Xxhash: Document meaning of "non-portable"
I think it would be good to specify (in xxhash.h) what exactly is meant by portable.
For example there is:
The algorithm is portable: Like XXH32 and XXH64, it generates the same hash on all platforms.
Under the heading:
XXH3 is a new hash algorithm featuring:
Notably this doesn't mention the various vectorized implementations of XXH3, which other references (readme, github, etc) refer to as "non-portable".
My understanding is that "non-portable" only means that the code requires specific instruction set support in order to execute, not that the API will output different digests, given the same input. Is that correct? If so, perhaps different terms should be used, as "portable" and "non-portable" are not actually being used as inverses of each other.
p.s. That part of the header still references pre-0.8.0 release, and sounds as if the XXH3 implementation is still unstable...
shuffle2
All 6 comments
That is a good idea.
Basically, XXH3 is portable in that should compile on any compiler that can compile XXH64 — any sane C90 or C++98 compiler with a long long extension.
However, unlike XXH32 and XXH64, XXH3 can be further optimized with non-portable extensions like SIMD or 64-bit to 128-bit multiplication.
easyaspi314
on 6 Sep 2020
It's important to note that what I care about is if the XXH3 algorithm, for all runtime implementations, is deterministic. For example, is there a guarantee that given identical inputs, outputs produced on a system using the NEON implementation will match those from an AVX2 system? (the comparison should hold for the full matrix of platforms supported by xxhash).
Otherwise, outputs from xxhash cannot be shared between systems using differently accelerated versions of the library.
I think/hope it is deterministic in this way, but it doesn't seem to be explicitly stated anywhere...
shuffle2
on 6 Sep 2020
Yes. All implementations return the same value.
easyaspi314
on 6 Sep 2020
The XXH3 presentation paragraph has been reworded,
hopefully reducing risks of confusion on this topic.
Cyan4973
on 15 Sep 2020
Follow up question: have XXH32 and XXH64 been deterministic with respect to inputs and output values since release r42?
atomtx
on 22 Sep 2020
yes
Cyan4973
on 22 Sep 2020
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Most helpful comment
Yes. All implementations return the same value.