SOP: Adding rustc_codegen_tuffy Codegen Tests

Purpose

This SOP defines the standard procedure for adding new codegen tests to rustc_codegen_tuffy/tests/codegen/. These tests verify MIR-to-IR translation correctness.

When to Use

• After fixing a bug in rustc_codegen_tuffy/mir_to_ir/ (regression test)
• When adding new translation logic for MIR constructs
• When modifying existing translation behavior

Prerequisites

• Backend must be built: cd rustc_codegen_tuffy && cargo build
• Understand which mir_to_ir/ module your test targets

Procedure

1. Determine Test Location

Organize tests by the mir_to_ir/ module being tested:

2. Create Test File

Create a new .rs file in the appropriate directory with a descriptive name:

tests/codegen/rvalue/my_feature.rs

3. Write Test Structure

Use this template:

#![allow(unused)]
fn main() {
// compile-flags: -Zmir-opt-level=3 -C debug-assertions=off

#![crate_type = "lib"]
#![no_std]

#[no_mangle]
pub fn test_function(a: i32) -> i32 {
    // Minimal code that exercises the feature
    a + 1
}
}

Key requirements:

• Use standard compile flags: -Zmir-opt-level=3 -C debug-assertions=off
• Include #![crate_type = "lib"] and #![no_std]
• Use #[no_mangle] on test functions
• Keep code minimal and focused
• Use meaningful identifier names

4. Generate CHECK Lines

Run the update script to automatically generate CHECK annotations:

cd rustc_codegen_tuffy
./tests/update-codegen-test.sh tests/codegen/rvalue/my_feature.rs

This will:

• Compile the test with the tuffy backend
• Capture MIR and IR output
• Insert CHECK lines into the test file

CRITICAL: Never write CHECK lines manually. Always use update-codegen-test.sh.

5. Verify Test Passes

Run the codegen test suite:

cd rustc_codegen_tuffy
./tests/run-codegen-tests.sh

Verify your new test passes.

6. Review Generated Output

Open the test file and review:

• CHECK lines match expected IR structure
• MIR output is as expected
• Test exercises the intended code path

7. Commit

Commit the test file with an appropriate message:

test(rustc-codegen): add codegen test for <feature>

Add regression test for <bug> or test coverage for <feature>.
Verifies that <specific behavior> translates correctly to IR.

Notes

• CHECK lines use exact string matching (not regex)
• Indentation in CHECK lines must match IR output exactly
• Each CHECK line must appear in order in the generated output
• Empty CHECK lines (// CHECK:) preserve blank lines in output
• The script automatically filters out panic messages from IR output

Common Patterns

Testing Multiple Functions

#![allow(unused)]
fn main() {
#[no_mangle]
pub fn func1(a: i32) -> i32 { a + 1 }

#[no_mangle]
pub fn func2(a: u64) -> u64 { a * 2 }
}

Each function will have separate MIR and IR CHECK blocks.

Testing with Specific Compile Flags

#![allow(unused)]
fn main() {
// compile-flags: -Zmir-opt-level=0 -C opt-level=0
}

Only use non-standard flags when testing specific behaviors (e.g., debug assertions, unoptimized codegen).

Testing Intrinsics

#![allow(unused)]
fn main() {
#[no_mangle]
pub fn test_rotate(x: u32, n: u32) -> u32 {
    x.rotate_left(n)
}
}

The test will capture both the high-level intrinsic call and its lowered implementation.

References

• rustc_codegen_tuffy/README.md — Component architecture and testing conventions
• rustc_codegen_tuffy/tests/update-codegen-test.sh — CHECK line generation script
• rustc_codegen_tuffy/tests/run-codegen-tests.sh — Test runner