1.. role:: raw-html(raw) 2 :format: html 3 4================================= 5LLVM Code Coverage Mapping Format 6================================= 7 8.. contents:: 9 :local: 10 11Introduction 12============ 13 14LLVM's code coverage mapping format is used to provide code coverage 15analysis using LLVM's and Clang's instrumenation based profiling 16(Clang's ``-fprofile-instr-generate`` option). 17 18This document is aimed at those who use LLVM's code coverage mapping to provide 19code coverage analysis for their own programs, and for those who would like 20to know how it works under the hood. A prior knowledge of how Clang's profile 21guided optimization works is useful, but not required. 22 23We start by showing how to use LLVM and Clang for code coverage analysis, 24then we briefly desribe LLVM's code coverage mapping format and the 25way that Clang and LLVM's code coverage tool work with this format. After 26the basics are down, more advanced features of the coverage mapping format 27are discussed - such as the data structures, LLVM IR representation and 28the binary encoding. 29 30Quick Start 31=========== 32 33Here's a short story that describes how to generate code coverage overview 34for a sample source file called *test.c*. 35 36* First, compile an instrumented version of your program using Clang's 37 ``-fprofile-instr-generate`` option with the additional ``-fcoverage-mapping`` 38 option: 39 40 ``clang -o test -fprofile-instr-generate -fcoverage-mapping test.c`` 41* Then, run the instrumented binary. The runtime will produce a file called 42 *default.profraw* containing the raw profile instrumentation data: 43 44 ``./test`` 45* After that, merge the profile data using the *llvm-profdata* tool: 46 47 ``llvm-profdata merge -o test.profdata default.profraw`` 48* Finally, run LLVM's code coverage tool (*llvm-cov*) to produce the code 49 coverage overview for the sample source file: 50 51 ``llvm-cov show ./test -instr-profile=test.profdata test.c`` 52 53High Level Overview 54=================== 55 56LLVM's code coverage mapping format is designed to be a self contained 57data format, that can be embedded into the LLVM IR and object files. 58It's described in this document as a **mapping** format because its goal is 59to store the data that is required for a code coverage tool to map between 60the specific source ranges in a file and the execution counts obtained 61after running the instrumented version of the program. 62 63The mapping data is used in two places in the code coverage process: 64 651. When clang compiles a source file with ``-fcoverage-mapping``, it 66 generates the mapping information that describes the mapping between the 67 source ranges and the profiling instrumentation counters. 68 This information gets embedded into the LLVM IR and conveniently 69 ends up in the final executable file when the program is linked. 70 712. It is also used by *llvm-cov* - the mapping information is extracted from an 72 object file and is used to associate the execution counts (the values of the 73 profile instrumentation counters), and the source ranges in a file. 74 After that, the tool is able to generate various code coverage reports 75 for the program. 76 77The coverage mapping format aims to be a "universal format" that would be 78suitable for usage by any frontend, and not just by Clang. It also aims to 79provide the frontend the possibility of generating the minimal coverage mapping 80data in order to reduce the size of the IR and object files - for example, 81instead of emitting mapping information for each statement in a function, the 82frontend is allowed to group the statements with the same execution count into 83regions of code, and emit the mapping information only for those regions. 84 85Advanced Concepts 86================= 87 88The remainder of this guide is meant to give you insight into the way the 89coverage mapping format works. 90 91The coverage mapping format operates on a per-function level as the 92profile instrumentation counters are associated with a specific function. 93For each function that requires code coverage, the frontend has to create 94coverage mapping data that can map between the source code ranges and 95the profile instrumentation counters for that function. 96 97Mapping Region 98-------------- 99 100The function's coverage mapping data contains an array of mapping regions. 101A mapping region stores the `source code range`_ that is covered by this region, 102the `file id <coverage file id_>`_, the `coverage mapping counter`_ and 103the region's kind. 104There are several kinds of mapping regions: 105 106* Code regions associate portions of source code and `coverage mapping 107 counters`_. They make up the majority of the mapping regions. They are used 108 by the code coverage tool to compute the execution counts for lines, 109 highlight the regions of code that were never executed, and to obtain 110 the various code coverage statistics for a function. 111 For example: 112 113 :raw-html:`<pre class='highlight' style='line-height:initial;'><span>int main(int argc, const char *argv[]) </span><span style='background-color:#4A789C'>{ </span> <span class='c1'>// Code Region from 1:40 to 9:2</span> 114 <span style='background-color:#4A789C'> </span> 115 <span style='background-color:#4A789C'> if (argc > 1) </span><span style='background-color:#85C1F5'>{ </span> <span class='c1'>// Code Region from 3:17 to 5:4</span> 116 <span style='background-color:#85C1F5'> printf("%s\n", argv[1]); </span> 117 <span style='background-color:#85C1F5'> }</span><span style='background-color:#4A789C'> else </span><span style='background-color:#F6D55D'>{ </span> <span class='c1'>// Code Region from 5:10 to 7:4</span> 118 <span style='background-color:#F6D55D'> printf("\n"); </span> 119 <span style='background-color:#F6D55D'> }</span><span style='background-color:#4A789C'> </span> 120 <span style='background-color:#4A789C'> return 0; </span> 121 <span style='background-color:#4A789C'>}</span> 122 </pre>` 123* Skipped regions are used to represent source ranges that were skipped 124 by Clang's preprocessor. They don't associate with 125 `coverage mapping counters`_, as the frontend knows that they are never 126 executed. They are used by the code coverage tool to mark the skipped lines 127 inside a function as non-code lines that don't have execution counts. 128 For example: 129 130 :raw-html:`<pre class='highlight' style='line-height:initial;'><span>int main() </span><span style='background-color:#4A789C'>{ </span> <span class='c1'>// Code Region from 1:12 to 6:2</span> 131 <span style='background-color:#85C1F5'>#ifdef DEBUG </span> <span class='c1'>// Skipped Region from 2:1 to 4:2</span> 132 <span style='background-color:#85C1F5'> printf("Hello world"); </span> 133 <span style='background-color:#85C1F5'>#</span><span style='background-color:#4A789C'>endif </span> 134 <span style='background-color:#4A789C'> return 0; </span> 135 <span style='background-color:#4A789C'>}</span> 136 </pre>` 137* Expansion regions are used to represent Clang's macro expansions. They 138 have an additional property - *expanded file id*. This property can be 139 used by the code coverage tool to find the mapping regions that are created 140 as a result of this macro expansion, by checking if their file id matches the 141 expanded file id. They don't associate with `coverage mapping counters`_, 142 as the code coverage tool can determine the execution count for this region 143 by looking up the execution count of the first region with a corresponding 144 file id. 145 For example: 146 147 :raw-html:`<pre class='highlight' style='line-height:initial;'><span>int func(int x) </span><span style='background-color:#4A789C'>{ </span> 148 <span style='background-color:#4A789C'> #define MAX(x,y) </span><span style='background-color:#85C1F5'>((x) > (y)? </span><span style='background-color:#F6D55D'>(x)</span><span style='background-color:#85C1F5'> : </span><span style='background-color:#F4BA70'>(y)</span><span style='background-color:#85C1F5'>)</span><span style='background-color:#4A789C'> </span> 149 <span style='background-color:#4A789C'> return </span><span style='background-color:#7FCA9F'>MAX</span><span style='background-color:#4A789C'>(x, 42); </span> <span class='c1'>// Expansion Region from 3:10 to 3:13</span> 150 <span style='background-color:#4A789C'>}</span> 151 </pre>` 152 153.. _source code range: 154 155Source Range: 156^^^^^^^^^^^^^ 157 158The source range record contains the starting and ending location of a certain 159mapping region. Both locations include the line and the column numbers. 160 161.. _coverage file id: 162 163File ID: 164^^^^^^^^ 165 166The file id an integer value that tells us 167in which source file or macro expansion is this region located. 168It enables Clang to produce mapping information for the code 169defined inside macros, like this example demonstrates: 170 171:raw-html:`<pre class='highlight' style='line-height:initial;'><span>void func(const char *str) </span><span style='background-color:#4A789C'>{ </span> <span class='c1'>// Code Region from 1:28 to 6:2 with file id 0</span> 172<span style='background-color:#4A789C'> #define PUT </span><span style='background-color:#85C1F5'>printf("%s\n", str)</span><span style='background-color:#4A789C'> </span> <span class='c1'>// 2 Code Regions from 2:15 to 2:34 with file ids 1 and 2</span> 173<span style='background-color:#4A789C'> if(*str) </span> 174<span style='background-color:#4A789C'> </span><span style='background-color:#F6D55D'>PUT</span><span style='background-color:#4A789C'>; </span> <span class='c1'>// Expansion Region from 4:5 to 4:8 with file id 0 that expands a macro with file id 1</span> 175<span style='background-color:#4A789C'> </span><span style='background-color:#F6D55D'>PUT</span><span style='background-color:#4A789C'>; </span> <span class='c1'>// Expansion Region from 5:3 to 5:6 with file id 0 that expands a macro with file id 2</span> 176<span style='background-color:#4A789C'>}</span> 177</pre>` 178 179.. _coverage mapping counter: 180.. _coverage mapping counters: 181 182Counter: 183^^^^^^^^ 184 185A coverage mapping counter can represents a reference to the profile 186instrumentation counter. The execution count for a region with such counter 187is determined by looking up the value of the corresponding profile 188instrumentation counter. 189 190It can also represent a binary arithmetical expression that operates on 191coverage mapping counters or other expressions. 192The execution count for a region with an expression counter is determined by 193evaluating the expression's arguments and then adding them together or 194subtracting them from one another. 195In the example below, a subtraction expression is used to compute the execution 196count for the compound statement that follows the *else* keyword: 197 198:raw-html:`<pre class='highlight' style='line-height:initial;'><span>int main(int argc, const char *argv[]) </span><span style='background-color:#4A789C'>{ </span> <span class='c1'>// Region's counter is a reference to the profile counter #0</span> 199<span style='background-color:#4A789C'> </span> 200<span style='background-color:#4A789C'> if (argc > 1) </span><span style='background-color:#85C1F5'>{ </span> <span class='c1'>// Region's counter is a reference to the profile counter #1</span> 201<span style='background-color:#85C1F5'> printf("%s\n", argv[1]); </span><span> </span> 202<span style='background-color:#85C1F5'> }</span><span style='background-color:#4A789C'> else </span><span style='background-color:#F6D55D'>{ </span> <span class='c1'>// Region's counter is an expression (reference to the profile counter #0 - reference to the profile counter #1)</span> 203<span style='background-color:#F6D55D'> printf("\n"); </span> 204<span style='background-color:#F6D55D'> }</span><span style='background-color:#4A789C'> </span> 205<span style='background-color:#4A789C'> return 0; </span> 206<span style='background-color:#4A789C'>}</span> 207</pre>` 208 209Finally, a coverage mapping counter can also represent an execution count of 210of zero. The zero counter is used to provide coverage mapping for 211unreachable statements and expressions, like in the example below: 212 213:raw-html:`<pre class='highlight' style='line-height:initial;'><span>int main() </span><span style='background-color:#4A789C'>{ </span> 214<span style='background-color:#4A789C'> return 0; </span> 215<span style='background-color:#4A789C'> </span><span style='background-color:#85C1F5'>printf("Hello world!\n")</span><span style='background-color:#4A789C'>; </span> <span class='c1'>// Unreachable region's counter is zero</span> 216<span style='background-color:#4A789C'>}</span> 217</pre>` 218 219The zero counters allow the code coverage tool to display proper line execution 220counts for the unreachable lines and highlight the unreachable code. 221Without them, the tool would think that those lines and regions were still 222executed, as it doesn't possess the frontend's knowledge. 223 224LLVM IR Representation 225====================== 226 227The coverage mapping data is stored in the LLVM IR using a single global 228constant structure variable called *__llvm_coverage_mapping* 229with the *__llvm_covmap* section specifier. 230 231For example, let’s consider a C file and how it gets compiled to LLVM: 232 233.. _coverage mapping sample: 234 235.. code-block:: c 236 237 int foo() { 238 return 42; 239 } 240 int bar() { 241 return 13; 242 } 243 244The coverage mapping variable generated by Clang has 3 fields: 245 246* Coverage mapping header. 247 248* An array of function records. 249 250* Coverage mapping data which is an array of bytes. Zero paddings are added at the end to force 8 byte alignment. 251 252.. code-block:: llvm 253 254 @__llvm_coverage_mapping = internal constant { { i32, i32, i32, i32 }, [2 x { i8*, i32, i32 }], [40 x i8] } 255 { 256 { i32, i32, i32, i32 } ; Coverage map header 257 { 258 i32 2, ; The number of function records 259 i32 20, ; The length of the string that contains the encoded translation unit filenames 260 i32 20, ; The length of the string that contains the encoded coverage mapping data 261 i32 0, ; Coverage mapping format version 262 }, 263 [2 x { i8*, i32, i32 }] [ ; Function records 264 { i8*, i32, i32 } { i8* getelementptr inbounds ([3 x i8]* @__llvm_profile_name_foo, i32 0, i32 0), ; Function's name 265 i32 3, ; Function's name length 266 i32 9 ; Function's encoded coverage mapping data string length 267 }, 268 { i8*, i32, i32 } { i8* getelementptr inbounds ([3 x i8]* @__llvm_profile_name_bar, i32 0, i32 0), ; Function's name 269 i32 3, ; Function's name length 270 i32 9 ; Function's encoded coverage mapping data string length 271 }], 272 [40 x i8] c"..." ; Encoded data (dissected later) 273 }, section "__llvm_covmap", align 8 274 275Coverage Mapping Header: 276------------------------ 277 278The coverage mapping header has the following fields: 279 280* The number of function records. 281 282* The length of the string in the third field of *__llvm_coverage_mapping* that contains the encoded translation unit filenames. 283 284* The length of the string in the third field of *__llvm_coverage_mapping* that contains the encoded coverage mapping data. 285 286* The format version. 0 is the first (current) version of the coverage mapping format. 287 288.. _function records: 289 290Function record: 291---------------- 292 293A function record is a structure of the following type: 294 295.. code-block:: llvm 296 297 { i8*, i32, i32 } 298 299It contains the pointer to the function's name, function's name length, 300and the length of the encoded mapping data for that function. 301 302Encoded data: 303------------- 304 305The encoded data is stored in a single string that contains 306the encoded filenames used by this translation unit and the encoded coverage 307mapping data for each function in this translation unit. 308 309The encoded data has the following structure: 310 311``[filenames, coverageMappingDataForFunctionRecord0, coverageMappingDataForFunctionRecord1, ..., padding]`` 312 313If necessary, the encoded data is padded with zeroes so that the size 314of the data string is rounded up to the nearest multiple of 8 bytes. 315 316Dissecting the sample: 317^^^^^^^^^^^^^^^^^^^^^^ 318 319Here's an overview of the encoded data that was stored in the 320IR for the `coverage mapping sample`_ that was shown earlier: 321 322* The IR contains the following string constant that represents the encoded 323 coverage mapping data for the sample translation unit: 324 325 .. code-block:: llvm 326 327 c"\01\12/Users/alex/test.c\01\00\00\01\01\01\0C\02\02\01\00\00\01\01\04\0C\02\02\00\00" 328 329* The string contains values that are encoded in the LEB128 format, which is 330 used throughout for storing integers. It also contains a string value. 331 332* The length of the substring that contains the encoded translation unit 333 filenames is the value of the second field in the *__llvm_coverage_mapping* 334 structure, which is 20, thus the filenames are encoded in this string: 335 336 .. code-block:: llvm 337 338 c"\01\12/Users/alex/test.c" 339 340 This string contains the following data: 341 342 * Its first byte has a value of ``0x01``. It stores the number of filenames 343 contained in this string. 344 * Its second byte stores the length of the first filename in this string. 345 * The remaining 18 bytes are used to store the first filename. 346 347* The length of the substring that contains the encoded coverage mapping data 348 for the first function is the value of the third field in the first 349 structure in an array of `function records`_ stored in the 350 third field of the *__llvm_coverage_mapping* structure, which is the 9. 351 Therefore, the coverage mapping for the first function record is encoded 352 in this string: 353 354 .. code-block:: llvm 355 356 c"\01\00\00\01\01\01\0C\02\02" 357 358 This string consists of the following bytes: 359 360 +----------+-------------------------------------------------------------------------------------------------------------------------+ 361 | ``0x01`` | The number of file ids used by this function. There is only one file id used by the mapping data in this function. | 362 +----------+-------------------------------------------------------------------------------------------------------------------------+ 363 | ``0x00`` | An index into the filenames array which corresponds to the file "/Users/alex/test.c". | 364 +----------+-------------------------------------------------------------------------------------------------------------------------+ 365 | ``0x00`` | The number of counter expressions used by this function. This function doesn't use any expressions. | 366 +----------+-------------------------------------------------------------------------------------------------------------------------+ 367 | ``0x01`` | The number of mapping regions that are stored in an array for the function's file id #0. | 368 +----------+-------------------------------------------------------------------------------------------------------------------------+ 369 | ``0x01`` | The coverage mapping counter for the first region in this function. The value of 1 tells us that it's a coverage | 370 | | mapping counter that is a reference to the profile instrumentation counter with an index of 0. | 371 +----------+-------------------------------------------------------------------------------------------------------------------------+ 372 | ``0x01`` | The starting line of the first mapping region in this function. | 373 +----------+-------------------------------------------------------------------------------------------------------------------------+ 374 | ``0x0C`` | The starting column of the first mapping region in this function. | 375 +----------+-------------------------------------------------------------------------------------------------------------------------+ 376 | ``0x02`` | The ending line of the first mapping region in this function. | 377 +----------+-------------------------------------------------------------------------------------------------------------------------+ 378 | ``0x02`` | The ending column of the first mapping region in this function. | 379 +----------+-------------------------------------------------------------------------------------------------------------------------+ 380 381* The length of the substring that contains the encoded coverage mapping data 382 for the second function record is also 9. It's structured like the mapping data 383 for the first function record. 384 385* The two trailing bytes are zeroes and are used to pad the coverage mapping 386 data to give it the 8 byte alignment. 387 388Encoding 389======== 390 391The per-function coverage mapping data is encoded as a stream of bytes, 392with a simple structure. The structure consists of the encoding 393`types <cvmtypes_>`_ like variable-length unsigned integers, that 394are used to encode `File ID Mapping`_, `Counter Expressions`_ and 395the `Mapping Regions`_. 396 397The format of the structure follows: 398 399 ``[file id mapping, counter expressions, mapping regions]`` 400 401The translation unit filenames are encoded using the same encoding 402`types <cvmtypes_>`_ as the per-function coverage mapping data, with the 403following structure: 404 405 ``[numFilenames : LEB128, filename0 : string, filename1 : string, ...]`` 406 407.. _cvmtypes: 408 409Types 410----- 411 412This section describes the basic types that are used by the encoding format 413and can appear after ``:`` in the ``[foo : type]`` description. 414 415.. _LEB128: 416 417LEB128 418^^^^^^ 419 420LEB128 is an unsigned interger value that is encoded using DWARF's LEB128 421encoding, optimizing for the case where values are small 422(1 byte for values less than 128). 423 424.. _Strings: 425 426Strings 427^^^^^^^ 428 429``[length : LEB128, characters...]`` 430 431String values are encoded with a `LEB value <LEB128_>`_ for the length 432of the string and a sequence of bytes for its characters. 433 434.. _file id mapping: 435 436File ID Mapping 437--------------- 438 439``[numIndices : LEB128, filenameIndex0 : LEB128, filenameIndex1 : LEB128, ...]`` 440 441File id mapping in a function's coverage mapping stream 442contains the indices into the translation unit's filenames array. 443 444Counter 445------- 446 447``[value : LEB128]`` 448 449A `coverage mapping counter`_ is stored in a single `LEB value <LEB128_>`_. 450It is composed of two things --- the `tag <counter-tag_>`_ 451which is stored in the lowest 2 bits, and the `counter data`_ which is stored 452in the remaining bits. 453 454.. _counter-tag: 455 456Tag: 457^^^^ 458 459The counter's tag encodes the counter's kind 460and, if the counter is an expression, the expression's kind. 461The possible tag values are: 462 463* 0 - The counter is zero. 464 465* 1 - The counter is a reference to the profile instrumentation counter. 466 467* 2 - The counter is a subtraction expression. 468 469* 3 - The counter is an addition expression. 470 471.. _counter data: 472 473Data: 474^^^^^ 475 476The counter's data is interpreted in the following manner: 477 478* When the counter is a reference to the profile instrumentation counter, 479 then the counter's data is the id of the profile counter. 480* When the counter is an expression, then the counter's data 481 is the index into the array of counter expressions. 482 483.. _Counter Expressions: 484 485Counter Expressions 486------------------- 487 488``[numExpressions : LEB128, expr0LHS : LEB128, expr0RHS : LEB128, expr1LHS : LEB128, expr1RHS : LEB128, ...]`` 489 490Counter expressions consist of two counters as they 491represent binary arithmetic operations. 492The expression's kind is determined from the `tag <counter-tag_>`_ of the 493counter that references this expression. 494 495.. _Mapping Regions: 496 497Mapping Regions 498--------------- 499 500``[numRegionArrays : LEB128, regionsForFile0, regionsForFile1, ...]`` 501 502The mapping regions are stored in an array of sub-arrays where every 503region in a particular sub-array has the same file id. 504 505The file id for a sub-array of regions is the index of that 506sub-array in the main array e.g. The first sub-array will have the file id 507of 0. 508 509Sub-Array of Regions 510^^^^^^^^^^^^^^^^^^^^ 511 512``[numRegions : LEB128, region0, region1, ...]`` 513 514The mapping regions for a specific file id are stored in an array that is 515sorted in an ascending order by the region's starting location. 516 517Mapping Region 518^^^^^^^^^^^^^^ 519 520``[header, source range]`` 521 522The mapping region record contains two sub-records --- 523the `header`_, which stores the counter and/or the region's kind, 524and the `source range`_ that contains the starting and ending 525location of this region. 526 527.. _header: 528 529Header 530^^^^^^ 531 532``[counter]`` 533 534or 535 536``[pseudo-counter]`` 537 538The header encodes the region's counter and the region's kind. 539 540The value of the counter's tag distinguishes between the counters and 541pseudo-counters --- if the tag is zero, than this header contains a 542pseudo-counter, otherwise this header contains an ordinary counter. 543 544Counter: 545"""""""" 546 547A mapping region whose header has a counter with a non-zero tag is 548a code region. 549 550Pseudo-Counter: 551""""""""""""""" 552 553``[value : LEB128]`` 554 555A pseudo-counter is stored in a single `LEB value <LEB128_>`_, just like 556the ordinary counter. It has the following interpretation: 557 558* bits 0-1: tag, which is always 0. 559 560* bit 2: expansionRegionTag. If this bit is set, then this mapping region 561 is an expansion region. 562 563* remaining bits: data. If this region is an expansion region, then the data 564 contains the expanded file id of that region. 565 566 Otherwise, the data contains the region's kind. The possible region 567 kind values are: 568 569 * 0 - This mapping region is a code region with a counter of zero. 570 * 2 - This mapping region is a skipped region. 571 572.. _source range: 573 574Source Range 575^^^^^^^^^^^^ 576 577``[deltaLineStart : LEB128, columnStart : LEB128, numLines : LEB128, columnEnd : LEB128]`` 578 579The source range record contains the following fields: 580 581* *deltaLineStart*: The difference between the starting line of the 582 current mapping region and the starting line of the previous mapping region. 583 584 If the current mapping region is the first region in the current 585 sub-array, then it stores the starting line of that region. 586 587* *columnStart*: The starting column of the mapping region. 588 589* *numLines*: The difference between the ending line and the starting line 590 of the current mapping region. 591 592* *columnEnd*: The ending column of the mapping region. 593