postopt pass.

The postopt pass is run on each function, and then results are run through filecheck.

test compile

Test the whole code generation pipeline.

Each function is passed through the full Context::compile() function which is normally used to compile code. This type of test often depends on assertions or verifier errors, but it is also possible to use filecheck directives which will be matched against the final form of the Cranelift IR right before binary machine code emission.

test run

Compile and execute a function.

Add a ; run directive after each function that should be executed. These functions must have the signature () -> bNN where bNN is some sort of boolean, e.g. b1 or b32. A true value is interpreted as a successful test execution, whereas a false value is interpreted as a failed test.


test run

function %trivial_test() -> b1 {
    v0 = bconst.b1 true
    return v0
; run