Re: is Vax addressing sane today

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Sujet : Re: is Vax addressing sane today
De : already5chosen (at) *nospam* yahoo.com (Michael S)
Groupes : comp.arch
Date : 13. Oct 2024, 11:00:14
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Organisation : A noiseless patient Spider
Message-ID : <20241013130014.000013bb@yahoo.com>
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On Sat, 12 Oct 2024 10:23:18 GMT
anton@mips.complang.tuwien.ac.at (Anton Ertl) wrote:

Michael S <already5chosen@yahoo.com> writes:
That's correct about intrinsics, but incorrect about ADCX/ADOX.
The later can be moderately helpful in special situuations, esp.
128b * 128b => 256b multiplication, but it is never necessary
and for addition/sbtraction is not needed at all. 
 
They are useful if there are two strings of additions.  This happens
naturally in wide multiplication (also beyond 256b results).  But it
also happens when you add three multi-precision numbers (say, X, Y,
Z): You need C for the carry of XYi=X[i]+Y[i]+C, and O for the carry
of XYZ[i]=XYi+Z[i]+O.  If you have ADCX/ADOX, you can do both
additions in one loop, so XYi can be in a register and does not need
to be stored .  If you don't have these instructions, only ADC, you
need one loop to compute X+Y and store the result in memory, and one
loop to compute XY+Z, i.e., the lack of ADCX/ADOX results in
substantial additional cost.
 
If you add 4 multi-precision numbers, AMD64 with ADX runs out of carry
bits, so you have to spend the overhead of an additional loop (but not
of two additional loops as without ADCX/ADOX).
 
With carry bits in the general purpose registers
<https://www.complang.tuwien.ac.at/anton/tmp/carry.pdf> and 30 GPRs
(one is zero, one is sp), you can add 14 multi-precision numbers per
loop: 14 GPRs for source addresses, 1 GPR for the target address, 1
for the loop counter, 13 registers for loop-carried carry flags.
 
Of course, the question is if this kind of computation is needed
frequently enough to justify this kind of extension.  For
multi-precision multiplication and squaring, Intel considered the
frequency relevant enough to introduce ADCX/ADOX/MULX.
 
- anton

That's not bad. I think, you see yourself that spill and context switch
parts could benefit from more work.
But I suspect that the main opposition you'll face in RISC-V
organization will center not on that, but on fear of increase in cycle
time, no matter if proven or not with hard numbers.



Date Sujet#  Auteur
5 Sep 24 * is Vax adressing sane today336Brett
5 Sep 24 +* Re: is Vax adressing sane today324John Dallman
6 Sep 24 i+- Re: is Vax adressing sane today1Lawrence D'Oliveiro
6 Sep 24 i`* Re: is Vax adressing sane today322Anton Ertl
6 Sep 24 i +- Re: is Vax adressing sane today1Lawrence D'Oliveiro
6 Sep 24 i +* Re: is Vax adressing sane today5MitchAlsup1
7 Sep 24 i i`* Re: is Vax adressing sane today4Anton Ertl
7 Sep 24 i i `* Re: is Vax adressing sane today3Anton Ertl
7 Sep 24 i i  `* Re: is Vax addressing sane today2John Dallman
7 Sep 24 i i   `- Re: is Vax addressing sane today1Anton Ertl
7 Sep 24 i +* Re: is Vax adressing sane today313John Levine
8 Sep 24 i i`* Re: is Vax adressing sane today312Anton Ertl
8 Sep 24 i i +* Re: is Vax adressing sane today304MitchAlsup1
8 Sep 24 i i i`* Re: is Vax addressing sane today303Lawrence D'Oliveiro
9 Sep 24 i i i +* Re: is Vax addressing sane today231MitchAlsup1
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11 Sep 24 i i i i i +- Re: is Vax addressing sane today1Lawrence D'Oliveiro
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25 Sep 24 i i i i i  `* Re: is Vax addressing sane today3Michael S
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11 Sep 24 i i i i  i `- Re: is Vax addressing sane today1Michael S
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