w2c2

Translates WebAssembly modules to portable C. Inspired by wabt's wasm2c.

Working towards WebAssembly as the Elusive Universal Binary:

                                          ↗ different
  source code     →  WebAssembly  →  C89  → OSes,
(C++, Rust, ...)                          ↘ CPUs

Features

• Implements the WebAssembly Core Specification 1.0
• Implements several extensions:
  • Threads and atomics
  • Bulk memory operations
  • Conditional data segment initialization
  • Sign-extension operators
  • Non-trapping float-to-int conversions
• Passes 99.9% of the WebAssembly core semantics test suite
• Written in (mostly) C89 and generates (mostly) C89 (some common extensions are used and not all limits are followed, see e.g. #98)
• Support for many operating systems (e.g. Mac OS X, Mac OS 9, Haiku, Rhapsody, OPENSTEP, NeXTSTEP, DOS, Windows XP, etc.)
• Support for many architectures (e.g. x86, ARM, PowerPC, SPARC, PA-RISC, etc.)
• Support for big-endian systems (e.g. PowerPC, SPARC, PA-RISC, etc.)
• Support for various compilers (e.g. old GCC, MSVC, CodeWarrior, etc.)
• Streaming/single-pass compilation, low memory usage
• Separate compilation into multiple files
• Parallel compilation
• Assist with incremental compilation: Separate static and dynamic functions by comparing with another module
• Support for multiple modules and instances
• Support for generating debug information:
  • Function names, if function names are provided in the names custom section
  • Source line mapping, if DWARF line information is provided in the .debug_line custom section. Requires libdwarf to be installed. See instructions below.
• WASI implementation
  • Able to run clang and Python
  • Support for many operating systems and architectures, support for big-endian systems
  • Implements the threads proposal

Performance

• Coremark 1.0: ~7% slower than native

Compilation

If your system is supported by at least CMake 2.8.12, prefer using CMake to detect features. On systems without CMake you can also use Make.

w2c2

cd w2c2
cmake -B build
cmake --build build

wasi

cd wasi
cmake -B build
cmake --build build

Usage

For example, to compile module.wasm to module.c (and module.h):

./w2c2 module.wasm module.c

Separate Compilation

w2c2 is able to compile a module into separate C files. This is recommended when compiling large modules and on hosts with limited resources.

For example, to compile module.wasm (and module.h), into multiple files with 100 functions each:

./w2c2 -f 100 module.wasm module.c

Parallel Compilation

When w2c2 was built with threading support, it is able to compile a module in parallel. By default, w2c2 spawns as many worker threads as CPU cores are available.

To manually specify the number of worker threads, pass the number using the -t flag.

For example, to compile using 2 threads:

./w2c2 -t 2 module.wasm module.c

Examples

Coremark:

cd examples/coremark
make
./coremark

Testing

Requires Python 3 and wabt (for wast2json).

cd tests
make gen
make run-tests

WASI Status

• args_get
• args_sizes_get
• clock_res_get
• clock_time_get
• environ_get
• environ_sizes_get
• fd_advise
• fd_allocate
• fd_close
• fd_datasync
• fd_fdstat_get
• fd_fdstat_set_flags
• fd_fdstat_set_rights
• fd_filestat_get
• fd_filestat_set_size
• fd_filestat_set_times
• fd_pread
• fd_prestat_get
• fd_prestat_dir_name
• fd_pwrite
• fd_read
• fd_readdir
• fd_renumber
• fd_seek
• fd_sync
• fd_tell
• fd_write
• path_create_directory
• path_filestat_get
• path_filestat_set_times
• path_link
• path_open
• path_readlink
• path_remove_directory
• path_rename
• path_symlink
• path_unlink_file
• poll_oneoff
• proc_exit
• random_get
• sched_yield
• sock_recv
• sock_send
• sock_shutdown
• thread-spawn (from the threads proposal)

Development

To build a debug release, pass BUILD=debug to make.

To enable sanitizers, list them in the SANITIZERS variable passed to make, e.g. make BUILD=debug SANITIZERS="base clang address thread".

• base enables the Undefined Behavior Sanitizer
• clang enables Clang-specific sanitizers
• thread enables the Thread Sanitizer
• address enables the Address Sanitizer

Installing libdwarf (required for source line mapping)

• On Linux, try installing a package named like libdwarf-dev

• On macOS, you can use Homebrew and install libdwarf (not dwarf!)

• w2c2 currently defaults to using the libdwarf API of >=v0.4.2. v0.6.0 has been tested to work successfully too.

• If using a version <0.4.2, try passing -DDWARF_OLD=1 to CMake. Version 20200114 is known to work.

• Since version 0.1.1, libdwarf ships with a pkg-config file, which CMake should be able to detect automatically.

  If libdwarf cannot be automatically found by CMake, you get the following message:

  -- Checking for module 'libdwarf'
  --   No package 'libdwarf' found
  

  In that case you can still provide the necessary information manually by passing a variation of the following options:

  -DDWARF_FOUND=1 -DDWARF_LIBRARIES=-ldwarf -DDWARF_LIBRARY_DIRS=/usr/lib -DDWARF_INCLUDE_DIRS=/usr/include/libdwarf