2. Some platforms prohibit creating new executable pages, which prevents JITing.
3. Memory savings!
> Bytecode/opcodes are translated into more efficient "operations" during a compilation pass, generating pages of meta-machine code
WASM compiled to a novel bytecode format aimed at efficient interpretation.
> Commonly occurring sequences of operations can can also be optimized into a "fused" operation.
Peephole optimizations producing fused opcodes, makes sense.
> In M3/Wasm, the stack machine model is translated into a more direct and efficient "register file" approach
WASM translated to register-based bytecode. That's awesome!
> Since operations all have a standardized signature and arguments are tail-call passed through to the next, the M3 "virtual" machine registers end up mapping directly to real CPU registers.
This is some black magic, if it works!
Regardless, kudos to the authors and nice to see a fast wasm interpreter done well.
Translating the stack machine into registers was always a core part of the model but it's interesting to me that even interpreters are doing it. The necessity of doing coloring to assign registers efficiently is kind of unfortunate, I feel like the WASM compiler would have been the right place to do this offline.
Register based VMs like Lua don't do this. The register allocation is incredibly simple https://github.com/LuaJIT/LuaJIT/blob/v2.1/src/lj_parse.c#L3...
You don't have to do register allocation with coloring; it's just that most implementations do.
If the hardware executing this code is "stack-based" (or, does not offer enough general purpose registers to accomodate the funtion call) - this will need to be converted back to a stack-based function call (either at runtime, or beforehand). Wouldn't this intermediate WASM-to-register-based-bytecode translation be redundant then?
I would guess negligible.
It would be interesting to see how this is designed for security in mind.
Struggling to see it.
1. Control flow is always checked. You can't jump to an arbitrary address, you jump to index N in a control flow table.
2. Calls out of the sandbox are also table based.
3. Indexed accesses are bounds checked. On 64 bit platforms, this is achieved by demoting the wasm to 32 bit and using big guard pages. On 32 bit platforms, it's explicit compares.
The result is something which may become internally inconsistent (can Heartbleed) but cannot perform arbitrary accesses to host memory.
Needs to be memory safe otherwise a wasm program can execute arbitrary code, access memory that it should not, etc.
Tbh, I couldn't get the eureka moment though. Might try to read in the AM ;)
https://en.wikipedia.org/wiki/Threaded_code
http://www.complang.tuwien.ac.at/forth/threaded-code.html
You can see an example of this particular implementation style (where each operation is a tail call to a C function, passing the registers as arguments) at the second link above, under "continuation-passing style".
One of the big advantages of a threaded interpreter is relatively good branch prediction. A simple switch-based dispatch loop has a single indirect jump at its core, which is almost entirely unpredictable -- whereas threaded dispatch puts a copy of that indirect jump at the end of each opcode's implementation, giving the branch predictor way more data to work with. Effectively, you're letting it use the current opcode to help predict the next opcode!
> Because operations end with a call to the next function, the C compiler will tail-call optimize most operations.
It appears that this relies on tail-call optimization to avoid overflowing the stack. Unfortunately this means you probably can't run it in debug mode.
https://github.com/wasm3/wasm3/blob/master/test/benchmark/co...
59.5x faster than node.js at what? Executing WebAssembly?
