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Shih-wei Liao | cecdab2 | 2011-01-10 02:10:29 -0800 | [diff] [blame] | 7 | <title>libbcc: A Versatile Bitcode Execution Engine for Mobile Devices</title> |
Shih-wei Liao | e961c66 | 2011-01-09 04:42:39 -0800 | [diff] [blame] | 8 | <style type="text/css"> |
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Shih-wei Liao | cecdab2 | 2011-01-10 02:10:29 -0800 | [diff] [blame] | 307 | <div class="document" id="libbcc-a-versatile-bitcode-execution-engine-for-mobile-devices"> |
| 308 | <h1 class="title">libbcc: A Versatile Bitcode Execution Engine for Mobile Devices</h1> |
Shih-wei Liao | e961c66 | 2011-01-09 04:42:39 -0800 | [diff] [blame] | 309 | |
| 310 | <div class="section" id="introduction"> |
| 311 | <h1>Introduction</h1> |
Shih-wei Liao | 7fff228 | 2011-01-09 13:57:01 -0800 | [diff] [blame] | 312 | <p>libbcc is an LLVM bitcode execution engine that compiles the bitcode |
Shih-wei Liao | cecdab2 | 2011-01-10 02:10:29 -0800 | [diff] [blame] | 313 | to an in-memory executable. libbcc is versatile because:</p> |
| 314 | <ul class="simple"> |
Shih-wei Liao | be84944 | 2011-01-20 05:19:00 -0800 | [diff] [blame] | 315 | <li>it implements both AOT (Ahead-of-Time) and JIT (Just-in-Time) |
| 316 | compilation.</li> |
| 317 | <li>Android devices demand fast start-up time, small size, and high |
| 318 | performance <em>at the same time</em>. libbcc attempts to address these |
| 319 | design constraints.</li> |
| 320 | <li>it supports on-device linking. Each device vendor can supply his or |
| 321 | her own runtime bitcode library (lib*.bc) that differentiates his or |
| 322 | her system. Specialization becomes ecosystem-friendly.</li> |
Shih-wei Liao | cecdab2 | 2011-01-10 02:10:29 -0800 | [diff] [blame] | 323 | </ul> |
Shih-wei Liao | 7fff228 | 2011-01-09 13:57:01 -0800 | [diff] [blame] | 324 | <p>libbcc provides:</p> |
Shih-wei Liao | e961c66 | 2011-01-09 04:42:39 -0800 | [diff] [blame] | 325 | <ul class="simple"> |
Shih-wei Liao | be84944 | 2011-01-20 05:19:00 -0800 | [diff] [blame] | 326 | <li>a <em>just-in-time bitcode compiler</em>, which translates the LLVM bitcode |
| 327 | into machine code</li> |
Shih-wei Liao | 7fff228 | 2011-01-09 13:57:01 -0800 | [diff] [blame] | 328 | <li>a <em>caching mechanism</em>, which can:<ul> |
Shih-wei Liao | be84944 | 2011-01-20 05:19:00 -0800 | [diff] [blame] | 329 | <li>after each compilation, serialize the in-memory executable into a |
| 330 | cache file. Note that the compilation is triggered by a cache |
| 331 | miss.</li> |
Shih-wei Liao | 7fff228 | 2011-01-09 13:57:01 -0800 | [diff] [blame] | 332 | <li>load from the cache file upon cache-hit.</li> |
| 333 | </ul> |
| 334 | </li> |
| 335 | </ul> |
Shih-wei Liao | 0dc51b1 | 2011-01-10 03:41:58 -0800 | [diff] [blame] | 336 | <p>Highlights of libbcc are:</p> |
Shih-wei Liao | be84944 | 2011-01-20 05:19:00 -0800 | [diff] [blame] | 337 | <ul> |
| 338 | <li><p class="first">libbcc supports bitcode from various language frontends, such as |
| 339 | RenderScript, GLSL (pixelflinger2).</p> |
| 340 | </li> |
| 341 | <li><p class="first">libbcc strives to balance between library size, launch time and |
| 342 | steady-state performance:</p> |
| 343 | <ul> |
| 344 | <li><p class="first">The size of libbcc is aggressively reduced for mobile devices. We |
| 345 | customize and improve upon the default Execution Engine from |
| 346 | upstream. Otherwise, libbcc's execution engine can easily become |
| 347 | at least 2 times bigger.</p> |
| 348 | </li> |
| 349 | <li><p class="first">To reduce launch time, we support caching of |
| 350 | binaries. Just-in-Time compilation are oftentimes Just-too-Late, |
| 351 | if the given apps are performance-sensitive. Thus, we implemented |
| 352 | AOT to get the best of both worlds: Fast launch time and high |
| 353 | steady-state performance.</p> |
| 354 | <p>AOT is also important for projects such as NDK on LLVM with |
| 355 | portability enhancement. Launch time reduction after we |
| 356 | implemented AOT is signficant:</p> |
| 357 | <pre class="literal-block"> |
| 358 | Apps libbcc without AOT libbcc with AOT |
| 359 | launch time in libbcc launch time in libbcc |
| 360 | App_1 1218ms 9ms |
| 361 | App_2 842ms 4ms |
| 362 | Wallpaper: |
| 363 | MagicSmoke 182ms 3ms |
| 364 | Halo 127ms 3ms |
| 365 | Balls 149ms 3ms |
| 366 | SceneGraph 146ms 90ms |
| 367 | Model 104ms 4ms |
| 368 | Fountain 57ms 3ms |
| 369 | </pre> |
| 370 | <p>AOT also masks the launching time overhead of on-device linking |
| 371 | and helps it become reality.</p> |
| 372 | </li> |
| 373 | <li><p class="first">For steady-state performance, we enable VFP3 and aggressive |
| 374 | optimizations.</p> |
| 375 | </li> |
Shih-wei Liao | e961c66 | 2011-01-09 04:42:39 -0800 | [diff] [blame] | 376 | </ul> |
| 377 | </li> |
Shih-wei Liao | be84944 | 2011-01-20 05:19:00 -0800 | [diff] [blame] | 378 | <li><p class="first">Currently we disable Lazy JITting.</p> |
| 379 | </li> |
Shih-wei Liao | e961c66 | 2011-01-09 04:42:39 -0800 | [diff] [blame] | 380 | </ul> |
| 381 | </div> |
| 382 | <div class="section" id="api"> |
| 383 | <h1>API</h1> |
Shih-wei Liao | 7fff228 | 2011-01-09 13:57:01 -0800 | [diff] [blame] | 384 | <p><strong>Basic:</strong></p> |
Shih-wei Liao | e961c66 | 2011-01-09 04:42:39 -0800 | [diff] [blame] | 385 | <ul class="simple"> |
| 386 | <li><strong>bccCreateScript</strong> - Create new bcc script</li> |
| 387 | <li><strong>bccRegisterSymbolCallback</strong> - Register the callback function for external |
| 388 | symbol lookup</li> |
| 389 | <li><strong>bccReadBC</strong> - Set the source bitcode for compilation</li> |
| 390 | <li><strong>bccReadModule</strong> - Set the llvm::Module for compilation</li> |
| 391 | <li><strong>bccLinkBC</strong> - Set the library bitcode for linking</li> |
Logan Chien | d9db38a | 2011-07-11 17:22:35 +0800 | [diff] [blame^] | 392 | <li><strong>bccPrepareExecutable</strong> - <em>deprecated</em> - Use bccPrepareExecutableEx instead</li> |
| 393 | <li><strong>bccPrepareExecutableEx</strong> - Create the in-memory executable by either |
Shih-wei Liao | e961c66 | 2011-01-09 04:42:39 -0800 | [diff] [blame] | 394 | just-in-time compilation or cache loading</li> |
Logan | f340bf7 | 2011-01-14 17:51:40 +0800 | [diff] [blame] | 395 | <li><strong>bccGetFuncAddr</strong> - Get the entry address of the function</li> |
| 396 | <li><strong>bccDisposeScript</strong> - Destroy bcc script and release the resources</li> |
| 397 | <li><strong>bccGetError</strong> - <em>deprecated</em> - Don't use this</li> |
Shih-wei Liao | e961c66 | 2011-01-09 04:42:39 -0800 | [diff] [blame] | 398 | </ul> |
Shih-wei Liao | 7fff228 | 2011-01-09 13:57:01 -0800 | [diff] [blame] | 399 | <p><strong>Reflection:</strong></p> |
Shih-wei Liao | e961c66 | 2011-01-09 04:42:39 -0800 | [diff] [blame] | 400 | <ul class="simple"> |
Logan | f340bf7 | 2011-01-14 17:51:40 +0800 | [diff] [blame] | 401 | <li><strong>bccGetExportVarCount</strong> - Get the count of exported variables</li> |
| 402 | <li><strong>bccGetExportVarList</strong> - Get the addresses of exported variables</li> |
| 403 | <li><strong>bccGetExportFuncCount</strong> - Get the count of exported functions</li> |
| 404 | <li><strong>bccGetExportFuncList</strong> - Get the addresses of exported functions</li> |
| 405 | <li><strong>bccGetPragmaCount</strong> - Get the count of pragmas</li> |
| 406 | <li><strong>bccGetPragmaList</strong> - Get the pragmas</li> |
Shih-wei Liao | e961c66 | 2011-01-09 04:42:39 -0800 | [diff] [blame] | 407 | </ul> |
Shih-wei Liao | 7fff228 | 2011-01-09 13:57:01 -0800 | [diff] [blame] | 408 | <p><strong>Debug:</strong></p> |
Shih-wei Liao | e961c66 | 2011-01-09 04:42:39 -0800 | [diff] [blame] | 409 | <ul class="simple"> |
Logan | f340bf7 | 2011-01-14 17:51:40 +0800 | [diff] [blame] | 410 | <li><strong>bccGetFuncCount</strong> - Get the count of functions (including non-exported)</li> |
| 411 | <li><strong>bccGetFuncInfoList</strong> - Get the function information (name, base, size)</li> |
Shih-wei Liao | e961c66 | 2011-01-09 04:42:39 -0800 | [diff] [blame] | 412 | </ul> |
| 413 | </div> |
| 414 | <div class="section" id="cache-file-format"> |
| 415 | <h1>Cache File Format</h1> |
| 416 | <p>A cache file (denoted as *.oBCC) for libbcc consists of several sections: |
| 417 | header, string pool, dependencies table, relocation table, exported |
| 418 | variable list, exported function list, pragma list, function information |
| 419 | table, and bcc context. Every section should be aligned to a word size. |
| 420 | Here is the brief description of each sections:</p> |
| 421 | <ul class="simple"> |
| 422 | <li><strong>Header</strong> (OBCC_Header) - The header of a cache file. It contains the |
Shih-wei Liao | cecdab2 | 2011-01-10 02:10:29 -0800 | [diff] [blame] | 423 | magic word, version, machine integer type information (the endianness, |
| 424 | the size of off_t, size_t, and ptr_t), and the size |
| 425 | and offset of other sections. The header section is guaranteed |
Shih-wei Liao | e961c66 | 2011-01-09 04:42:39 -0800 | [diff] [blame] | 426 | to be at the beginning of the cache file.</li> |
| 427 | <li><strong>String Pool</strong> (OBCC_StringPool) - A collection of serialized variable |
| 428 | length strings. The strp_index in the other part of the cache file |
| 429 | represents the index of such string in this string pool.</li> |
| 430 | <li><strong>Dependencies Table</strong> (OBCC_DependencyTable) - The dependencies table. |
Shih-wei Liao | 8538dfa | 2011-01-09 15:42:28 -0800 | [diff] [blame] | 431 | This table stores the resource name (or file path), the resource |
Shih-wei Liao | e961c66 | 2011-01-09 04:42:39 -0800 | [diff] [blame] | 432 | type (rather in APK or on the file system), and the SHA1 checksum.</li> |
| 433 | <li><strong>Relocation Table</strong> (OBCC_RelocationTable) - <em>not enabled</em></li> |
Shih-wei Liao | ac4e458 | 2011-01-10 02:59:54 -0800 | [diff] [blame] | 434 | <li><strong>Exported Variable List</strong> (OBCC_ExportVarList) - |
| 435 | The list of the addresses of exported variables.</li> |
| 436 | <li><strong>Exported Function List</strong> (OBCC_ExportFuncList) - |
| 437 | The list of the addresses of exported functions.</li> |
Shih-wei Liao | e961c66 | 2011-01-09 04:42:39 -0800 | [diff] [blame] | 438 | <li><strong>Pragma List</strong> (OBCC_PragmaList) - The list of pragma key-value pair.</li> |
| 439 | <li><strong>Function Information Table</strong> (OBCC_FuncTable) - This is a table of |
| 440 | function information, such as function name, function entry address, |
| 441 | and function binary size. Besides, the table should be ordered by |
| 442 | function name.</li> |
| 443 | <li><strong>Context</strong> - The context of the in-memory executable, including |
| 444 | the code and the data. The offset of context should aligned to |
Shih-wei Liao | 0dc51b1 | 2011-01-10 03:41:58 -0800 | [diff] [blame] | 445 | a page size, so that we can mmap the context directly into memory.</li> |
Shih-wei Liao | e961c66 | 2011-01-09 04:42:39 -0800 | [diff] [blame] | 446 | </ul> |
| 447 | <p>For furthur information, you may read <a class="reference external" href="include/bcc/bcc_cache.h">bcc_cache.h</a>, |
| 448 | <a class="reference external" href="lib/bcc/CacheReader.cpp">CacheReader.cpp</a>, and |
| 449 | <a class="reference external" href="lib/bcc/CacheWriter.cpp">CacheWriter.cpp</a> for details.</p> |
| 450 | </div> |
| 451 | <div class="section" id="jit-ed-code-calling-conventions"> |
| 452 | <h1>JIT'ed Code Calling Conventions</h1> |
| 453 | <ol class="arabic"> |
| 454 | <li><p class="first">Calls from Execution Environment or from/to within script:</p> |
| 455 | <p>On ARM, the first 4 arguments will go into r0, r1, r2, and r3, in that order. |
| 456 | The remaining (if any) will go through stack.</p> |
| 457 | <p>For ext_vec_types such as float2, a set of registers will be used. In the case |
| 458 | of float2, a register pair will be used. Specifically, if float2 is the first |
| 459 | argument in the function prototype, float2.x will go into r0, and float2.y, |
| 460 | r1.</p> |
| 461 | <p>Note: stack will be aligned to the coarsest-grained argument. In the case of |
| 462 | float2 above as an argument, parameter stack will be aligned to an 8-byte |
| 463 | boundary (if the sizes of other arguments are no greater than 8.)</p> |
| 464 | </li> |
| 465 | <li><p class="first">Calls from/to a separate compilation unit: (E.g., calls to Execution |
| 466 | Environment if those runtime library callees are not compiled using LLVM.)</p> |
| 467 | <p>On ARM, we use hardfp. Note that double will be placed in a register pair.</p> |
| 468 | </li> |
| 469 | </ol> |
| 470 | </div> |
| 471 | </div> |
| 472 | </body> |
| 473 | </html> |