vendor/clang/clang-release_39-r276489

1 files changed, 237 insertions, 0 deletions

diff --git a/docs/SourceBasedCodeCoverage.rst b/docs/SourceBasedCodeCoverage.rst
new file mode 100644
index 000000000000..8d0a51fd3315
--- /dev/null
+++ b/docs/SourceBasedCodeCoverage.rst
@@ -0,0 +1,237 @@
+==========================
+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.
+
+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: any uninstrumented code simply won't be accounted for.
+
+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.
+
+.. code-block:: console
+
+    # Step 2: Run the program.
+    % LLVM_PROFILE_FILE="foo.profraw" ./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``, so
+named because it can combine and index profiles 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. One 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
+
+To demangle any C++ identifiers in the output, use:
+
+.. code-block:: console
+
+    % llvm-cov show ./foo -instr-profile=foo.profdata | c++filt -n
+
+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
+
+       20|    1|#define BAR(x) ((x) || (x))
+                               ^20     ^2
+        2|    2|template <typename T> void foo(T x) {
+       22|    3|  for (unsigned I = 0; I < 10; ++I) { BAR(I); }
+                                       ^22     ^20  ^20^20
+        2|    4|}
+    ------------------
+    | void foo<int>(int):
+    |      1|    2|template <typename T> void foo(T x) {
+    |     11|    3|  for (unsigned I = 0; I < 10; ++I) { BAR(I); }
+    |                                     ^11     ^10  ^10^10
+    |      1|    4|}
+    ------------------
+    | void foo<float>(int):
+    |      1|    2|template <typename T> void foo(T x) {
+    |     11|    3|  for (unsigned I = 0; I < 10; ++I) { BAR(I); }
+    |                                     ^11     ^10  ^10^10
+    |      1|    4|}
+    ------------------
+
+It's possible to generate a file-level summary of coverage statistics (instead
+of a line-oriented report) with:
+
+.. code-block:: console
+
+    # Step 3(c): Create a coverage summary.
+    % llvm-cov report ./foo -instr-profile=foo.profdata
+    Filename                    Regions    Miss   Cover Functions  Executed
+    -----------------------------------------------------------------------
+    /tmp/foo.cc                      13       0 100.00%         3   100.00%
+    -----------------------------------------------------------------------
+    TOTAL                            13       0 100.00%         3   100.00%
+
+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
+
+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).
+
+* There is a third format in play: the format of the coverage mappings emitted
+  into instrumented binaries. Tools must retain **backwards** compatibility
+  with these formats. These formats are not forwards-compatible.
+
+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.
+
+Drawbacks and limitations
+=========================
+
+* 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.