474 lines
14 KiB
HTML
474 lines
14 KiB
HTML
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<body>
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<div class="document" id="libbcc-a-versatile-bitcode-execution-engine-for-mobile-devices">
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<h1 class="title">libbcc: A Versatile Bitcode Execution Engine for Mobile Devices</h1>
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<div class="section" id="introduction">
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<h1>Introduction</h1>
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<p>libbcc is an LLVM bitcode execution engine that compiles the bitcode
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to an in-memory executable. libbcc is versatile because:</p>
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<ul class="simple">
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<li>it implements both AOT (Ahead-of-Time) and JIT (Just-in-Time)
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compilation.</li>
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<li>Android devices demand fast start-up time, small size, and high
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performance <em>at the same time</em>. libbcc attempts to address these
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design constraints.</li>
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<li>it supports on-device linking. Each device vendor can supply their
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own runtime bitcode library (lib*.bc) that differentiates their
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system. Specialization becomes ecosystem-friendly.</li>
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</ul>
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<p>libbcc provides:</p>
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<ul class="simple">
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<li>a <em>just-in-time bitcode compiler</em>, which translates the LLVM bitcode
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into machine code</li>
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<li>a <em>caching mechanism</em>, which can:<ul>
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<li>after each compilation, serialize the in-memory executable into a
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cache file. Note that the compilation is triggered by a cache
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miss.</li>
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<li>load from the cache file upon cache-hit.</li>
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</ul>
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</li>
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</ul>
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<p>Highlights of libbcc are:</p>
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<ul>
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<li><p class="first">libbcc supports bitcode from various language frontends, such as
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Renderscript, GLSL (pixelflinger2).</p>
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</li>
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<li><p class="first">libbcc strives to balance between library size, launch time and
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steady-state performance:</p>
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<ul>
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<li><p class="first">The size of libbcc is aggressively reduced for mobile devices. We
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customize and improve upon the default Execution Engine from
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upstream. Otherwise, libbcc's execution engine can easily become
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at least 2 times bigger.</p>
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</li>
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<li><p class="first">To reduce launch time, we support caching of
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binaries. Just-in-Time compilation are oftentimes Just-too-Late,
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if the given apps are performance-sensitive. Thus, we implemented
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AOT to get the best of both worlds: Fast launch time and high
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steady-state performance.</p>
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<p>AOT is also important for projects such as NDK on LLVM with
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portability enhancement. Launch time reduction after we
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implemented AOT is signficant:</p>
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<pre class="literal-block">
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Apps libbcc without AOT libbcc with AOT
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launch time in libbcc launch time in libbcc
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App_1 1218ms 9ms
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App_2 842ms 4ms
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Wallpaper:
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MagicSmoke 182ms 3ms
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Halo 127ms 3ms
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Balls 149ms 3ms
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SceneGraph 146ms 90ms
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Model 104ms 4ms
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Fountain 57ms 3ms
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</pre>
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<p>AOT also masks the launching time overhead of on-device linking
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and helps it become reality.</p>
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</li>
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<li><p class="first">For steady-state performance, we enable VFP3 and aggressive
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optimizations.</p>
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</li>
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</ul>
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</li>
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<li><p class="first">Currently we disable Lazy JITting.</p>
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</li>
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</ul>
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</div>
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<div class="section" id="api">
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<h1>API</h1>
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<p><strong>Basic:</strong></p>
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<ul class="simple">
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<li><strong>bccCreateScript</strong> - Create new bcc script</li>
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<li><strong>bccRegisterSymbolCallback</strong> - Register the callback function for external
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symbol lookup</li>
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<li><strong>bccReadBC</strong> - Set the source bitcode for compilation</li>
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<li><strong>bccReadModule</strong> - Set the llvm::Module for compilation</li>
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<li><strong>bccLinkBC</strong> - Set the library bitcode for linking</li>
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<li><strong>bccPrepareExecutable</strong> - <em>deprecated</em> - Use bccPrepareExecutableEx instead</li>
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<li><strong>bccPrepareExecutableEx</strong> - Create the in-memory executable by either
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just-in-time compilation or cache loading</li>
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<li><strong>bccGetFuncAddr</strong> - Get the entry address of the function</li>
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<li><strong>bccDisposeScript</strong> - Destroy bcc script and release the resources</li>
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<li><strong>bccGetError</strong> - <em>deprecated</em> - Don't use this</li>
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</ul>
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<p><strong>Reflection:</strong></p>
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<ul class="simple">
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<li><strong>bccGetExportVarCount</strong> - Get the count of exported variables</li>
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<li><strong>bccGetExportVarList</strong> - Get the addresses of exported variables</li>
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<li><strong>bccGetExportFuncCount</strong> - Get the count of exported functions</li>
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<li><strong>bccGetExportFuncList</strong> - Get the addresses of exported functions</li>
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<li><strong>bccGetPragmaCount</strong> - Get the count of pragmas</li>
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<li><strong>bccGetPragmaList</strong> - Get the pragmas</li>
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</ul>
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<p><strong>Debug:</strong></p>
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<ul class="simple">
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<li><strong>bccGetFuncCount</strong> - Get the count of functions (including non-exported)</li>
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<li><strong>bccGetFuncInfoList</strong> - Get the function information (name, base, size)</li>
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</ul>
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</div>
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<div class="section" id="cache-file-format">
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<h1>Cache File Format</h1>
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<p>A cache file (denoted as *.oBCC) for libbcc consists of several sections:
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header, string pool, dependencies table, relocation table, exported
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variable list, exported function list, pragma list, function information
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table, and bcc context. Every section should be aligned to a word size.
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Here is the brief description of each sections:</p>
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<ul class="simple">
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<li><strong>Header</strong> (MCO_Header) - The header of a cache file. It contains the
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magic word, version, machine integer type information (the endianness,
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the size of off_t, size_t, and ptr_t), and the size
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and offset of other sections. The header section is guaranteed
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to be at the beginning of the cache file.</li>
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<li><strong>String Pool</strong> (MCO_StringPool) - A collection of serialized variable
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length strings. The strp_index in the other part of the cache file
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represents the index of such string in this string pool.</li>
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<li><strong>Dependencies Table</strong> (MCO_DependencyTable) - The dependencies table.
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This table stores the resource name (or file path), the resource
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type (rather in APK or on the file system), and the SHA1 checksum.</li>
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<li><strong>Relocation Table</strong> (MCO_RelocationTable) - <em>not enabled</em></li>
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<li><strong>Exported Variable List</strong> (MCO_ExportVarList) -
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The list of the addresses of exported variables.</li>
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<li><strong>Exported Function List</strong> (MCO_ExportFuncList) -
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The list of the addresses of exported functions.</li>
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<li><strong>Pragma List</strong> (MCO_PragmaList) - The list of pragma key-value pair.</li>
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<li><strong>Function Information Table</strong> (MCO_FuncTable) - This is a table of
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function information, such as function name, function entry address,
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and function binary size. Besides, the table should be ordered by
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function name.</li>
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<li><strong>Context</strong> - The context of the in-memory executable, including
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the code and the data. The offset of context should aligned to
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a page size, so that we can mmap the context directly into memory.</li>
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</ul>
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<p>For furthur information, you may read <a class="reference external" href="include/bcc/bcc_cache.h">bcc_cache.h</a>,
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<a class="reference external" href="lib/bcc/CacheReader.cpp">CacheReader.cpp</a>, and
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<a class="reference external" href="lib/bcc/CacheWriter.cpp">CacheWriter.cpp</a> for details.</p>
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</div>
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<div class="section" id="jit-ed-code-calling-conventions">
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<h1>JIT'ed Code Calling Conventions</h1>
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<ol class="arabic">
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<li><p class="first">Calls from Execution Environment or from/to within script:</p>
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<p>On ARM, the first 4 arguments will go into r0, r1, r2, and r3, in that order.
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The remaining (if any) will go through stack.</p>
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<p>For ext_vec_types such as float2, a set of registers will be used. In the case
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of float2, a register pair will be used. Specifically, if float2 is the first
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argument in the function prototype, float2.x will go into r0, and float2.y,
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r1.</p>
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<p>Note: stack will be aligned to the coarsest-grained argument. In the case of
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float2 above as an argument, parameter stack will be aligned to an 8-byte
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boundary (if the sizes of other arguments are no greater than 8.)</p>
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</li>
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<li><p class="first">Calls from/to a separate compilation unit: (E.g., calls to Execution
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Environment if those runtime library callees are not compiled using LLVM.)</p>
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<p>On ARM, we use hardfp. Note that double will be placed in a register pair.</p>
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</li>
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</ol>
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</div>
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</div>
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