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Show HN: (bits) of a Libc, Optimized for Wasm
by ncruces
I make a no-CGO Go SQLite driver, by compiling the amalgamation to Wasm, then loading the result with wazero (a CGO-free Wasm runtime).
To compile SQLite, I use wasi-sdk, which uses wasi-libc, which is based on musl. It's been said that musl is slow(er than glibc), which is true, to a point.
musl uses SWAR on a size_t to implement various functions in string.h. This is fine, except size_t is just 32-bit on Wasm.
I found that implementing a few of those functions with Wasm SIMD128 can make them go around 4x faster.
Other functions don't even use SWAR; redoing those can make them 16x faster.
Smooth sort also has trouble pulling its own weight; a Shell sort seems both simpler and faster, while similarly avoiding recursion, allocations and the addressable stack.
I found that using SIMD intrinsics (rather than SWAR) makes it easier to avoid UB, but the code would definitely benefit from more eyeballs.
See this for some benchmarks on both x86-64 and Aarch64: https://github.com/ncruces/go-sqlite3/actions/runs/145169318...
Wasm intrinsics look neat as a higher-level fixed size SIMD abstraction. I wonder how good the compilers can do if using them for AOT targets with libraries like simd-everywhere.
string.h is missing strstr(), there's an algorithm of similar complexity you might consider: http://0x80.pl/notesen/2016-11-28-simd-strfind.html
Yeah, so far I did exactly the ones (my build of) SQLite needed and not others.
If there's interest, the set of implemented functions can definitely be extended.
Would you consider writing some blog posts or other resources about WASM? I was experimenting recently with WIT, and ran into a mountain of issues. There's a lot of jargon that could do with some untangling.
It took me a lot longer than it should have to put together this basic module, and even then there's this shared library I had to download to build it, and I couldn't figure out why this requires a libc:
This looks like a nice approach to making wasi-libc faster. Could you submit these changes upstream?
I'd like to be a little more sure that I'm not totally messing things up before doing that, but yes, eventually, that would be a nice outcome.
I've also only really tested wazero. I can't know for sure that this is a straight improvement for other runtimes and architectures.
For instance, the code delays using wasm_i8x16_bitmask as much as possible, because on Aarch64 it can be slower than not using SIMD at all, whereas it's plenty fast on x86-64.
The maintainers of wasi-libc are some of the best people to review this, and I don’t think it would be wasting their time to ask them to look at a PR.
A PR is a significant investment from me. I'd have to figure out where something like this is supposed to fit, how the build infra works, etc.
One of the nice things about Go is how much that's a solved issue out of the box, compared to almost everything else; certainly compared to C.
Pinging them in an issue: https://github.com/WebAssembly/wasi-libc/issues/580
What is SWAR?
SIMD within a register: https://en.wikipedia.org/wiki/SWAR
It's generally used for techniques that apply SIMD principles within general-purpose registers and instructions.
Assume you've loaded a 64-bit register (a uint64_t) with 8 bytes (unsigned char) of data. Can you answer the question “is any of these 8 bytes zero (the NUL terminator)?”
If you find a cheap way to do it, you can make strlen go faster by consuming 8 bytes at a time.
Et voilà:
#define ONES ((uint64_t)-1/UCHAR_MAX)
#define HIGHS (ONES \* (UCHAR_MAX/2+1))
#define HASZERO(x) ((x)-ONES & ~(x) & HIGHS)It is still a bit early, but I'm majorly bullish on WASM for multiple use cases:
1. Client side browser polyglot "applets" (Java applets were ahead of their time IMO)
2. Server side polyglot "servlets" (Node.js, embedded runtimes, etc.)
3. Language interop/FFI (Lang A -> WASM -> Lang B, like wasm2c)
Why is #3 so interesting? The hardest thing in language conversion is the library calls. WASI standardizes that, so all the proprietary libs will eventually compile down to WASI as a sort of POSIX/libc like layer. In addition, WASM standardizes calling convention. The resulting new source code may not look like much, but it will solve the FFI calling convention/marshalling/library issues nicely.
I’m not sure how it solves the FFI problem. Lowest common denominator calling conventions don’t make it any easier to bridge languages than it already is.
C calling conventions are already the standard for FFI in native code, and that means dropping down to what can be expressed in C if you want to cross that boundary.
As far as Go is concerned, the Wasm sandbox makes the (addressable, C) stack explicit, which solves at least some of the issues CGO has to deal with.
It's not a panacea, though; it introduces other issues.
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Crafted by Rajat
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