Branch/jump target prediction -8

By: Maynard Handley (name99.delete@this.name99.org), August 23, 2016 6:49 am
Room: Moderated Discussions
David Kanter (dkanter.delete@this.realworldtech.com) on August 23, 2016 12:05 am wrote:
> Maynard Handley (name99.delete@this.name99.org) on August 19, 2016 8:41 am wrote:
> > Ricardo B (ricardo.b.delete@this.xxxxx.xx) on August 19, 2016 7:18 am wrote:
> > > Etienne (etienne_lorrain.delete@this.yahoo.fr) on August 19, 2016 1:25 am wrote:
> > > > Maynard Handley (name99.delete@this.name99.org) on August 18, 2016 10:02 am wrote:
> > > > > What provides more value is fixing the remaining problems
> > > > > in this fetch pipeline. These might include things like
> > > >
> > > > Wouldn't it be worthwhile, because it seems technology has reach a point where fetching
> > > > those "unit of code" (i.e. assembly instructions) very optimised, to make more work
> > > > in each "unit of code" (so they can take more time to do more usefull stuff).
> > > > Obviously it would not be with the same assembly instruction set, but if you had integer operations
> > > > with saturation (could also be programmable saturation at for instance [0..4095]), integer division
> > > > with round-up, round-down and round nearest - then the compiler would not need to generate so many
> > > > simple assembly instructions with so many jumps, which seems so difficult to fetch.
> > >
> > > Yes, it's worthwhile.
> > > But it has already been done.
> > >
> > > Eg, any modern architecture has conditional move or min/max instructions which the
> > > compilers can and do use to implement saturation arithmetic without branches.
> > > Some had even more sophisticated methods in the form of general purpose predication.
> > >
> > > Eg, GCC output for adding two numbers and saturating the result into a range:
> > >
> > > leal (%rsi,%rdi), %edi
> > > cmpl %edi, %ecx
> > > movl %edi, %eax
> > > cmovle %ecx, %eax
> > > movl %edx, %ecx
> > > negl %ecx
> > > cmpl %ecx, %eax
> > > cmovle %edx, %eax
> > > ret
> > >

> > >
> > > The problem is that for anything more than this type of minimal use, it's faster to branch
> > > and risk the penalty than to execute a number of potentially useless instructions.
> > >
> >
> > [[
> > Simon Farnsworth (simon.delete@this.farnz.org.uk) on August 19, 2016 4:17 am wrote:
> > >
> > > I'm not entirely clear on what code constructs to avoid with OOO, beyond the obvious of keeping
> > > your dependency chains under control - OOO can't help you if, within the OOO window, it can't find
> > > enough µops to execute. OTOH, compilers are now pretty good at helping you out here, so...
> > >
> >
> > That's one of the reasons why extensive use of branch avoidance techniques,
> > as suggested above by Maynard, is not a very good idea.
> > ]]
> >
> > WTF do you (Farnsworth) insist on misrepresenting what I said? What value is there in this?
> >
> > Did I talk about "extensive use of branch avoidance techniques"?
> > 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?
> >
>
> Maynard,
>
> If you are going to misattribute a quote to Simon that is from someone else
> and use that as a basis to gripe about Simon - I think it would be nice for
> you to apologize to Simon when you realize it was someone else entirely.
>
>
> David

I was going to then we got that bizarre racist rant thrown in by god knows who, and it seems safest to just ignore the whole damn thread and whoever was trying to cause trouble.
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                            The (apparent) state of trace caches on modern CPUsanon2016/08/24 07:26 PM
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                      The (wrong) state of trace caches on modern CPUsEric Bron2016/08/25 06:12 AM
                      The (wrong) state of trace caches on modern CPUsMaynard Handley2016/08/25 08:50 AM
                        The (wrong) state of trace caches on modern CPUsMichael S2016/08/25 09:36 AM
                          The (wrong) state of trace caches on modern CPUsExophase2016/08/25 10:32 AM
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                          The (wrong) state of trace caches on modern CPUsMaynard Handley2016/08/25 11:01 AM
                            The (wrong) state of trace caches on modern CPUsEric Bron2016/08/25 11:20 AM
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