src/llvm-coverage-instrumentation.md

# LLVM source-based code coverage

<!-- toc -->

`rustc` supports detailed source-based code and test coverage analysis
with a command line option (`-C instrument-coverage`) that instruments Rust
libraries and binaries with additional instructions and data, at compile time.

The coverage instrumentation injects calls to the LLVM intrinsic instruction
[`llvm.instrprof.increment`][llvm-instrprof-increment] at code branches
(based on a MIR-based control flow analysis), and LLVM converts these to
instructions that increment static counters, when executed. The LLVM coverage
instrumentation also requires a [Coverage Map] that encodes source metadata,
mapping counter IDs--directly and indirectly--to the file locations (with
start and end line and column).

Rust libraries, with or without coverage instrumentation, can be linked into
instrumented binaries. When the program is executed and cleanly terminates,
LLVM libraries write the final counter values to a file (`default.profraw` or
a custom file set through environment variable `LLVM_PROFILE_FILE`).

Developers use existing LLVM coverage analysis tools to decode `.profraw`
files, with corresponding Coverage Maps (from matching binaries that produced
them), and generate various reports for analysis, for example:

<img alt="Screenshot of sample `llvm-cov show` result, for function add_quoted_string"
 src="img/llvm-cov-show-01.png" class="center"/>
<br/>

Detailed instructions and examples are documented in the
[rustc book][rustc-book-instrument-coverage].

[llvm-instrprof-increment]: https://llvm.org/docs/LangRef.html#llvm-instrprof-increment-intrinsic
[coverage map]: https://llvm.org/docs/CoverageMappingFormat.html
[rustc-book-instrument-coverage]: https://doc.rust-lang.org/nightly/rustc/instrument-coverage.html

## Recommended `bootstrap.toml` settings

When working on the coverage instrumentation code, it is usually necessary to
**enable the profiler runtime** by setting `profiler = true` in `[build]`.
This allows the compiler to produce instrumented binaries, and makes it possible
to run the full coverage test suite.

Enabling debug assertions in the compiler and in LLVM is recommended, but not
mandatory.

```toml
# Similar to the "compiler" profile, but also enables debug assertions in LLVM.
# These assertions can detect malformed coverage mappings in some cases.
profile = "codegen"

[build]
# IMPORTANT: This tells the build system to build the LLVM profiler runtime.
# Without it, the compiler can't produce coverage-instrumented binaries,
# and many of the coverage tests will be skipped.
profiler = true

[rust]
# Enable debug assertions in the compiler.
debug-assertions = true
```

## Rust symbol mangling

`-C instrument-coverage` automatically enables Rust symbol mangling `v0` (as
if the user specified `-C symbol-mangling-version=v0` option when invoking
`rustc`) to ensure consistent and reversible name mangling. This has two
important benefits:

1. LLVM coverage tools can analyze coverage over multiple runs, including some
   changes to source code; so mangled names must be consistent across compilations.
2. LLVM coverage reports can report coverage by function, and even separates
   out the coverage counts of each unique instantiation of a generic function,
   if invoked with multiple type substitution variations.

## The LLVM profiler runtime

Coverage data is only generated by running the executable Rust program. `rustc`
statically links coverage-instrumented binaries with LLVM runtime code
([compiler-rt][compiler-rt-profile]) that implements program hooks
(such as an `exit` hook) to write the counter values to the `.profraw` file.

In the `rustc` source tree,
`library/profiler_builtins` bundles the LLVM `compiler-rt` code into a Rust library crate.
Note that when building `rustc`,
`profiler_builtins` is only included when `build.profiler = true` is set in `bootstrap.toml`.

When compiling with `-C instrument-coverage`,
[`CrateLoader::postprocess()`][crate-loader-postprocess] dynamically loads
`profiler_builtins` by calling `inject_profiler_runtime()`.

[compiler-rt-profile]: https://github.com/llvm/llvm-project/tree/main/compiler-rt/lib/profile
[crate-loader-postprocess]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_metadata/creader/struct.CrateLoader.html#method.postprocess

## Testing coverage instrumentation

[(See also the compiletest documentation for the `tests/coverage`
test suite.)](./tests/compiletest.md#coverage-tests)

Coverage instrumentation in the MIR is validated by a `mir-opt` test:
[`tests/mir-opt/coverage/instrument_coverage.rs`].

Coverage instrumentation in LLVM IR is validated by the [`tests/coverage`]
test suite in `coverage-map` mode.
These tests compile a test program to LLVM IR assembly, and then
use the [`src/tools/coverage-dump`] tool to extract and pretty-print the
coverage mappings that would be embedded in the final binary.

End-to-end testing of coverage instrumentation and coverage reporting is
performed by the [`tests/coverage`] test suite in `coverage-run` mode,
and by the [`tests/coverage-run-rustdoc`] test suite.
These tests compile and run a test program with coverage
instrumentation, then use LLVM tools to convert the coverage data into a
human-readable coverage report.

> Tests in `coverage-run` mode have an implicit `//@ needs-profiler-runtime`
> directive, so they will be skipped if the profiler runtime has not been
> [enabled in `bootstrap.toml`](#recommended-configtoml-settings).

Finally, the [`tests/codegen/instrument-coverage/testprog.rs`] test compiles a simple Rust program
with `-C instrument-coverage` and compares the compiled program's LLVM IR to
expected LLVM IR instructions and structured data for a coverage-enabled
program, including various checks for Coverage Map-related metadata and the LLVM
intrinsic calls to increment the runtime counters.

Expected results for the `coverage`, `coverage-run-rustdoc`,
and `mir-opt` tests can be refreshed by running:

```shell
./x test coverage --bless
./x test coverage-run-rustdoc --bless
./x test tests/mir-opt --bless
```

[`tests/mir-opt/coverage/instrument_coverage.rs`]: https://github.com/rust-lang/rust/blob/master/tests/mir-opt/coverage/instrument_coverage.rs
[`tests/coverage`]: https://github.com/rust-lang/rust/tree/master/tests/coverage
[`src/tools/coverage-dump`]: https://github.com/rust-lang/rust/tree/master/src/tools/coverage-dump
[`tests/coverage-run-rustdoc`]: https://github.com/rust-lang/rust/tree/master/tests/coverage-run-rustdoc
[`tests/codegen/instrument-coverage/testprog.rs`]: https://github.com/rust-lang/rust/blob/master/tests/mir-opt/coverage/instrument_coverage.rs