clang/docs/SourceBasedCodeCoverage.rst

==========================
Source-based Code Coverage
==========================

.. contents::
   :local:

Introduction
============

This document explains how to use clang's source-based code coverage feature.
It's called "source-based" because it operates on AST and preprocessor
information directly. This allows it to generate very precise coverage data.

Clang ships two other code coverage implementations:

* :doc:`SanitizerCoverage` - A low-overhead tool meant for use alongside the
  various sanitizers. It can provide up to edge-level coverage.

* gcov - A GCC-compatible coverage implementation which operates on DebugInfo.
  This is enabled by ``-ftest-coverage`` or ``--coverage``.

From this point onwards "code coverage" will refer to the source-based kind.

The code coverage workflow
==========================

The code coverage workflow consists of three main steps:

* Compiling with coverage enabled.

* Running the instrumented program.

* Creating coverage reports.

The next few sections work through a complete, copy-'n-paste friendly example
based on this program:

.. code-block:: cpp

    % cat <<EOF > foo.cc
    #define BAR(x) ((x) || (x))
    template <typename T> void foo(T x) {
      for (unsigned I = 0; I < 10; ++I) { BAR(I); }
    }
    int main() {
      foo<int>(0);
      foo<float>(0);
      return 0;
    }
    EOF

Compiling with coverage enabled
===============================

To compile code with coverage enabled, pass ``-fprofile-instr-generate
-fcoverage-mapping`` to the compiler:

.. code-block:: console

    # Step 1: Compile with coverage enabled.
    % clang++ -fprofile-instr-generate -fcoverage-mapping foo.cc -o foo

Note that linking together code with and without coverage instrumentation is
supported. Uninstrumented code simply won't be accounted for in reports.

Running the instrumented program
================================

The next step is to run the instrumented program. When the program exits it
will write a **raw profile** to the path specified by the ``LLVM_PROFILE_FILE``
environment variable. If that variable does not exist, the profile is written
to ``default.profraw`` in the current directory of the program. If
``LLVM_PROFILE_FILE`` contains a path to a non-existent directory, the missing
directory structure will be created.  Additionally, the following special
**pattern strings** are rewritten:

* "%p" expands out to the process ID.

* "%h" expands out to the hostname of the machine running the program.

* "%Nm" expands out to the instrumented binary's signature. When this pattern
  is specified, the runtime creates a pool of N raw profiles which are used for
  on-line profile merging. The runtime takes care of selecting a raw profile
  from the pool, locking it, and updating it before the program exits.  If N is
  not specified (i.e the pattern is "%m"), it's assumed that ``N = 1``. N must
  be between 1 and 9. The merge pool specifier can only occur once per filename
  pattern.

* "%c" expands out to nothing, but enables a mode in which profile counter
  updates are continuously synced to a file. This means that if the
  instrumented program crashes, or is killed by a signal, perfect coverage
  information can still be recovered. Continuous mode does not support value
  profiling for PGO, and is only supported on Darwin at the moment. Support for
  Linux may be mostly complete but requires testing, and support for Windows
  may require more extensive changes: please get involved if you are interested
  in porting this feature.

.. code-block:: console

    # Step 2: Run the program.
    % LLVM_PROFILE_FILE="foo.profraw" ./foo

Note that continuous mode is also used on Fuchsia where it's the only supported
mode, but the implementation is different. The Darwin and Linux implementation
relies on padding and the ability to map a file over the existing memory
mapping which is generally only available on POSIX systems and isn't suitable
for other platforms.

On Fuchsia, we rely on the ability to relocate counters at runtime using a
level of indirection. On every counter access, we add a bias to the counter
address. This bias is stored in ``__llvm_profile_counter_bias`` symbol that's
provided by the profile runtime and is initially set to zero, meaning no
relocation. The runtime can map the profile into memory at arbitrary locations,
and set bias to the offset between the original and the new counter location,
at which point every subsequent counter access will be to the new location,
which allows updating profile directly akin to the continuous mode.

The advantage of this approach is that doesn't require any special OS support.
The disadvantage is the extra overhead due to additional instructions required
for each counter access (overhead both in terms of binary size and performance)
plus duplication of counters (i.e. one copy in the binary itself and another
copy that's mapped into memory). This implementation can be also enabled for
other platforms by passing the ``-runtime-counter-relocation`` option to the
backend during compilation.

.. code-block:: console

    % clang++ -fprofile-instr-generate -fcoverage-mapping -mllvm -runtime-counter-relocation foo.cc -o foo

Creating coverage reports
=========================

Raw profiles have to be **indexed** before they can be used to generate
coverage reports. This is done using the "merge" tool in ``llvm-profdata``
(which can combine multiple raw profiles and index them at the same time):

.. code-block:: console

    # Step 3(a): Index the raw profile.
    % llvm-profdata merge -sparse foo.profraw -o foo.profdata

There are multiple different ways to render coverage reports. The simplest
option is to generate a line-oriented report:

.. code-block:: console

    # Step 3(b): Create a line-oriented coverage report.
    % llvm-cov show ./foo -instr-profile=foo.profdata

This report includes a summary view as well as dedicated sub-views for
templated functions and their instantiations. For our example program, we get
distinct views for ``foo<int>(...)`` and ``foo<float>(...)``.  If
``-show-line-counts-or-regions`` is enabled, ``llvm-cov`` displays sub-line
region counts (even in macro expansions):

.. code-block:: none

        1|   20|#define BAR(x) ((x) || (x))
                               ^20     ^2
        2|    2|template <typename T> void foo(T x) {
        3|   22|  for (unsigned I = 0; I < 10; ++I) { BAR(I); }
                                       ^22     ^20  ^20^20
        4|    2|}
    ------------------
    | void foo<int>(int):
    |      2|    1|template <typename T> void foo(T x) {
    |      3|   11|  for (unsigned I = 0; I < 10; ++I) { BAR(I); }
    |                                     ^11     ^10  ^10^10
    |      4|    1|}
    ------------------
    | void foo<float>(int):
    |      2|    1|template <typename T> void foo(T x) {
    |      3|   11|  for (unsigned I = 0; I < 10; ++I) { BAR(I); }
    |                                     ^11     ^10  ^10^10
    |      4|    1|}
    ------------------

To generate a file-level summary of coverage statistics instead of a
line-oriented report, try:

.. code-block:: console

    # Step 3(c): Create a coverage summary.
    % llvm-cov report ./foo -instr-profile=foo.profdata
    Filename           Regions    Missed Regions     Cover   Functions  Missed Functions  Executed       Lines      Missed Lines     Cover
    --------------------------------------------------------------------------------------------------------------------------------------
    /tmp/foo.cc             13                 0   100.00%           3                 0   100.00%          13                 0   100.00%
    --------------------------------------------------------------------------------------------------------------------------------------
    TOTAL                   13                 0   100.00%           3                 0   100.00%          13                 0   100.00%

The ``llvm-cov`` tool supports specifying a custom demangler, writing out
reports in a directory structure, and generating html reports. For the full
list of options, please refer to the `command guide
<https://llvm.org/docs/CommandGuide/llvm-cov.html>`_.

A few final notes:

* The ``-sparse`` flag is optional but can result in dramatically smaller
  indexed profiles. This option should not be used if the indexed profile will
  be reused for PGO.

* Raw profiles can be discarded after they are indexed. Advanced use of the
  profile runtime library allows an instrumented program to merge profiling
  information directly into an existing raw profile on disk. The details are
  out of scope.

* The ``llvm-profdata`` tool can be used to merge together multiple raw or
  indexed profiles. To combine profiling data from multiple runs of a program,
  try e.g:

  .. code-block:: console

      % llvm-profdata merge -sparse foo1.profraw foo2.profdata -o foo3.profdata

Exporting coverage data
=======================

Coverage data can be exported into JSON using the ``llvm-cov export``
sub-command. There is a comprehensive reference which defines the structure of
the exported data at a high level in the llvm-cov source code.

Interpreting reports
====================

There are four statistics tracked in a coverage summary:

* Function coverage is the percentage of functions which have been executed at
  least once. A function is considered to be executed if any of its
  instantiations are executed.

* Instantiation coverage is the percentage of function instantiations which
  have been executed at least once. Template functions and static inline
  functions from headers are two kinds of functions which may have multiple
  instantiations.

* Line coverage is the percentage of code lines which have been executed at
  least once. Only executable lines within function bodies are considered to be
  code lines.

* Region coverage is the percentage of code regions which have been executed at
  least once. A code region may span multiple lines (e.g in a large function
  body with no control flow). However, it's also possible for a single line to
  contain multiple code regions (e.g in "return x || y && z").

Of these four statistics, function coverage is usually the least granular while
region coverage is the most granular. The project-wide totals for each
statistic are listed in the summary.

Format compatibility guarantees
===============================

* There are no backwards or forwards compatibility guarantees for the raw
  profile format. Raw profiles may be dependent on the specific compiler
  revision used to generate them. It's inadvisable to store raw profiles for
  long periods of time.

* Tools must retain **backwards** compatibility with indexed profile formats.
  These formats are not forwards-compatible: i.e, a tool which uses format
  version X will not be able to understand format version (X+k).

* Tools must also retain **backwards** compatibility with the format of the
  coverage mappings emitted into instrumented binaries. These formats are not
  forwards-compatible.

* The JSON coverage export format has a (major, minor, patch) version triple.
  Only a major version increment indicates a backwards-incompatible change. A
  minor version increment is for added functionality, and patch version
  increments are for bugfixes.

Using the profiling runtime without static initializers
=======================================================

By default the compiler runtime uses a static initializer to determine the
profile output path and to register a writer function. To collect profiles
without using static initializers, do this manually:

* Export a ``int __llvm_profile_runtime`` symbol from each instrumented shared
  library and executable. When the linker finds a definition of this symbol, it
  knows to skip loading the object which contains the profiling runtime's
  static initializer.

* Forward-declare ``void __llvm_profile_initialize_file(void)`` and call it
  once from each instrumented executable. This function parses
  ``LLVM_PROFILE_FILE``, sets the output path, and truncates any existing files
  at that path. To get the same behavior without truncating existing files,
  pass a filename pattern string to ``void __llvm_profile_set_filename(char
  *)``.  These calls can be placed anywhere so long as they precede all calls
  to ``__llvm_profile_write_file``.

* Forward-declare ``int __llvm_profile_write_file(void)`` and call it to write
  out a profile. This function returns 0 when it succeeds, and a non-zero value
  otherwise. Calling this function multiple times appends profile data to an
  existing on-disk raw profile.

In C++ files, declare these as ``extern "C"``.

Collecting coverage reports for the llvm project
================================================

To prepare a coverage report for llvm (and any of its sub-projects), add
``-DLLVM_BUILD_INSTRUMENTED_COVERAGE=On`` to the cmake configuration. Raw
profiles will be written to ``$BUILD_DIR/profiles/``. To prepare an html
report, run ``llvm/utils/prepare-code-coverage-artifact.py``.

To specify an alternate directory for raw profiles, use
``-DLLVM_PROFILE_DATA_DIR``. To change the size of the profile merge pool, use
``-DLLVM_PROFILE_MERGE_POOL_SIZE``.

Drawbacks and limitations
=========================

* Prior to version 2.26, the GNU binutils BFD linker is not able link programs
  compiled with ``-fcoverage-mapping`` in its ``--gc-sections`` mode.  Possible
  workarounds include disabling ``--gc-sections``, upgrading to a newer version
  of BFD, or using the Gold linker.

* Code coverage does not handle unpredictable changes in control flow or stack
  unwinding in the presence of exceptions precisely. Consider the following
  function:

  .. code-block:: cpp

      int f() {
        may_throw();
        return 0;
      }

  If the call to ``may_throw()`` propagates an exception into ``f``, the code
  coverage tool may mark the ``return`` statement as executed even though it is
  not. A call to ``longjmp()`` can have similar effects.

Clang implementation details
============================

This section may be of interest to those wishing to understand or improve
the clang code coverage implementation.

Gap regions
-----------

Gap regions are source regions with counts. A reporting tool cannot set a line
execution count to the count from a gap region unless that region is the only
one on a line.

Gap regions are used to eliminate unnatural artifacts in coverage reports, such
as red "unexecuted" highlights present at the end of an otherwise covered line,
or blue "executed" highlights present at the start of a line that is otherwise
not executed.

Switch statements
-----------------

The region mapping for a switch body consists of a gap region that covers the
entire body (starting from the '{' in 'switch (...) {', and terminating where the
last case ends). This gap region has a zero count: this causes "gap" areas in
between case statements, which contain no executable code, to appear uncovered.

When a switch case is visited, the parent region is extended: if the parent
region has no start location, its start location becomes the start of the case.
This is used to support switch statements without a ``CompoundStmt`` body, in
which the switch body and the single case share a count.

For switches with ``CompoundStmt`` bodies, a new region is created at the start
of each switch case.