Known problems

Known problems of the current release, with detection and workaround. Entries are removed when a release fixes them; problems in the underlying LLVM tools stay listed with their upstream references.

Problem	Detection	Workaround / status
Per-instantiation counting in llvm-cov. Template-heavy C++ files can show lower line coverage than under gcov; LF/LH totals may disagree with the file’s own DA rows.	Compare the per-file summary with the DA rows of the LCOV.	Upstream behaviour (llvm-project #93843, #111743, #119299). The error is towards under-reporting; numbers are reported as llvm-cov produces them.
Rust branch coverage needs an unstable rustc flag. -Zcoverage-options=branch works on rolling (nightly-based) Ferrocene builds only.	Rust rows show - in the branch columns.	On a stable-channel Ferrocene drop the flag; Rust branch coverage is then not available.
Clang warns where GCC does not. Repositories with treat_warnings_as_errors fail to compile under the coverage config.	-Werror failures only under --config=llvm_cov.	Add --features=-treat_warnings_as_errors and --host_features=-treat_warnings_as_errors to the coverage config.
Containerised tests produce no coverage. Instrumented binaries inside stock containers lack runtime dependencies and profraw files never reach the host.	Exit 127 in the test log; no profraw.	Exclude containerised or system tests from the coverage run; they keep running in the regular test jobs.
QNX on-target coverage covers C++ only. rustc emits no gcov counters, and the LLVM profile transport from the QEMU guest is not established yet (tooling issue #427, track 2).	Rust sources listed as not-instrumented in unmapped_files.txt of a QNX report.	Measure Rust with the Linux (LLVM) run of the same tree.
Vendored external headers have no data on the gcov backend. Bazel’s per-test collector keeps only files of its instrumented-files manifest, which never lists sources from external repositories, although gcov itself recorded them.	Such a header is no-data in a QNX report and measured in the Linux report of the same tree.	Known limitation of Bazel’s collector; use the Linux report for those headers.
gcov and LLVM count different lines. gcov reports only lines the compiler emitted code for: unused inline functions and closing braces have no line, while LLVM’s mapping keeps unused functions at 0 %.	Different totals for the same file on QNX and Linux.	Expected; compare the two reports per file, do not merge them.
An archive is rejected by llvm-cov because one member has no coverage mapping: the lib.rmeta of a Rust rlib, or the object of an empty translation unit.	no coverage data found on an archive; untested files of that library missing from the report.	Handled since the pipeline passes only members with a mapping to llvm-cov; if seen, the installed version predates the fix.
A header compiled under two different paths is reported once. When a translation unit includes a header through its _virtual_includes/ path and another through the declared path, the compiler produces two coverage entries for one file; the reporter keeps the declared-path variant and drops the other.	WARNING: <file> is compiled under several paths in the reporter log.	Expected; hits recorded only through the dropped variant are not counted. Include the header consistently.
An in-scope header is absent from the report. A header no translation unit includes, or one that contains only templates that are never instantiated, produces no code and therefore no coverage mapping; llvm-cov cannot show it, not even at 0 %.	The file is named in the job summary under “In-scope files without coverage data”, in unmapped_files.txt of the archive and in a reporter WARNING. Headers whose same-named source file has data (declaration-only) and placeholder sources that were compiled but hold no code of their own are listed in the same file under their own category and are not findings.	Decide per file: write a test that instantiates it (it is shipped API), or remove it from the target’s hdrs (it is not needed).
A header reached through several targets is compiled under several names. Virtual-include trees of targets outside the scope are resolved to the declared header by their path tail; if two in-scope files share that tail the header stays unresolved.	WARNING: ... matches several in-scope files in the reporter log; the header appears as no-data.	Rename one of the files, or declare the header only once.
A source could not be staged for llvm-cov. The reporter reads the sources from the scope’s exported files; a file that is neither there nor in the workspace directory gets no HTML page (its numbers stay in the index and the LCOV).	WARNING: N in-scope sources were not found in the reporter log, an index row without a link.	Report it; every declared source is expected to be exported.
Libraries tested from a test subpackage show 0 % or no-data. Bazel guesses --instrumentation_filter from the packages of the test targets and strips only a trailing /tests; score/os tested from score/os/test is outside the guess. On the gcov backend Bazel’s collector then drops the library’s counters; on both backends a library without an instrumented direct dep is compiled without counters.	Whole directories at 0 % on QNX that are covered on Linux; WARNING: N in-scope files have no test data although their directory is tested from a test/ or tests/ subdirectory in the reporter log; Using default value for --instrumentation_filter in the Bazel output.	Set --instrumentation_filter=^//<root>[/:] in every coverage config (user manual, step 4).