[fpc-devel] x86_64 SHA1 implementation

[J. Gareth Moreton]

I missed this post - thanks Florian!

Indeed, SHA-1 is deprecated at least as far as being a cryptographic 
algorithm is concerned, but it still has some uses in data verification 
in a similar vein to MD5.  I know git uses it internally so server 
branches can't be corrupted.

I have probably spent too much time on SHA-1 already - its awkward size 
of 160 bits has always irked me... not a clean power of two!

Speaking of the Intel SHA instructions, can I introduce a merge request 
that adds "CPUX86_HAS_SHA" as a feature flag?  I know to add it for 
"cpu_zen" and later, but I'm not sure what the equivalent Intel 
processor is... is "cpu_core_avx2" okay or does there need to be a new one?

Kit

On 15/09/2023 22:48, Florian Klämpfl via fpc-devel wrote:
> Am 16.09.23 um 15:13 schrieb J. Gareth Moreton via fpc-devel:
>> Hi everyone,
>>
>> So this past week I've been building on Rika's work by adding an 
>> assembly version of SHA-1 for x86_64 to complement Rika's i386 
>> version. So far I've successfully made a version that runs twice as 
>> fast as the Pascal code.  I hoped to go even faster by making use of 
>> the SSE2 instruction set, but currently the end result is slower even 
>> though computing the common parts of 4 rounds simultaneously should 
>> be much faster.  This occurs even when I forgo writing to the stack 
>> and keep pretty much all of the state within registers.  Preliminary 
>> investigation suggests that the slowdown comes from using MOVD/Q to 
>> transfer data between the XMM registers and general-purpose 
>> registers, since they are different parts of the CPU.  I'm still 
>> amazed it causes this much latency though.
>>
>> I'll keep investigating and seeing if I can squeeze out more 
>> performance, but otherwise I may just have to fall back on a 
>> non-SIMD-optimised implementation.
>
> As SHA-1 is basically deprecated and not recommended to be used 
> anymore, I wouldn't spend too much into this. Besides this, for SHA-1 
> and SHA-256, it might be even more useful to use the SHA CPU 
> extensions if available. While they are only introduced in Ice Lake 
> and Zen, they will get more and more available in the future.
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