Re: Byte Addressability And Beyond

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Sujet : Re: Byte Addressability And Beyond
De : terje.mathisen (at) *nospam* tmsw.no (Terje Mathisen)
Groupes : comp.arch
Date : 06. Jun 2024, 09:10:38
Autres entêtes
Organisation : A noiseless patient Spider
Message-ID : <v3rndf$1dejp$1@dont-email.me>
References : 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
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MitchAlsup1 wrote:
Terje Mathisen wrote:
 
MitchAlsup1 wrote:
>
>
I.e. h.264 CABAC decoding has three branches per bit decoded, at least one of them impossible to predict or work around with clever coding.
>
How many instructions in the then-clause and in the else-clause ??
If these are smaller than 8, My 66000 can process them without
"branching" using predication.
 
No, the real problem is the context branching: After doing the 50% branch you pick up one of two alternative contexts and follow totally different paths, i.e. you cannot simply use the branch bit as an index.
 If the number of instructions in the combined then and else clauses is
lower than a certain number, it is equally efficient to deal with the
branch as if it were later nullification rather than a redirection of
the fetch end of the pipeline. Here, NO prediction is required and there is no chance of misprediction without regard to the
predictability
of the control flow point. The whole point is that if the fetch end
of the pipeline will reach the convergence point before the branch
is fully resolved, then "don't branch" nullify. it saves cycles and
keeps unpredictable branches out of the branch predictor--even if the apparent takenness of the branch is completely random--improving
the prediction accuracy of "real branches".
 So, for example, let us postulate a 1-wide machine fetching 4 words per
clock and a then clause of 3 instructions and an else clause of 4 inst.
By the time the pseudo branch instruction enters execution, both the
then and the else have already been fetched, parsed, and are flowing
through decode. The execution of the branch merely decides which inst
survive the pipeline and there are no misprediction stalls. {{On a
wider machine, the fetch is even wider and the parse/decode BW is
still higher, so the mispredicted control flow point does not suffer
misprediction repair costs.}}
 Oddly enough, this is how predication works on My 66000.
 
I found ways to bypass the issues with the other two branches but this one is fundamental.
 It is fundamental only on ISAs that perform predication improperly
or does not have predication, or use the predictor when predicating.
My 66000 is not one of them.
 I return to the question posed earlier::
How many instructions in the then-clause and in the else-clause ??
 From 100++ to 10K+? Effectively no path merge within any kind a visible window.
I.e. decoding CABAC is running a state machine with tens to hundreds (afair) different states, with close to zero commonality between the code for individual paths. There is almost zero if/then/else/endif local branching at this level.
I could see absolutely no way to avoid biting the bullet and actually branch to the relevant code path.
Like I've written before, it is almost as if CABAC was designed to be as hard as possible for a sw decoder.
Terje
--
- <Terje.Mathisen at tmsw.no>
"almost all programming can be viewed as an exercise in caching"

Date Sujet#  Auteur
1 May 24 * Byte Addressability And Beyond590Lawrence D'Oliveiro
1 May 24 +* Re: Byte Addressability And Beyond431John Levine
1 May 24 i+* Re: Byte Addressability And Beyond409Lawrence D'Oliveiro
1 May 24 ii+* Re: Byte Addressability And Beyond3John Levine
1 May 24 iii+- Re: Byte Addressability And Beyond1John Levine
1 May 24 iii`- Re: Byte Addressability And Beyond1Lawrence D'Oliveiro
1 May 24 ii+- Re: Byte Addressability And Beyond1Michael S
1 May 24 ii`* Re: Byte Addressability And Beyond404John Levine
2 May 24 ii +* Re: Byte Addressability And Beyond382Lawrence D'Oliveiro
2 May 24 ii i+* Re: Byte Addressability And Beyond4John Levine
2 May 24 ii ii`* Re: Byte Addressability And Beyond3Lawrence D'Oliveiro
2 May 24 ii ii `* Re: Byte Addressability And Beyond2John Levine
5 May 24 ii ii  `- Re: Byte Addressability And Beyond1Lawrence D'Oliveiro
2 May 24 ii i+* Re: Byte Addressability And Beyond367John Savard
2 May 24 ii ii+* Re: Byte Addressability And Beyond2MitchAlsup1
11 May 24 ii iii`- Re: Byte Addressability And Beyond1John Savard
4 May 24 ii ii`* Re: Byte Addressability And Beyond364Lawrence D'Oliveiro
8 May 24 ii ii `* Re: Byte Addressability And Beyond363John Savard
8 May 24 ii ii  +* Re: Byte Addressability And Beyond2Lawrence D'Oliveiro
10 May 24 ii ii  i`- Re: Byte Addressability And Beyond1David Brown
8 May 24 ii ii  `* Re: Byte Addressability And Beyond360MitchAlsup1
8 May 24 ii ii   `* Re: Byte Addressability And Beyond359John Levine
8 May 24 ii ii    +* Re: Byte Addressability And Beyond357Lawrence D'Oliveiro
9 May 24 ii ii    i`* Re: Byte Addressability And Beyond356John Levine
10 May 24 ii ii    i +* Re: Byte Addressability And Beyond354David Brown
10 May 24 ii ii    i i`* Re: Byte Addressability And Beyond353Anton Ertl
11 May 24 ii ii    i i `* Re: Byte Addressability And Beyond352David Brown
11 May 24 ii ii    i i  `* Re: Byte Addressability And Beyond351Anton Ertl
11 May 24 ii ii    i i   +* Re: Byte Addressability And Beyond158David Brown
11 May 24 ii ii    i i   i+- Re: Byte Addressability And Beyond1Anton Ertl
27 May 24 ii ii    i i   i`* Re: Byte Addressability And Beyond156Lawrence D'Oliveiro
27 May 24 ii ii    i i   i `* Re: Byte Addressability And Beyond155John Levine
27 May 24 ii ii    i i   i  `* Re: Byte Addressability And Beyond154Lawrence D'Oliveiro
27 May 24 ii ii    i i   i   `* Re: Byte Addressability And Beyond153John Levine
27 May 24 ii ii    i i   i    +* Re: Byte Addressability And Beyond149John Levine
27 May 24 ii ii    i i   i    i+- Re: Byte Addressability And Beyond1MitchAlsup1
28 May 24 ii ii    i i   i    i`* Re: Byte Addressability And Beyond147Lawrence D'Oliveiro
28 May 24 ii ii    i i   i    i +- Re: encoding conversion, Byte Addressability And Beyond1John Levine
28 May 24 ii ii    i i   i    i `* Re: Byte Addressability And Beyond145Thomas Koenig
29 May 24 ii ii    i i   i    i  +* Re: Byte Addressability And Beyond137Lawrence D'Oliveiro
29 May 24 ii ii    i i   i    i  i`* Re: Byte Addressability And Beyond136Anton Ertl
29 May 24 ii ii    i i   i    i  i +* Re: Byte Addressability And Beyond12Stefan Monnier
29 May 24 ii ii    i i   i    i  i i+* Re: Byte Addressability And Beyond10Stefan Monnier
29 May 24 ii ii    i i   i    i  i ii+* Re: Byte Addressability And Beyond3John Levine
30 May 24 ii ii    i i   i    i  i iii`* Re: Byte Addressability And Beyond2George Neuner
4 Jun 24 ii ii    i i   i    i  i iii `- Re: Byte Addressability And Beyond1George Neuner
30 May 24 ii ii    i i   i    i  i ii`* Re: Byte Addressability And Beyond6Anton Ertl
4 Jun 24 ii ii    i i   i    i  i ii +- Re: Byte Addressability And Beyond1Lawrence D'Oliveiro
4 Jun 24 ii ii    i i   i    i  i ii `* Re: Byte Addressability And Beyond4Stefan Monnier
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6 Jun 24 ii ii    i i   i    i  i ii  ii   +- Re: architectural goals, Byte Addressability And Beyond1Lynn Wheeler
6 Jun 24 ii ii    i i   i    i  i ii  ii   +- Re: architectural goals, Byte Addressability And Beyond1OrangeFish
7 Jun 24 ii ii    i i   i    i  i ii  ii   `- Re: architectural goals, Byte Addressability And Beyond1Lawrence D'Oliveiro
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30 May 24 ii ii    i i   i    i  i i`- Re: Byte Addressability And Beyond1Anton Ertl
30 May 24 ii ii    i i   i    i  i +* Re: Byte Addressability And Beyond2Lawrence D'Oliveiro
30 May 24 ii ii    i i   i    i  i `* Re: Byte Addressability And Beyond4Terje Mathisen
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12 May 24 ii ii    i i   `* Re: Byte Addressability And Beyond178Thomas Koenig
27 May 24 ii ii    i `- Re: Byte Addressability And Beyond1Lawrence D'Oliveiro
8 May 24 ii ii    `- Re: Byte Addressability And Beyond1Michael S
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2 May 24 ii +* Re: Byte Addressability And Beyond3Michael S
2 May 24 ii `* Re: Byte Addressability And Beyond18Anton Ertl
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1 May 24 +* Re: Byte Addressability And Beyond40MitchAlsup1
1 May 24 +* Re: Byte Addressability And Beyond15Thomas Koenig
1 May 24 +* Re: Byte Addressability And Beyond3Michael S
2 May 24 +* Re: Byte Addressability And Beyond4Lawrence D'Oliveiro
3 May 24 +* Re: Byte Addressability And Beyond75Anton Ertl
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5 May 24 `- Re: Byte Addressability And Beyond1John Savard

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