[libc-commits] [libc] [libc][docs] Add developer guide for source-level code coverage (PR #214526)

Fri Aug 7 02:04:19 PDT 2026

llvmorg-github-actions[bot] wrote:




@llvm/pr-subscribers-libc

Author: Tapiwa Gonga (tapiwagonga)

<details>
<summary>Changes</summary>

This patch adds official developer documentation explaining how to configure, collect, and analyze source-level code coverage for LLVM-libc.

**Motivation:**
Because LLVM-libc is built as a freestanding (-nostdlib) library, standard compiler-rt coverage workflows fail out of the box. This guide provides developers with the exact build configuration, test execution commands, and profile-merging steps required to generate accurate coverage reports across the library.

**Content Summary:**
1. libc/docs/dev/code_coverage.md:
- Scope and Exclusions: Documents Linux platform support, execution boundaries (-DLIBC_TEST_UNIT_TEST_ONLY=ON), and report filtering (-ignore-filename-regex=".*(test|utils).*").
- CMake Configuration: Lists required flags (-DLLVM_LIBC_ENABLE_COVERAGE=ON -DLIBC_TEST_UNIT_TEST_ONLY=ON).
- Execution Options: Details whole-codebase execution (libc-unit-tests) alongside isolated single/targeted test execution (libc.test.src.<subsystem>.<function>_test.__unit__).
- Reporting Options: Provides commands for terminal text summaries (llvm-cov report) and interactive HTML dashboards (llvm-cov show -format=html).

2. libc/docs/dev/index.md:
- Registers code_coverage in the Sphinx toctree so the guide is automatically rendered in the navigation menu of libc.llvm.org/dev/.

---
Full diff: https://github.com/llvm/llvm-project/pull/214526.diff


2 Files Affected:

- (added) libc/docs/dev/code_coverage.md (+110) 
- (modified) libc/docs/dev/index.md (+1) 


``````````diff

diff --git a/libc/docs/dev/code_coverage.md b/libc/docs/dev/code_coverage.md
new file mode 100644
index 0000000000000..9d96d5634fc53
--- /dev/null
+++ b/libc/docs/dev/code_coverage.md
@@ -0,0 +1,110 @@
+(code_coverage)=
+
+# Source-Level Code Coverage
+
+LLVM-libc supports source-level code coverage for its unit tests.
+
+Because `llvm-libc` unit tests are built as freestanding binaries (`-nostdlib`), standard `compiler-rt` coverage workflows fail in this environment as they inherently rely on the host's standard C library (`fopen`, `fwrite`), which is intentionally omitted to prevent host contamination.
+
+To bypass this constraint, the `LibcTestMain.cpp` test harness implements a custom profiling dumper:
+
+1. It silences the default `compiler-rt` dumper by overriding the global symbol: `extern "C" char __llvm_profile_filename[] = "/dev/null";`.
+2. It hooks into `atexit()` to dump coverage before the process ends. *(Note: Death tests that terminate via `_exit()`, `abort()`, or unhandled signals inherently bypass this hook. The parent test runner still correctly dumps its overall profile).*
+3. It determines the required buffer size via `__llvm_profile_get_size_for_buffer()` and allocates memory using `LIBC_NAMESPACE::linux_syscalls::mmap`.
+4. It extracts the raw profiling data from the compiler into the memory segment using `__llvm_profile_write_buffer()`.
+5. It writes the segment to a `.profraw` file using internal Linux syscall wrappers (`open`, `write`, `close`, `munmap`).
+
+## Limitations
+
+- **OS Support:** Because this relies on Linux system call wrappers, coverage extraction is strictly gated behind `#if defined(__linux__)`. On macOS or Windows builds, the coverage dumping step is gracefully bypassed, allowing the tests to compile normally.
+
+## 1. Setup and Configuration
+
+Before running any tests, you must clear old profile data and configure your CMake build directory to generate coverage instrumentation.
+
+```bash
+# 1. Clear previous profile artifacts
+find . -name "libc_cov_*.profraw" -delete
+rm -f libc_full.profdata profraw_list.txt
+
+# 2. Configure the build directory
+cmake -G Ninja -S runtimes -B build-cov \
+  -DLLVM_ENABLE_RUNTIMES="libc" \
+  -DLLVM_LIBC_FULL_BUILD=ON \
+  -DLLVM_LIBC_ENABLE_COVERAGE=ON \
+  -DCMAKE_CXX_COMPILER=clang++-19 \
+  -DCMAKE_C_COMPILER=clang-19
+```
+
+## 2. Build and Run Tests
+
+You have three options depending on how much of the library you want to test. (Targeted testing is significantly faster for local debugging).
+
+Test targets in `llvm-libc` follow a strict naming convention based on their directory structure: `libc.test.<directory_path>.<test_name>.__unit__`.
+
+### Option A: Whole Codebase
+To run the entire unit test suite (approx. 1,776 tests) and generate a massive, whole-codebase report:
+
+```bash
+ninja -C build-cov check-libc
+
+# If using Option A, do not define FILES_TO_REPORT in Step 3.
+```
+
+### Option B: Single Target
+To instantly run coverage on a single test, specify its exact Ninja target. For example, to test `isalpha`:
+
+```bash
+# Run only the specific target
+ninja -C build-cov libc.test.src.ctype.isalpha_test.__unit__
+
+# Define the source file to filter the report in Step 3
+FILES_TO_REPORT="libc/src/ctype/isalpha.cpp"
+```
+
+### Option C: Multiple Targets
+To run multiple isolated tests simultaneously, pass them as a space-separated list. For example, testing `isalpha` and `isdigit`:
+
+```bash
+# Define your targets 
+TARGETS="libc.test.src.ctype.isalpha_test.__unit__ libc.test.src.ctype.isdigit_test.__unit__"
+
+# Run the targets
+ninja -C build-cov $TARGETS
+
+# Define the source files to filter the report in Step 3
+FILES_TO_REPORT="libc/src/ctype/isalpha.cpp libc/src/ctype/isdigit.cpp"
+```
+
+## 3. Generate the Report
+
+Once your tests have finished running, merge the raw profile data and extract the executables to map the coverage back to the source code.
+
+```bash
+# 1. Merge raw profiles
+find . -name "libc_cov_*.profraw" > profraw_list.txt
+llvm-profdata-19 merge -sparse --input-files=profraw_list.txt -o libc_full.profdata
+
+# 2. Extract executables
+EXECUTABLES=($(find build-cov -type f -executable -name "*__build__"))
+OBJECTS=("${EXECUTABLES[@]:1}")
+OBJECTS=("${OBJECTS[@]/#/-object=}")
+```
+
+### Choose Your Output Type
+
+You can specify the format of your final coverage report by changing the output command.
+
+**Option A: Terminal Summary Table (Text)**
+This provides a quick text-based summary of your coverage percentages directly in the terminal:
+```bash
+llvm-cov-19 report -instr-profile=libc_full.profdata "${EXECUTABLES[0]}" "${OBJECTS[@]}" $FILES_TO_REPORT
+```
+
+**Option B: Line-by-Line Interactive Webpage (HTML)**
+This generates an interactive HTML website so you can visually inspect exactly which lines of code are missing coverage. You can change the `OUTPUT_DIR` variable to save it wherever you prefer:
+```bash
+OUTPUT_DIR="coverage_html"
+llvm-cov-19 show -instr-profile=libc_full.profdata -format=html -output-dir=$OUTPUT_DIR "${EXECUTABLES[0]}" "${OBJECTS[@]}" $FILES_TO_REPORT
+```
+*(After running this, open `coverage_html/index.html` in your web browser).*
diff --git a/libc/docs/dev/index.md b/libc/docs/dev/index.md
index 4ce78540c8fad..be0d0018df249 100644
--- a/libc/docs/dev/index.md
+++ b/libc/docs/dev/index.md
@@ -8,6 +8,7 @@ Navigate to the links below for information on the respective topics:
 :maxdepth: 1
 
 building_docs
+code_coverage
 code_style
 source_tree_layout
 entrypoints

``````````

</details>


https://github.com/llvm/llvm-project/pull/214526
