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