1 .. role:: raw-html(raw) 2 :format: html 3 4 ================================= 5 LLVM Code Coverage Mapping Format 6 ================================= 7 8 .. contents:: 9 :local: 10 11 Introduction 12 ============ 13 14 LLVM's code coverage mapping format is used to provide code coverage 15 analysis using LLVM's and Clang's instrumenation based profiling 16 (Clang's ``-fprofile-instr-generate`` option). 17 18 This document is aimed at those who use LLVM's code coverage mapping to provide 19 code coverage analysis for their own programs, and for those who would like 20 to know how it works under the hood. A prior knowledge of how Clang's profile 21 guided optimization works is useful, but not required. 22 23 We start by showing how to use LLVM and Clang for code coverage analysis, 24 then we briefly desribe LLVM's code coverage mapping format and the 25 way that Clang and LLVM's code coverage tool work with this format. After 26 the basics are down, more advanced features of the coverage mapping format 27 are discussed - such as the data structures, LLVM IR representation and 28 the binary encoding. 29 30 Quick Start 31 =========== 32 33 Here's a short story that describes how to generate code coverage overview 34 for 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 53 High Level Overview 54 =================== 55 56 LLVM's code coverage mapping format is designed to be a self contained 57 data format, that can be embedded into the LLVM IR and object files. 58 It's described in this document as a **mapping** format because its goal is 59 to store the data that is required for a code coverage tool to map between 60 the specific source ranges in a file and the execution counts obtained 61 after running the instrumented version of the program. 62 63 The mapping data is used in two places in the code coverage process: 64 65 1. 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 71 2. 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 77 The coverage mapping format aims to be a "universal format" that would be 78 suitable for usage by any frontend, and not just by Clang. It also aims to 79 provide the frontend the possibility of generating the minimal coverage mapping 80 data in order to reduce the size of the IR and object files - for example, 81 instead of emitting mapping information for each statement in a function, the 82 frontend is allowed to group the statements with the same execution count into 83 regions of code, and emit the mapping information only for those regions. 84 85 Advanced Concepts 86 ================= 87 88 The remainder of this guide is meant to give you insight into the way the 89 coverage mapping format works. 90 91 The coverage mapping format operates on a per-function level as the 92 profile instrumentation counters are associated with a specific function. 93 For each function that requires code coverage, the frontend has to create 94 coverage mapping data that can map between the source code ranges and 95 the profile instrumentation counters for that function. 96 97 Mapping Region 98 -------------- 99 100 The function's coverage mapping data contains an array of mapping regions. 101 A mapping region stores the `source code range`_ that is covered by this region, 102 the `file id <coverage file id_>`_, the `coverage mapping counter`_ and 103 the region's kind. 104 There 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: 154 155 Source Range: 156 ^^^^^^^^^^^^^ 157 158 The source range record contains the starting and ending location of a certain 159 mapping region. Both locations include the line and the column numbers. 160 161 .. _coverage file id: 162 163 File ID: 164 ^^^^^^^^ 165 166 The file id an integer value that tells us 167 in which source file or macro expansion is this region located. 168 It enables Clang to produce mapping information for the code 169 defined 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: 181 182 Counter: 183 ^^^^^^^^ 184 185 A coverage mapping counter can represents a reference to the profile 186 instrumentation counter. The execution count for a region with such counter 187 is determined by looking up the value of the corresponding profile 188 instrumentation counter. 189 190 It can also represent a binary arithmetical expression that operates on 191 coverage mapping counters or other expressions. 192 The execution count for a region with an expression counter is determined by 193 evaluating the expression's arguments and then adding them together or 194 subtracting them from one another. 195 In the example below, a subtraction expression is used to compute the execution 196 count 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>` 208 209 Finally, a coverage mapping counter can also represent an execution count of 210 of zero. The zero counter is used to provide coverage mapping for 211 unreachable 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>` 218 219 The zero counters allow the code coverage tool to display proper line execution 220 counts for the unreachable lines and highlight the unreachable code. 221 Without them, the tool would think that those lines and regions were still 222 executed, as it doesn't possess the frontend's knowledge. 223 224 LLVM IR Representation 225 ====================== 226 227 The coverage mapping data is stored in the LLVM IR using a single global 228 constant structure variable called *__llvm_coverage_mapping* 229 with the *__llvm_covmap* section specifier. 230 231 For example, lets 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 } 243 244 The coverage mapping variable generated by Clang is: 245 246 .. code-block:: llvm 247 248 @__llvm_coverage_mapping = internal constant { i32, i32, i32, i32, [2 x { i8*, i32, i32 }], [40 x i8] } 249 { i32 2, ; The number of function records 250 i32 20, ; The length of the string that contains the encoded translation unit filenames 251 i32 20, ; The length of the string that contains the encoded coverage mapping data 252 i32 0, ; Coverage mapping format version 253 [2 x { i8*, i32, i32 }] [ ; Function records 254 { i8*, i32, i32 } { i8* getelementptr inbounds ([3 x i8]* @__llvm_profile_name_foo, i32 0, i32 0), ; Function's name 255 i32 3, ; Function's name length 256 i32 9 ; Function's encoded coverage mapping data string length 257 }, 258 { i8*, i32, i32 } { i8* getelementptr inbounds ([3 x i8]* @__llvm_profile_name_bar, i32 0, i32 0), ; Function's name 259 i32 3, ; Function's name length 260 i32 9 ; Function's encoded coverage mapping data string length 261 }], 262 [40 x i8] c"..." ; Encoded data (dissected later) 263 }, section "__llvm_covmap", align 8 264 265 Version: 266 -------- 267 268 The coverage mapping version number can have the following values: 269 270 * 0 The first (current) version of the coverage mapping format. 271 272 .. _function records: 273 274 Function record: 275 ---------------- 276 277 A function record is a structure of the following type: 278 279 .. code-block:: llvm 280 281 { i8*, i32, i32 } 282 283 It contains the pointer to the function's name, function's name length, 284 and the length of the encoded mapping data for that function. 285 286 Encoded data: 287 ------------- 288 289 The encoded data is stored in a single string that contains 290 the encoded filenames used by this translation unit and the encoded coverage 291 mapping data for each function in this translation unit. 292 293 The encoded data has the following structure: 294 295 ``[filenames, coverageMappingDataForFunctionRecord0, coverageMappingDataForFunctionRecord1, ..., padding]`` 296 297 If necessary, the encoded data is padded with zeroes so that the size 298 of the data string is rounded up to the nearest multiple of 8 bytes. 299 300 Dissecting the sample: 301 ^^^^^^^^^^^^^^^^^^^^^^ 302 303 Here's an overview of the encoded data that was stored in the 304 IR for the `coverage mapping sample`_ that was shown earlier: 305 306 * The IR contains the following string constant that represents the encoded 307 coverage mapping data for the sample translation unit: 308 309 .. code-block:: llvm 310 311 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" 312 313 * The string contains values that are encoded in the LEB128 format, which is 314 used throughout for storing integers. It also contains a string value. 315 316 * The length of the substring that contains the encoded translation unit 317 filenames is the value of the second field in the *__llvm_coverage_mapping* 318 structure, which is 20, thus the filenames are encoded in this string: 319 320 .. code-block:: llvm 321 322 c"\01\12/Users/alex/test.c" 323 324 This string contains the following data: 325 326 * Its first byte has a value of ``0x01``. It stores the number of filenames 327 contained in this string. 328 * Its second byte stores the length of the first filename in this string. 329 * The remaining 18 bytes are used to store the first filename. 330 331 * The length of the substring that contains the encoded coverage mapping data 332 for the first function is the value of the third field in the first 333 structure in an array of `function records`_ stored in the 334 fifth field of the *__llvm_coverage_mapping* structure, which is the 9. 335 Therefore, the coverage mapping for the first function record is encoded 336 in this string: 337 338 .. code-block:: llvm 339 340 c"\01\00\00\01\01\01\0C\02\02" 341 342 This string consists of the following bytes: 343 344 +----------+-------------------------------------------------------------------------------------------------------------------------+ 345 | ``0x01`` | The number of file ids used by this function. There is only one file id used by the mapping data in this function. | 346 +----------+-------------------------------------------------------------------------------------------------------------------------+ 347 | ``0x00`` | An index into the filenames array which corresponds to the file "/Users/alex/test.c". | 348 +----------+-------------------------------------------------------------------------------------------------------------------------+ 349 | ``0x00`` | The number of counter expressions used by this function. This function doesn't use any expressions. | 350 +----------+-------------------------------------------------------------------------------------------------------------------------+ 351 | ``0x01`` | The number of mapping regions that are stored in an array for the function's file id #0. | 352 +----------+-------------------------------------------------------------------------------------------------------------------------+ 353 | ``0x01`` | The coverage mapping counter for the first region in this function. The value of 1 tells us that it's a coverage | 354 | | mapping counter that is a reference ot the profile instrumentation counter with an index of 0. | 355 +----------+-------------------------------------------------------------------------------------------------------------------------+ 356 | ``0x01`` | The starting line of the first mapping region in this function. | 357 +----------+-------------------------------------------------------------------------------------------------------------------------+ 358 | ``0x0C`` | The starting column of the first mapping region in this function. | 359 +----------+-------------------------------------------------------------------------------------------------------------------------+ 360 | ``0x02`` | The ending line of the first mapping region in this function. | 361 +----------+-------------------------------------------------------------------------------------------------------------------------+ 362 | ``0x02`` | The ending column of the first mapping region in this function. | 363 +----------+-------------------------------------------------------------------------------------------------------------------------+ 364 365 * The length of the substring that contains the encoded coverage mapping data 366 for the second function record is also 9. It's structured like the mapping data 367 for the first function record. 368 369 * The two trailing bytes are zeroes and are used to pad the coverage mapping 370 data to give it the 8 byte alignment. 371 372 Encoding 373 ======== 374 375 The per-function coverage mapping data is encoded as a stream of bytes, 376 with a simple structure. The structure consists of the encoding 377 `types <cvmtypes_>`_ like variable-length unsigned integers, that 378 are used to encode `File ID Mapping`_, `Counter Expressions`_ and 379 the `Mapping Regions`_. 380 381 The format of the structure follows: 382 383 ``[file id mapping, counter expressions, mapping regions]`` 384 385 The translation unit filenames are encoded using the same encoding 386 `types <cvmtypes_>`_ as the per-function coverage mapping data, with the 387 following structure: 388 389 ``[numFilenames : LEB128, filename0 : string, filename1 : string, ...]`` 390 391 .. _cvmtypes: 392 393 Types 394 ----- 395 396 This section describes the basic types that are used by the encoding format 397 and can appear after ``:`` in the ``[foo : type]`` description. 398 399 .. _LEB128: 400 401 LEB128 402 ^^^^^^ 403 404 LEB128 is an unsigned interger value that is encoded using DWARF's LEB128 405 encoding, optimizing for the case where values are small 406 (1 byte for values less than 128). 407 408 .. _Strings: 409 410 Strings 411 ^^^^^^^ 412 413 ``[length : LEB128, characters...]`` 414 415 String values are encoded with a `LEB value <LEB128_>`_ for the length 416 of the string and a sequence of bytes for its characters. 417 418 .. _file id mapping: 419 420 File ID Mapping 421 --------------- 422 423 ``[numIndices : LEB128, filenameIndex0 : LEB128, filenameIndex1 : LEB128, ...]`` 424 425 File id mapping in a function's coverage mapping stream 426 contains the indices into the translation unit's filenames array. 427 428 Counter 429 ------- 430 431 ``[value : LEB128]`` 432 433 A `coverage mapping counter`_ is stored in a single `LEB value <LEB128_>`_. 434 It is composed of two things --- the `tag <counter-tag_>`_ 435 which is stored in the lowest 2 bits, and the `counter data`_ which is stored 436 in the remaining bits. 437 438 .. _counter-tag: 439 440 Tag: 441 ^^^^ 442 443 The counter's tag encodes the counter's kind 444 and, if the counter is an expression, the expression's kind. 445 The possible tag values are: 446 447 * 0 - The counter is zero. 448 449 * 1 - The counter is a reference to the profile instrumentation counter. 450 451 * 2 - The counter is a subtraction expression. 452 453 * 3 - The counter is an addition expression. 454 455 .. _counter data: 456 457 Data: 458 ^^^^^ 459 460 The counter's data is interpreted in the following manner: 461 462 * When the counter is a reference to the profile instrumentation counter, 463 then the counter's data is the id of the profile counter. 464 * When the counter is an expression, then the counter's data 465 is the index into the array of counter expressions. 466 467 .. _Counter Expressions: 468 469 Counter Expressions 470 ------------------- 471 472 ``[numExpressions : LEB128, expr0LHS : LEB128, expr0RHS : LEB128, expr1LHS : LEB128, expr1RHS : LEB128, ...]`` 473 474 Counter expressions consist of two counters as they 475 represent binary arithmetic operations. 476 The expression's kind is determined from the `tag <counter-tag_>`_ of the 477 counter that references this expression. 478 479 .. _Mapping Regions: 480 481 Mapping Regions 482 --------------- 483 484 ``[numRegionArrays : LEB128, regionsForFile0, regionsForFile1, ...]`` 485 486 The mapping regions are stored in an array of sub-arrays where every 487 region in a particular sub-array has the same file id. 488 489 The file id for a sub-array of regions is the index of that 490 sub-array in the main array e.g. The first sub-array will have the file id 491 of 0. 492 493 Sub-Array of Regions 494 ^^^^^^^^^^^^^^^^^^^^ 495 496 ``[numRegions : LEB128, region0, region1, ...]`` 497 498 The mapping regions for a specific file id are stored in an array that is 499 sorted in an ascending order by the region's starting location. 500 501 Mapping Region 502 ^^^^^^^^^^^^^^ 503 504 ``[header, source range]`` 505 506 The mapping region record contains two sub-records --- 507 the `header`_, which stores the counter and/or the region's kind, 508 and the `source range`_ that contains the starting and ending 509 location of this region. 510 511 .. _header: 512 513 Header 514 ^^^^^^ 515 516 ``[counter]`` 517 518 or 519 520 ``[pseudo-counter]`` 521 522 The header encodes the region's counter and the region's kind. 523 524 The value of the counter's tag distinguishes between the counters and 525 pseudo-counters --- if the tag is zero, than this header contains a 526 pseudo-counter, otherwise this header contains an ordinary counter. 527 528 Counter: 529 """""""" 530 531 A mapping region whose header has a counter with a non-zero tag is 532 a code region. 533 534 Pseudo-Counter: 535 """"""""""""""" 536 537 ``[value : LEB128]`` 538 539 A pseudo-counter is stored in a single `LEB value <LEB128_>`_, just like 540 the ordinary counter. It has the following interpretation: 541 542 * bits 0-1: tag, which is always 0. 543 544 * bit 2: expansionRegionTag. If this bit is set, then this mapping region 545 is an expansion region. 546 547 * remaining bits: data. If this region is an expansion region, then the data 548 contains the expanded file id of that region. 549 550 Otherwise, the data contains the region's kind. The possible region 551 kind values are: 552 553 * 0 - This mapping region is a code region with a counter of zero. 554 * 2 - This mapping region is a skipped region. 555 556 .. _source range: 557 558 Source Range 559 ^^^^^^^^^^^^ 560 561 ``[deltaLineStart : LEB128, columnStart : LEB128, numLines : LEB128, columnEnd : LEB128]`` 562 563 The source range record contains the following fields: 564 565 * *deltaLineStart*: The difference between the starting line of the 566 current mapping region and the starting line of the previous mapping region. 567 568 If the current mapping region is the first region in the current 569 sub-array, then it stores the starting line of that region. 570 571 * *columnStart*: The starting column of the mapping region. 572 573 * *numLines*: The difference between the ending line and the starting line 574 of the current mapping region. 575 576 * *columnEnd*: The ending column of the mapping region. 577
