[Libre-soc-dev] Rust's SIMD project group

Luke Kenneth Casson Leighton lkcl at lkcl.net
Wed Sep 30 11:15:01 BST 2020

On Wed, Sep 30, 2020 at 9:53 AM Jacob Lifshay <programmerjake at gmail.com> wrote:

> https://zulip-archive.rust-lang.org/257879projectportablesimd/96123LibreSOCsSimpleV.html

okaaay nooow i can reply :)  (and i'll investigate an account later today)

although it is... how do we say... "inevitable" that, as it matures, a
given language would expose SIMD up to developers (gcc intrinsics) the
consequences are... always going to be harmful to those developers

a recent implementation of strncpy in PowerISA VSX was a whopping 250
hand-coded assembly instructions yet SV and RVV it is around 14.

note that, just as outlined in the sigarch article, preparation takes
vast swathes of detection for whether it's "ok" to actually proceed at
all with the 16-byte-wide vector copy, followed by further vast
swathes of "cleanup" cascade-detecting the 8, 4 2 and final 1 bytes
remaining that cannot be copied by a 16-byte-wide vector ISA.

the fact remains - sadly - that despite this god-awful mess - repeated
unavoidably *everywhere* that SIMD is attempted to be used for
general-purpose code - there's actually nothing that rust-lang can do
about it, at a *language* level.  SIMD exists, therefore it needs to
be supported by the language.  going into that eyes-open however will
help avoid some of the... debates.

sadly, if predication was available on SIMD as the "norm" this would
be a different matter entirely.


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