1 ========================== 2 Source-based Code Coverage 3 ========================== 4 5 .. contents:: 6 :local: 7 8 Introduction 9 ============ 10 11 This document explains how to use clang's source-based code coverage feature. 12 It's called "source-based" because it operates on AST and preprocessor 13 information directly. This allows it to generate very precise coverage data. 14 15 Clang ships two other code coverage implementations: 16 17 * :doc:`SanitizerCoverage` - A low-overhead tool meant for use alongside the 18 various sanitizers. It can provide up to edge-level coverage. 19 20 * gcov - A GCC-compatible coverage implementation which operates on DebugInfo. 21 22 From this point onwards "code coverage" will refer to the source-based kind. 23 24 The code coverage workflow 25 ========================== 26 27 The code coverage workflow consists of three main steps: 28 29 * Compiling with coverage enabled. 30 31 * Running the instrumented program. 32 33 * Creating coverage reports. 34 35 The next few sections work through a complete, copy-'n-paste friendly example 36 based on this program: 37 38 .. code-block:: cpp 39 40 % cat <<EOF > foo.cc 41 #define BAR(x) ((x) || (x)) 42 template <typename T> void foo(T x) { 43 for (unsigned I = 0; I < 10; ++I) { BAR(I); } 44 } 45 int main() { 46 foo<int>(0); 47 foo<float>(0); 48 return 0; 49 } 50 EOF 51 52 Compiling with coverage enabled 53 =============================== 54 55 To compile code with coverage enabled, pass ``-fprofile-instr-generate 56 -fcoverage-mapping`` to the compiler: 57 58 .. code-block:: console 59 60 # Step 1: Compile with coverage enabled. 61 % clang++ -fprofile-instr-generate -fcoverage-mapping foo.cc -o foo 62 63 Note that linking together code with and without coverage instrumentation is 64 supported: any uninstrumented code simply won't be accounted for. 65 66 Running the instrumented program 67 ================================ 68 69 The next step is to run the instrumented program. When the program exits it 70 will write a **raw profile** to the path specified by the ``LLVM_PROFILE_FILE`` 71 environment variable. If that variable does not exist, the profile is written 72 to ``default.profraw`` in the current directory of the program. If 73 ``LLVM_PROFILE_FILE`` contains a path to a non-existent directory, the missing 74 directory structure will be created. Additionally, the following special 75 **pattern strings** are rewritten: 76 77 * "%p" expands out to the process ID. 78 79 * "%h" expands out to the hostname of the machine running the program. 80 81 * "%Nm" expands out to the instrumented binary's signature. When this pattern 82 is specified, the runtime creates a pool of N raw profiles which are used for 83 on-line profile merging. The runtime takes care of selecting a raw profile 84 from the pool, locking it, and updating it before the program exits. If N is 85 not specified (i.e the pattern is "%m"), it's assumed that ``N = 1``. N must 86 be between 1 and 9. The merge pool specifier can only occur once per filename 87 pattern. 88 89 .. code-block:: console 90 91 # Step 2: Run the program. 92 % LLVM_PROFILE_FILE="foo.profraw" ./foo 93 94 Creating coverage reports 95 ========================= 96 97 Raw profiles have to be **indexed** before they can be used to generate 98 coverage reports. This is done using the "merge" tool in ``llvm-profdata``, so 99 named because it can combine and index profiles at the same time: 100 101 .. code-block:: console 102 103 # Step 3(a): Index the raw profile. 104 % llvm-profdata merge -sparse foo.profraw -o foo.profdata 105 106 There are multiple different ways to render coverage reports. One option is to 107 generate a line-oriented report: 108 109 .. code-block:: console 110 111 # Step 3(b): Create a line-oriented coverage report. 112 % llvm-cov show ./foo -instr-profile=foo.profdata 113 114 To demangle any C++ identifiers in the output, use: 115 116 .. code-block:: console 117 118 % llvm-cov show ./foo -instr-profile=foo.profdata | c++filt -n 119 120 This report includes a summary view as well as dedicated sub-views for 121 templated functions and their instantiations. For our example program, we get 122 distinct views for ``foo<int>(...)`` and ``foo<float>(...)``. If 123 ``-show-line-counts-or-regions`` is enabled, ``llvm-cov`` displays sub-line 124 region counts (even in macro expansions): 125 126 .. code-block:: none 127 128 20| 1|#define BAR(x) ((x) || (x)) 129 ^20 ^2 130 2| 2|template <typename T> void foo(T x) { 131 22| 3| for (unsigned I = 0; I < 10; ++I) { BAR(I); } 132 ^22 ^20 ^20^20 133 2| 4|} 134 ------------------ 135 | void foo<int>(int): 136 | 1| 2|template <typename T> void foo(T x) { 137 | 11| 3| for (unsigned I = 0; I < 10; ++I) { BAR(I); } 138 | ^11 ^10 ^10^10 139 | 1| 4|} 140 ------------------ 141 | void foo<float>(int): 142 | 1| 2|template <typename T> void foo(T x) { 143 | 11| 3| for (unsigned I = 0; I < 10; ++I) { BAR(I); } 144 | ^11 ^10 ^10^10 145 | 1| 4|} 146 ------------------ 147 148 It's possible to generate a file-level summary of coverage statistics (instead 149 of a line-oriented report) with: 150 151 .. code-block:: console 152 153 # Step 3(c): Create a coverage summary. 154 % llvm-cov report ./foo -instr-profile=foo.profdata 155 Filename Regions Miss Cover Functions Executed 156 ----------------------------------------------------------------------- 157 /tmp/foo.cc 13 0 100.00% 3 100.00% 158 ----------------------------------------------------------------------- 159 TOTAL 13 0 100.00% 3 100.00% 160 161 A few final notes: 162 163 * The ``-sparse`` flag is optional but can result in dramatically smaller 164 indexed profiles. This option should not be used if the indexed profile will 165 be reused for PGO. 166 167 * Raw profiles can be discarded after they are indexed. Advanced use of the 168 profile runtime library allows an instrumented program to merge profiling 169 information directly into an existing raw profile on disk. The details are 170 out of scope. 171 172 * The ``llvm-profdata`` tool can be used to merge together multiple raw or 173 indexed profiles. To combine profiling data from multiple runs of a program, 174 try e.g: 175 176 .. code-block:: console 177 178 % llvm-profdata merge -sparse foo1.profraw foo2.profdata -o foo3.profdata 179 180 Format compatibility guarantees 181 =============================== 182 183 * There are no backwards or forwards compatibility guarantees for the raw 184 profile format. Raw profiles may be dependent on the specific compiler 185 revision used to generate them. It's inadvisable to store raw profiles for 186 long periods of time. 187 188 * Tools must retain **backwards** compatibility with indexed profile formats. 189 These formats are not forwards-compatible: i.e, a tool which uses format 190 version X will not be able to understand format version (X+k). 191 192 * There is a third format in play: the format of the coverage mappings emitted 193 into instrumented binaries. Tools must retain **backwards** compatibility 194 with these formats. These formats are not forwards-compatible. 195 196 Using the profiling runtime without static initializers 197 ======================================================= 198 199 By default the compiler runtime uses a static initializer to determine the 200 profile output path and to register a writer function. To collect profiles 201 without using static initializers, do this manually: 202 203 * Export a ``int __llvm_profile_runtime`` symbol from each instrumented shared 204 library and executable. When the linker finds a definition of this symbol, it 205 knows to skip loading the object which contains the profiling runtime's 206 static initializer. 207 208 * Forward-declare ``void __llvm_profile_initialize_file(void)`` and call it 209 once from each instrumented executable. This function parses 210 ``LLVM_PROFILE_FILE``, sets the output path, and truncates any existing files 211 at that path. To get the same behavior without truncating existing files, 212 pass a filename pattern string to ``void __llvm_profile_set_filename(char 213 *)``. These calls can be placed anywhere so long as they precede all calls 214 to ``__llvm_profile_write_file``. 215 216 * Forward-declare ``int __llvm_profile_write_file(void)`` and call it to write 217 out a profile. This function returns 0 when it succeeds, and a non-zero value 218 otherwise. Calling this function multiple times appends profile data to an 219 existing on-disk raw profile. 220 221 Drawbacks and limitations 222 ========================= 223 224 * Code coverage does not handle unpredictable changes in control flow or stack 225 unwinding in the presence of exceptions precisely. Consider the following 226 function: 227 228 .. code-block:: cpp 229 230 int f() { 231 may_throw(); 232 return 0; 233 } 234 235 If the call to ``may_throw()`` propagates an exception into ``f``, the code 236 coverage tool may mark the ``return`` statement as executed even though it is 237 not. A call to ``longjmp()`` can have similar effects. 238
