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Shih-wei Liaocecdab22011-01-10 02:10:29 -0800[diff] [blame]7<title>libbcc: A Versatile Bitcode Execution Engine for Mobile Devices</title>
Shih-wei Liaoe961c662011-01-09 04:42:39 -0800[diff] [blame]8<style type="text/css">
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Shih-wei Liaocecdab22011-01-10 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 Liaoe961c662011-01-09 04:42:39 -0800[diff] [blame]309
310<div class="section" id="introduction">
311<h1>Introduction</h1>
Shih-wei Liao7fff2282011-01-09 13:57:01 -0800[diff] [blame]312<p>libbcc is an LLVM bitcode execution engine that compiles the bitcode
Shih-wei Liaocecdab22011-01-10 02:10:29 -0800[diff] [blame]313to an in-memory executable. libbcc is versatile because:</p>
314<ul class="simple">
Shih-wei Liaobe849442011-01-20 05:19:00 -0800[diff] [blame]315<li>it implements both AOT (Ahead-of-Time) and JIT (Just-in-Time)
316compilation.</li>
317<li>Android devices demand fast start-up time, small size, and high
318performance <em>at the same time</em>. libbcc attempts to address these
319design constraints.</li>
320<li>it supports on-device linking. Each device vendor can supply his or
321her own runtime bitcode library (lib*.bc) that differentiates his or
322her system. Specialization becomes ecosystem-friendly.</li>
Shih-wei Liaocecdab22011-01-10 02:10:29 -0800[diff] [blame]323</ul>
Shih-wei Liao7fff2282011-01-09 13:57:01 -0800[diff] [blame]324<p>libbcc provides:</p>
Shih-wei Liaoe961c662011-01-09 04:42:39 -0800[diff] [blame]325<ul class="simple">
Shih-wei Liaobe849442011-01-20 05:19:00 -0800[diff] [blame]326<li>a <em>just-in-time bitcode compiler</em>, which translates the LLVM bitcode
327into machine code</li>
Shih-wei Liao7fff2282011-01-09 13:57:01 -0800[diff] [blame]328<li>a <em>caching mechanism</em>, which can:<ul>
Shih-wei Liaobe849442011-01-20 05:19:00 -0800[diff] [blame]329<li>after each compilation, serialize the in-memory executable into a
330cache file. Note that the compilation is triggered by a cache
331miss.</li>
Shih-wei Liao7fff2282011-01-09 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 Liao0dc51b12011-01-10 03:41:58 -0800[diff] [blame]336<p>Highlights of libbcc are:</p>
Shih-wei Liaobe849442011-01-20 05:19:00 -0800[diff] [blame]337<ul>
338<li><p class="first">libbcc supports bitcode from various language frontends, such as
339RenderScript, GLSL (pixelflinger2).</p>
340</li>
341<li><p class="first">libbcc strives to balance between library size, launch time and
342steady-state performance:</p>
343<ul>
344<li><p class="first">The size of libbcc is aggressively reduced for mobile devices. We
345customize and improve upon the default Execution Engine from
346upstream. Otherwise, libbcc's execution engine can easily become
347at least 2 times bigger.</p>
348</li>
349<li><p class="first">To reduce launch time, we support caching of
350binaries. Just-in-Time compilation are oftentimes Just-too-Late,
351if the given apps are performance-sensitive. Thus, we implemented
352AOT to get the best of both worlds: Fast launch time and high
353steady-state performance.</p>
354<p>AOT is also important for projects such as NDK on LLVM with
355portability enhancement. Launch time reduction after we
356implemented AOT is signficant:</p>
357<pre class="literal-block">
358Apps libbcc without AOT libbcc with AOT
359 launch time in libbcc launch time in libbcc
360App_1 1218ms 9ms
361App_2 842ms 4ms
362Wallpaper:
363 MagicSmoke 182ms 3ms
364 Halo 127ms 3ms
365Balls 149ms 3ms
366SceneGraph 146ms 90ms
367Model 104ms 4ms
368Fountain 57ms 3ms
369</pre>
370<p>AOT also masks the launching time overhead of on-device linking
371and helps it become reality.</p>
372</li>
373<li><p class="first">For steady-state performance, we enable VFP3 and aggressive
374optimizations.</p>
375</li>
Shih-wei Liaoe961c662011-01-09 04:42:39 -0800[diff] [blame]376</ul>
377</li>
Shih-wei Liaobe849442011-01-20 05:19:00 -0800[diff] [blame]378<li><p class="first">Currently we disable Lazy JITting.</p>
379</li>
Shih-wei Liaoe961c662011-01-09 04:42:39 -0800[diff] [blame]380</ul>
381</div>
382<div class="section" id="api">
383<h1>API</h1>
Shih-wei Liao7fff2282011-01-09 13:57:01 -0800[diff] [blame]384<p><strong>Basic:</strong></p>
Shih-wei Liaoe961c662011-01-09 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
388symbol 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>
Shih-wei Liao7fff2282011-01-09 13:57:01 -0800[diff] [blame]392<li><strong>bccPrepareExecutable</strong> - Create the in-memory executable by either
Shih-wei Liaoe961c662011-01-09 04:42:39 -0800[diff] [blame]393just-in-time compilation or cache loading</li>
Loganf340bf72011-01-14 17:51:40 +0800[diff] [blame]394<li><strong>bccGetFuncAddr</strong> - Get the entry address of the function</li>
395<li><strong>bccDisposeScript</strong> - Destroy bcc script and release the resources</li>
396<li><strong>bccGetError</strong> - <em>deprecated</em> - Don't use this</li>
Shih-wei Liaoe961c662011-01-09 04:42:39 -0800[diff] [blame]397</ul>
Shih-wei Liao7fff2282011-01-09 13:57:01 -0800[diff] [blame]398<p><strong>Reflection:</strong></p>
Shih-wei Liaoe961c662011-01-09 04:42:39 -0800[diff] [blame]399<ul class="simple">
Loganf340bf72011-01-14 17:51:40 +0800[diff] [blame]400<li><strong>bccGetExportVarCount</strong> - Get the count of exported variables</li>
401<li><strong>bccGetExportVarList</strong> - Get the addresses of exported variables</li>
402<li><strong>bccGetExportFuncCount</strong> - Get the count of exported functions</li>
403<li><strong>bccGetExportFuncList</strong> - Get the addresses of exported functions</li>
404<li><strong>bccGetPragmaCount</strong> - Get the count of pragmas</li>
405<li><strong>bccGetPragmaList</strong> - Get the pragmas</li>
Shih-wei Liaoe961c662011-01-09 04:42:39 -0800[diff] [blame]406</ul>
Shih-wei Liao7fff2282011-01-09 13:57:01 -0800[diff] [blame]407<p><strong>Debug:</strong></p>
Shih-wei Liaoe961c662011-01-09 04:42:39 -0800[diff] [blame]408<ul class="simple">
Loganf340bf72011-01-14 17:51:40 +0800[diff] [blame]409<li><strong>bccGetFuncCount</strong> - Get the count of functions (including non-exported)</li>
410<li><strong>bccGetFuncInfoList</strong> - Get the function information (name, base, size)</li>
Shih-wei Liaoe961c662011-01-09 04:42:39 -0800[diff] [blame]411</ul>
412</div>
413<div class="section" id="cache-file-format">
414<h1>Cache File Format</h1>
415<p>A cache file (denoted as *.oBCC) for libbcc consists of several sections:
416header, string pool, dependencies table, relocation table, exported
417variable list, exported function list, pragma list, function information
418table, and bcc context. Every section should be aligned to a word size.
419Here is the brief description of each sections:</p>
420<ul class="simple">
421<li><strong>Header</strong> (OBCC_Header) - The header of a cache file. It contains the
Shih-wei Liaocecdab22011-01-10 02:10:29 -0800[diff] [blame]422magic word, version, machine integer type information (the endianness,
423the size of off_t, size_t, and ptr_t), and the size
424and offset of other sections. The header section is guaranteed
Shih-wei Liaoe961c662011-01-09 04:42:39 -0800[diff] [blame]425to be at the beginning of the cache file.</li>
426<li><strong>String Pool</strong> (OBCC_StringPool) - A collection of serialized variable
427length strings. The strp_index in the other part of the cache file
428represents the index of such string in this string pool.</li>
429<li><strong>Dependencies Table</strong> (OBCC_DependencyTable) - The dependencies table.
Shih-wei Liao8538dfa2011-01-09 15:42:28 -0800[diff] [blame]430This table stores the resource name (or file path), the resource
Shih-wei Liaoe961c662011-01-09 04:42:39 -0800[diff] [blame]431type (rather in APK or on the file system), and the SHA1 checksum.</li>
432<li><strong>Relocation Table</strong> (OBCC_RelocationTable) - <em>not enabled</em></li>
Shih-wei Liaoac4e4582011-01-10 02:59:54 -0800[diff] [blame]433<li><strong>Exported Variable List</strong> (OBCC_ExportVarList) -
434The list of the addresses of exported variables.</li>
435<li><strong>Exported Function List</strong> (OBCC_ExportFuncList) -
436The list of the addresses of exported functions.</li>
Shih-wei Liaoe961c662011-01-09 04:42:39 -0800[diff] [blame]437<li><strong>Pragma List</strong> (OBCC_PragmaList) - The list of pragma key-value pair.</li>
438<li><strong>Function Information Table</strong> (OBCC_FuncTable) - This is a table of
439function information, such as function name, function entry address,
440and function binary size. Besides, the table should be ordered by
441function name.</li>
442<li><strong>Context</strong> - The context of the in-memory executable, including
443the code and the data. The offset of context should aligned to
Shih-wei Liao0dc51b12011-01-10 03:41:58 -0800[diff] [blame]444a page size, so that we can mmap the context directly into memory.</li>
Shih-wei Liaoe961c662011-01-09 04:42:39 -0800[diff] [blame]445</ul>
446<p>For furthur information, you may read <a class="reference external" href="include/bcc/bcc_cache.h">bcc_cache.h</a>,
447<a class="reference external" href="lib/bcc/CacheReader.cpp">CacheReader.cpp</a>, and
448<a class="reference external" href="lib/bcc/CacheWriter.cpp">CacheWriter.cpp</a> for details.</p>
449</div>
450<div class="section" id="jit-ed-code-calling-conventions">
451<h1>JIT'ed Code Calling Conventions</h1>
452<ol class="arabic">
453<li><p class="first">Calls from Execution Environment or from/to within script:</p>
454<p>On ARM, the first 4 arguments will go into r0, r1, r2, and r3, in that order.
455The remaining (if any) will go through stack.</p>
456<p>For ext_vec_types such as float2, a set of registers will be used. In the case
457of float2, a register pair will be used. Specifically, if float2 is the first
458argument in the function prototype, float2.x will go into r0, and float2.y,
459r1.</p>
460<p>Note: stack will be aligned to the coarsest-grained argument. In the case of
461float2 above as an argument, parameter stack will be aligned to an 8-byte
462boundary (if the sizes of other arguments are no greater than 8.)</p>
463</li>
464<li><p class="first">Calls from/to a separate compilation unit: (E.g., calls to Execution
465Environment if those runtime library callees are not compiled using LLVM.)</p>
466<p>On ARM, we use hardfp. Note that double will be placed in a register pair.</p>
467</li>
468</ol>
469</div>
470</div>
471</body>
472</html>