| Issue |
148499
|
| Summary |
GlobalMerge does not work well with comdat group sections, breaking coverage
|
| Labels |
new issue
|
| Assignees |
|
| Reporter |
arielb1
|
Sorry for posting a bug in Rust format, it would be hard to illustrate the problem with an LLVM IR example.
See rust-lang/rust#143892
On an aarch64/Graviton 2 (`m6g.2xlarge`) processor, doing the following steps
```
cat > coverage-test.rs << _EOF_
mod x {
pub(crate) fn foo<T>(t: T) {}
#[inline(never)]
pub(crate) fn zzz() {
foo(4u32);
}
}
#[test]
fn my_test() {
x::zzz();
}
_EOF_
rustc coverage-test.rs -C instrument-coverage -C opt-level=3 --test -C codegen-units=16
rm -f *.profraw # remove any pre-existing coverage files
./coverage-test
echo *.profraw | llvm-profdata merge -f - --binary -o prof.bin
$COV export coverage-test --instr-profile prof.bin --format text
```
results in a coverage of 33% (`"functions":{"count":3,"covered":1,"percent":33.333333333333329}`) rather than of 100%, as the lines inside the `mod x` are "skipped".
Using x86-64, or `-C opt-level=2`, or `-C llvm-args=-aarch64-enable-global-merge=false`, or `-C link-args=-Wl,-no-gc-sections`, or rustc 1.86, there is no problem.
Using nightly, the problem still exists.
### Meta
`rustc --version --verbose`:
```
rustc 1.87.0 (17067e9ac 2025-05-09)
binary: rustc
commit-hash: 17067e9ac6d7ecb70e50f92c1944e545188d2359
commit-date: 2025-05-09
host: aarch64-unknown-linux-gnu
release: 1.87.0
LLVM version: 20.1.1
```
### Investigation
See <https://maskray.me/blog/2021-07-25-comdat-and-section-group>
`-C instrument-coverage` generates section groups that look like this for every coverage counter:
```
group section [ 14] `.group' [__profc__RINvNtCseYCwLgNLg2S_13coverage_test1x3foomEB4_] contains 3 sections:
[Index] Name
[ 15] __llvm_prf_cnts
[ 16] __llvm_prf_data
[ 17] .rela__llvm_prf_data
```
The `__llvm_prf_data` contains metadata that needs to be present in order for coverage to count the lines. The `__llvm_prf_cnts` contains a counter that is incremented every time the expected code section is hit.
Coverage uses the comdat functionality to ensure that as long as the counter is compiled in, the data area is compiled in as well. The `__llvm_prf_data` is marked as `@llvm.compiler.used` to ensure only the linker can gc it.
When using the `GlobalMerge` pass, it merges the `__llvm_prf_cnts` into one big section, so the section group looks like this:
```
group section [ 13] `.group' [__profc__RINvNtCseYCwLgNLg2S_13coverage_test1x3foomEB4_] contains 2 sections:
[Index] Name
[ 14] __llvm_prf_data
[ 15] .rela__llvm_prf_data
```
As the `__llvm_prf_cnts` has been moved to one big section, and nothing prevents the `__llvm_prf_data` from being gc'd.
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