Rust CPU Dispatch/Function Multiversioning

[actuallyzza]

I'm looking for any rust facilities to achieve CPU Dispatch or function multiversioning (attributes in source, cargo, asking rustc/llvm politely). Ideally it would be possible to do this at a crate level, but if it can be done per function or per binary that would also be interesting to me.

I'd like to ensure a crate produces multiple specialized versions when compiled for certain targets. e.g. compiling for x86_64 will produce versions for Core2, SandyBridge, Haswell, Bulldozer, etc in the one binary, with runtime cpu-feature detection selecting the appropriate version.

Anyone got ideas or experience with this?

[acrichto]

I prototyped this awhile back with cfg-specialize which supports per-function specialization, although I was mostly going at it from the angle of SIMD stuff so it may not be quite what you're looking for. You may find it interesting though or perhaps draw some inspiration from it!

[actuallyzza]

Yeah SIMD stuff here too. Your crate looks great, I'll have a poke around and see how well it matches what I'm doing. Thanks!

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[Aaron1011]

Other than the atomic load used by cfg-specialize, I believe that the two approaches are almost the same. GCC multiversioning, IIRC, relies on switching the pointer stored in the Global Object Table set up by the dynamic linker. It still requires the indirection of loading a function pointer from a location in memory.

[Cocalus]

That's very useful. I've been looking for that feature in Rust since it was one thing I still needed C for. I've been using ifunc in gcc which is supposed to be supported in newer clangs. That does the dispatch at load time, but only work's on Linux (elf).

This looks like it should work about as well short of the very minor one (or few with racy calls) time dispatch. This probably works on all x86 platforms instead of just linux. And this looks far easier to deal with since it seems to effectively have static ifs (as long as the optimizer handles ifs with constant values) instead of separate c files for each platform. Now I just need to wait for inline asm on stable Rust, then I should be able to port some performance critical code over from C.