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For those wondering, Wasm3 describes itself as "the fastest WebAssembly interpreter". Other webassembly runtimes are JIT based, rather than being interpreters.
by nindalf 6y ago
For those wondering, Wasm3 describes itself as "the fastest WebAssembly interpreter". Other webassembly runtimes are JIT based, rather than being interpreters. The project's readme (https://github.com/wasm3/wasm3 https://github.com/wasm3/wasm3) talks more about this decision.
For more on the difference, and an explanation of what JIT is, check out this section of the book Crafting Interpreters (https://craftinginterpreters.com/a-map-of-the-territory.html#shortcuts-and-alternate-routes https://craftinginterpreters.com/a-map-of-the-territory.html...)
- Dylan16807 6y ago> Other webassembly runtimes are JIT based, rather than being interpreters. Do none of the notable ones do normal compilation?
- TazeTSchnitzel 6y agoWhat would the difference be between “JIT” and “normal” compilation for WebAssembly?
- smasher164 6y agoAn AOT compiler would do a one-time translation to a platform-specific binary. If the environment you run on doesn’t support JIT compilation (iOS for example), AOT compiling WASM is useful.
- Blikkentrekker 6y agoHow can a platform not support j.i.t. compilation? In what way does a platform need to coöperate with that?
- erik_seaberg 6y agoSome platforms are enforcing https://en.wikipedia.org/wiki/W%5EX https://en.wikipedia.org/wiki/W%5EX because malware also wants to create novel machine code within a running process.
- saagarjha 6y agoNote that it is possible to JIT even with W^X. What these platforms do is prevent memory that has been mapped writable and overwritten with new code from ever being mapped executable again.
- jlokier 6y agoA JIT generally means it compiles parts of the program to machine code on the fly before running them, as needed. To support that, the environment must allow the JIT to write machine code to memory, and then execute that same code. CPUs have memory protection flags to control which memory areas can be written, and which can be executed. The OS is in charge of setting those flags, on request from the application. Eg. mmap and mprotect system calls. iOS denies requests for memory that is both writable and executable at the same time. So applications cannot get the type of memory area a JIT needs. There are indirect methods where a file is written then mapped, like generating a small program or shared library on the fly. But iOS restricts these as well. There are workarounds for a developer's personal applications, used on their own registered iOS devices. But these workarounds cannot be run by everyone else, except people with a jailbroken iOS. They cannot be used in applications on the App Store.
- moonchild 6y agoW^X is not the issue here. JITs can deal with that, and in fact should do so even when it's not OS-enforced for security reasons. The problem on ios specifically is code signing, which is a problem for both JIT and AOT.
- jlokier 6y ago"There are indirect methods where a file is written then mapped, like generating a small program or shared library on the fly. But iOS restricts these as well."
- brundolf 6y agoAt that point, what's the purpose of using WASM at all?
- ReactiveJelly 6y agoYou can keep the sandbox intact after AOT. https://hacks.mozilla.org/2020/02/securing-firefox-with-webassembly/ https://hacks.mozilla.org/2020/02/securing-firefox-with-weba... > The core implementation idea behind wasm sandboxing is that you can compile C/C++ into wasm code, and then you can compile that wasm code into native code for the machine your program actually runs on. These steps are similar to what you’d do to run C/C++ applications in the browser, but we’re performing the wasm to native code translation ahead of time, when Firefox itself is built. It's a middle ground between sandboxing with subprocesses (which adds all the overhead of IPC) and switching to a fully memory-safe language (Rust, JS, etc.)
- smasher164 6y agoIf I were writing a sandboxing or instrumentation tool, operating on WASM would be much simpler than dealing with the quirks of assembly, syscalls, or some higher-level IR.
- seg_lol 6y agoControl Flow Integrity, https://en.wikipedia.org/wiki/Control-flow_integrity https://en.wikipedia.org/wiki/Control-flow_integrity
- Dylan16807 6y agoSo mostly I'm interested in compilers that compile the entire thing before any execution happens. Technically both AoT and JIT compilers can do that, but 95% of the things associated with "JIT" involve compiling the program one piece at a time. But even with the broad definition of JIT, one important difference is that AoT compilers don't have to mark pages executable.
- jedisct1 6y agoWAVM and SSVM do normal compilation.
- the_duke 6y agoSome runtimes like wasmtime support AOT compilation artifacts that can be loaded. But in general you can't "just compile" Webassembly, because you need a runtime. There is a C project that can compile an executable that includes the runtime and the compiled WASM, but the name is escaping me right now. Note that JIT in the WASM world doesn't quite mean the same thing as for eg Java. Almost all runtimes compile a whole module at once, not individual functions.
- pjmlp 6y agoCompiling individual functions was something in Java JITs like 20 years ago, they are a little bit more clever nowadays.
- nicoburns 6y agoI think the model is typically that optimisation applied in "normal compilation" are applied when the source language is translated to WASM, meaning that the WASM -> native translation can be quite straightforward and still performant.
- Others 6y agoI worked on one as an undergrad: https://github.com/gwsystems/aWsm https://github.com/gwsystems/aWsm Full AoT compilation, C programs run within 10% of native
- heavyset_go 6y ago10% difference from native, or 10% of native performance?
- Others 6y ago10% worse performance. Not 10x worse :D
- moonchild 6y agoSince it compiles using llvm, what's the reason for the discrepancy? Lost information about e.g. aliasing?
- Others 6y agoBounds checking, dynamic dispatch is more expensive, wasm semantics have no undefined behavior to exploit for optimizations. And there is bloat in the process. I’m sure losing information effects things as well
- vidarh 6y agowasm2c/WasmBoxC springs to mind: https://kripken.github.io/blog/wasm/2020/07/27/wasmboxc.html https://kripken.github.io/blog/wasm/2020/07/27/wasmboxc.html
- haberman 6y agoWasm3 calls its interpreter design a "meta machine": https://github.com/wasm3/wasm3/blob/main/docs/Interpreter.md#m3-massey-meta-machine https://github.com/wasm3/wasm3/blob/main/docs/Interpreter.md... It is heavily based around tail calls. I recently wrote a blog article about how we applied a similar tail-call-oriented strategy to accelerate protobuf parsing to 2+GB/s: https://blog.reverberate.org/2021/04/21/musttail-efficient-interpreters.html https://blog.reverberate.org/2021/04/21/musttail-efficient-i... I also recently landed a change in Clang trunk that offers guaranteed tail calls, so that this tail call design is safe in non-opt builds: https://reviews.llvm.org/D99517 https://reviews.llvm.org/D99517 I think wasm3 could benefit from using this attribute when it is available.
- glandium 6y agoHow do guaranteed tail calls work when you needs to put arguments on the stack, which usually prevents tail call? (and whether a specific call needs to pass arguments on the stack depends on the platform, and the number and type of arguments)
- haberman 6y agoThe musttail attribute performs several checks to ensure that a tail call can be guaranteed, and rejects the program if these rules are violated. The documentation has the details: https://github.com/llvm/llvm-project/blob/3b8ec86fd576b9808dc63da620d9a4f7bbe04372/clang/include/clang/Basic/AttrDocs.td#L449-L468 https://github.com/llvm/llvm-project/blob/3b8ec86fd576b9808d... These rules guarantee that the tail call is possible to perform on every platform. They end up being more strict than is necessary on some platforms and calling conventions.