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Update test coverage documentation to LLVM 19 #4265

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Nov 13, 2024
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Update test coverage documentation to LLVM 19 #4265

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26 changes: 7 additions & 19 deletions guide/src/wasm-bindgen-test/coverage.md
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Original file line number Diff line number Diff line change
Expand Up @@ -22,11 +22,7 @@ Currently it is particularly difficult to [deliver compile-line arguments to pro

Make sure you are using `RUSTFLAGS=-Cinstrument-coverage -Zno-profiler-runtime`.

Due to the current limitation of `llvm-cov`, we can't collect profiling symbols from the generated `.wasm` files. Instead, we can grab them from the LLVM IR with `--emit=llvm-ir` by using Clang. Additionally, the emitted LLVM IR files by Rust contain invalid code that can't be parsed by Clang, so they need to be adjusted.

At the time of writing Rust Nightly uses LLVM v19, however [minicov] only supports LLVM v18. Usage of Clang or any LLVM tools must match the version used by [minicov].

[minicov]: https://crates.io/crates/minicov
Due to the current limitation of `llvm-cov`, we can't collect profiling symbols from the generated `.wasm` files. Instead, we can grab them from the LLVM IR with `--emit=llvm-ir` by using Clang. Usage of Clang or any LLVM tools must match the LLVM version used by Rust.

### Arguments to the test runner

Expand Down Expand Up @@ -58,11 +54,9 @@ CARGO_HOST_RUSTFLAGS=--cfg=wasm_bindgen_unstable_test_coverage \
RUSTFLAGS="-Cinstrument-coverage -Zno-profiler-runtime --emit=llvm-ir --cfg=wasm_bindgen_unstable_test_coverage" \
CARGO_TARGET_WASM32_UNKNOWN_UNKNOWN_RUNNER=wasm-bindgen-test-runner \
cargo +nightly test -Ztarget-applies-to-host -Zhost-config --tests
# Adjust the LLVM IR and compile to object files:
# Compile to object files:
# - Extract a list of compiled artifacts from Cargo and filter them with `jq`.
# - Figure out the path to the LLVM IR file corresponding to an artifact.
# - Replace every function body with `unreachable`.
# - Remove Rust-specific `range` annotations from function signatures.
# - Compile to object file with Clang and store for later usage with `llvm-cov`.
crate_name=name_of_the_tested_crate_in_snake_case
objects=()
Expand All @@ -81,20 +75,14 @@ do
file=$(dirname $file)/$(basename $file .wasm).ll
fi

perl -i -p0e 's/(^define.*?$).*?^}/$1\nstart:\n unreachable\n}/gms' $file
counter=1
while (( counter != 0 )); do
counter=$(perl -i -p0e '$c+= s/(^(define|declare)(,? [^\n ]+)*),? range\(.*?\)/$1/gm; END{print "$c"}' $file)
done

output = $(basename $file .ll).o
clang-18 $input -Wno-override-module -c -o $output
objects+=(-object $output)
output = $(basename $file .ll).o
clang-19 $file -Wno-override-module -c -o $output
objects+=(-object $output)
done
# Merge all generated raw profiling data.
llvm-profdata-18 merge -sparse *.profraw -o coverage.profdata
llvm-profdata-19 merge -sparse *.profraw -o coverage.profdata
# Produce test coverage data in the HTML format and pass the object files we generated earlier.
llvm-cov-18 show -show-instantiations=false -Xdemangler=rustfilt -output-dir coverage -format=html -instr-profile=coverage.profdata ${objects[@]} -sources src
llvm-cov-19 show -show-instantiations=false -Xdemangler=rustfilt -output-dir coverage -format=html -instr-profile=coverage.profdata ${objects[@]} -sources src
```

[rustc book]: https://doc.rust-lang.org/nightly/rustc/instrument-coverage.html
Expand Down