site/source/docs/compiling/WebAssembly.rst
.. _WebAssembly:
WebAssembly is a binary format for executing code on the web, allowing fast start times (smaller download and much faster parsing in browsers when compared to JS or asm.js). Emscripten compiles to WebAssembly by default, but you can also compile to JS for older browsers.
For some historical background, see these slides <https://kripken.github.io/talks/wasm.html>_ and this blogpost <https://hacks.mozilla.org/2015/12/compiling-to-webassembly-its-happening/>_.
WebAssembly is emitted by default, without the need for any special flags.
.. note:: If you don't want WebAssembly, you can disable it with something like
::
emcc [..args..] -sWASM=0
.. note:: Deciding to compile to Wasm or JS can be done at the linking stage: it doesn't affect the object files.
Emscripten emits WebAssembly using the upstream LLVM Wasm backend, since
version 1.39.0 (October 2019). Previously emscripten also supported the
old fastcomp backend which was removed in 2.0.0 (August 2020).
There are some differences you may notice between the two backends, if you upgrade from fastcomp to upstream:
The Wasm backend is strict about linking files with different features sets - for example, if one file was built with atomics but another was not, it will error at link time. This prevents possible bugs, but may mean you need to make some build system fixes.
WASM=0 behaves differently in the two backends. In fastcomp we emit
asm.js, while in upstream we emit JS (since not all Wasm constructs can be
expressed in asm.js). Also, the JS support implements the same external
WebAssembly.* API, so in particular startup will be async just like Wasm
by default, and you can control that with WASM_ASYNC_COMPILATION (even
though WASM=0).
The Wasm backend uses Wasm object files by default. That means that it does codegen at the compile step, which makes the link step much faster - like a normal native compiler. For comparison, in fastcomp the compile step emits LLVM IR in object files.
You normally wouldn't notice this, but some compiler flags affect codegen,
like DISABLE_EXCEPTION_CATCHING. Such flags must be passed during
codegen. The simple and safe thing is to pass all -s flags at both
compile and link time.
You can enable Link Time Optimization (LTO) with the usual llvm flags
(-flto, -flto=full, -flto=thin, at both compile and link times;
note, however, that thin LTO is not heavily tested atm and so regular LTO
is recommended).
With fastcomp, LTO optimization passes were not be run by default; for that
it was necessary to pass --llvm-lto 1. With the llvm backend LTO passes
will be run on any object files that are in bitcode format.
Another thing you might notice is that fastcomp's link stage is able to perform some minor types of link time optimization even without LTO being set. The LLVM backend requires actually setting LTO for those things.
wasm-ld, the linker used by the Wasm backend, requires libraries (.a
archives) to contain symbol indexes. This matches the behaviour the native
GNU linker. While emar will create such indexes by default, native tools
such as GNU ar and GNU strip are not aware of the WebAssembly object
format and cannot create archive indexes. In particular, if you run GNU
strip on an archive file that contains WebAssembly object files it will
remove the index which makes the archive unusable at link time.
Also see the blocker bugs on the Wasm backend <https://github.com/emscripten-core/emscripten/projects/1>, and the Wasm backend tagged issues <https://github.com/emscripten-core/emscripten/issues?utf8=✓&q=is%3Aissue+is%3Aopen+label%3A"LLVM+wasm+backend">.
WebAssembly can trap - throw an exception - on things like division by zero, rounding a very large float to an int, and so forth. In asm.js such things were silently ignored, as in JavaScript they do not throw, so this is a difference between JavaScript and WebAssembly that you may notice, with the browser reporting an error like float unrepresentable in integer range, integer result unrepresentable, integer overflow, or Out of bounds Trunc operation.
The LLVM Wasm backend avoids traps by adding more code around each possible trap (basically clamping the value if it would trap). This can increase code size and decrease speed, if you don't need that extra code. The proper solution for this is to use newer Wasm instructions that do not trap, by calling emcc or clang with -mnontrapping-fptoint. That code may not run in older VMs, though.
When using emcc to build to WebAssembly, you will see a .wasm file containing that code, as well as the usual .js file that is the main target of compilation. Those two are built to work together: run the .js (or .html, if that's what you asked for) file, and it will load and set up the WebAssembly code for you, properly setting up imports and exports for it, etc. Basically, you don't need to care about whether the compiled code is asm.js or WebAssembly, it's just a compiler flag, and otherwise everything should just work (except the WebAssembly should be faster).
.wasm file is not standalone - it's not easy to manually run it without that .js code, as it depends on getting the proper imports that integrate with JS. For example, it receives imports for syscalls so that it can do things like print to the console. There is work in progress towards ways to create standalone .wasm files, see the WebAssembly Standalone page <https://github.com/emscripten-core/emscripten/wiki/WebAssembly-Standalone>_.You may also see additional files generated, like a .data file if you are preloading files into the virtual filesystem. All that is exactly the same as when building to asm.js. One difference you may notice is the lack of a .mem file, which for asm.js contains the static memory initialization data, which in WebAssembly we can pack more efficiently into the WebAssembly binary itself.
Since its original launch, WebAssembly has been expanded with various feature
extensions, which have been implemented in browsers. A list of features
(including already-shipped and in-progress) and details about browser versions
that support them can be found on
webassembly.org <https://webassembly.org/features/>_.
Several of these features can be used by Emscripten (or are by default) and can be enabled or disabled individually (using either Clang or emscripten flags) or by selecting which version of browsers Emscripten should target.
Examples:
exceptions for details).Porting SIMD code for details).-mno-nontrapping-fptoint to disable).
Clang will generate nontrapping (saturating) float-to-int conversion instructions for
C typecasts. This should have no effect on programs that do not have
undefined behavior but if the casted floating-point value is outside the range
of the target integer type, the result will be a number of the max or min value
instead of a trap. This also results in a small code size improvement because
of details of the LLVM IR semantics.-mno-bulk-memory-opt -mno-bulk-memory to disable). memory.copy
and memory.fill instructions are used in the implementation of C
memcpy and memset, and Clang may generate them elsewhere.-sWASM_BIGINT=0 :ref:setting <wasm_bigint>
to disable). This has the effect that Wasm i64 values are passed and returned
between Wasm and JS as BigInt values rather than being split by Binaryen into
pairs of Numbers.-mno-sign-ext to disable).For the features that are enabled by default (or will be when sufficient
browser support exists), it's also possible to control them by specifying
which browser versions you want to target. You can use the
-sMIN_FIREFOX_VERSION :ref:setting <min_firefox_version>
(and also MIN_CHROME_VERSION, MIN_SAFARI_VERSION and
MIN_NODE_VERSION). Setting a value lower than the default version will
disable features not supported by the specified version. Some features (e.g.
Exception handling and threads) are not enabled by default because they have
tradeoffs (e.g. binary size costs or restrictions on how the resulting wasm
can be used such as COEP headers). These are not controlled by the browser
version flags and must be enabled explicitly.
See the :ref:settings <min_firefox_version> page for details of the default
browser versions Emscripten targets.
.wasm files and compilationWebAssembly code is prepared somewhat differently than asm.js. asm.js can be bundled inside the main JS file, while as mentioned earlier WebAssembly is a binary file on the side, so you will have more than one file to distribute.
Another noticeable effect is that WebAssembly is compiled asynchronously by default, which means you must wait for compilation to complete before calling compiled code (by waiting for main(), or the onRuntimeInitialized callback, etc., which you also need to do when you have anything else that makes startup async, like a .mem file for asm.js, or preloaded file data, etc.). You can turn off async compilation by setting WASM_ASYNC_COMPILATION=0, but that may not work in Chrome due to current limitations there.
Module['wasmBinary'] and it will be used from there, and then (with async compilation off) compilation should be synchronous.To serve Wasm in the most efficient way over the network, make sure your web server has the proper MIME type for .wasm files, which is application/wasm. That will allow streaming compilation, where the browser can start to compile code as it downloads.
In Apache, you can do this with
.. code-block:: none
AddType application/wasm .wasm
Also make sure that gzip is enabled:
.. code-block:: none
AddOutputFilterByType DEFLATE application/wasm
If you serve large .wasm files, the webserver will consume CPU compressing them on the fly at each request.
Instead you can pre-compress them to .wasm.gz and use content negotiation:
.. code-block:: none
Options Multiviews
RemoveType .gz
AddEncoding x-gzip .gz
AddType application/wasm .wasm