Branch/jump target prediction -8

By: Maynard Handley (name99.delete@this.name99.org), August 19, 2016 12:47 pm
Room: Moderated Discussions
Michael S (already5chosen.delete@this.yahoo.com) on August 19, 2016 9:26 am wrote:
> Maynard Handley (name99.delete@this.name99.org) on August 19, 2016 8:41 am wrote:
> >
> > WTF do you (Farnsworth) insist on misrepresenting what I said? What value is there in this?
> >
>
> It's me, not Simon.
>
> > Did I talk about "extensive use of branch avoidance techniques"?
>
> Yes. Anything less than extensive use, wouldn't have measurable
> effect on average density of branches in instruction stream.
>
> > Or did I suggest exactly what Ricardo B is talking about --- extremely short instruction sequences
> > using csel/cmov to avoid likely unpredictable (because data-dependent) branches? Basically use csel/cmov
> > when the point of the branch is value calculation rather than control redirection?
> >
>
> Even for short unpredictable sequences branch avoidance can be sometimes counterproductive,
> specifically, when one of the inputs is a result of long dependency chain.
> With unpredictable branch you still have 50% chance of taking right path.
>
> I don't say that branch avoidance shell never be used, but that it shell be used with extreme care.
> In specific case presented by Ricardo, I can't say for sure, but would guess
> that it's a bad idea, because saturation tends to be predictable.
>

Why are you coupling together two distinct issues
- use of csel/cmov in certain limited precise circumstances?
- calculations at the end of a long dependency chain?

(a) I see no reason why the types of calculations I'm advocating are in some way particularly prone to being at the end of long dependency chains.

(b) Being at the end of a long dependency chain is not especially problematic if the result you calculate is not in turn critical. And once again I see no reason that the situations where I'm advocating the use of csel/cmov take that form.

(c) The correct solution to the generic problem of long dependency chains is value speculation, not replacing value calculations with control flow.
The point is that value prediction has the POTENTIAL to utilize a lighter weight replay mechanism than does control flow prediction. Control flow prediction, by definition, has to start everything again at the fetch point (and, unless you're using a strange ISA) doesn't have access to the join point at the end of a basic block, or a mechanism to know that perhaps the instructions after the join point don't need to be flushed. But value prediction IS amenable to lighter weight recovery mechanisms, if you're willing to put in the effort to do so.

(d) Your math makes no sense. You say "well, just guess the direction if you don't know. you have a 50% chance of being right".
OK, so 50% of the time the "obvious" calculation of something trivial (abs,max, maybe a test against saturation) costs say three instructions. And 50% of the time it flushes the pipeline and recovery costs us 20 cycles times say 4 to 6 to 8 instructions each cycle (depending on the exact CPU) lost. Average cost is basically 40, 60, 80 instructions per max/abs/sat calculation.
OR we could do some dicking around with csel/cmov and pay a cost of, depending on the details, maybe 4 to 8 instructions every time.
The correct choice seems pretty obvious to me.

Now maybe Ricardo's case of saturation is predictable; I have no idea, since I don't know what's he's doing and the data stream he is operating against.
I do know that in the use cases I cared about when I was doing this sort of thing (things like bit-twiddling to decode compressed data) I was dealing with maximally unpredictable data, and for the most part (usually, NOT always) each unpredictable branch was used to generate a value, not as control flow. ie exactly the situation I described in my original post.
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