Do we need branch prediction?

By: rwessel (, August 25, 2022 11:10 am
Room: Moderated Discussions
Kevin G ( on August 25, 2022 8:47 am wrote:
> Mark Roulo ( on August 24, 2022 4:25 pm wrote:
> > Kara ( on August 24, 2022 2:07 pm wrote:
> > >
> > > Since like my second year programming I stopped writing branched code totally,
> > > few tens of thousands lines of code in, I haven't encounter anything that
> > > intrinsically requires speculative branch prediction, like logically.
> > >
> > > But I write only numes.
> > >
> > > Is there any program that can't be written without a branching that can't
> > > be rephrased (literally, all that is is rephrasing) into branchless?
> > >
> > > Mind risc-v don't offer BPU, there's extensions but not in the requirements.
> >
> > How do you write code to do one thing if a condition holds and another thing if the condition doesn't hold?
> >
> > Example:

> > if (b)
> > printf("Hello");
> > else
> > printf("Goodbye");
> >
I'll note that the conditional operator (?) is a branch.
> For something like this, multiplication of the raw boolean and the pointer to the strings to print would
> work. Both "Hello" and "Goodbye" multiplications need to be processes and then summed to create a new pointer
> for the resulting value. This assumes that true is a simple bit of 1 and false is 0. There are three main
> operations using this style vs. two that are actually performed in the if-else statement.
> This technique was leveraged heavily for arcane platforms like IBM's Cell whose SPE units had utterly
> poor branching performance but could run through those calculations. Due to the awkward architecture,
> doing more but faster operations was faster in real time than simpler but costly branching.

Now why does that sound familiar...? ;-)

I'll add that the technique goes back a really long way. It's basically predication as supported by any vector machine, and was described, I think, by Von Neumann, perhaps in the context of EDVAC.
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TopicPosted ByDate
Do we need branch prediction?Kara2022/08/24 02:07 PM
  Do we need branch prediction?Rayla2022/08/24 03:04 PM
  Do we need branch prediction?Mark Roulo2022/08/24 04:25 PM
    Do we need branch prediction?Paul A. Clayton2022/08/24 06:28 PM
      Do we need branch prediction?Anon2022/08/24 06:52 PM
    Do we need branch prediction?rwessel2022/08/24 06:40 PM
    Do we need branch prediction?ananon2022/08/25 12:56 AM
      Do we need branch prediction?Carlie Coats2022/08/25 07:39 AM
        OT: You wanted rather than Mark Roulo2022/08/25 09:57 AM
          OT: You wanted "pre" rather than "code" rather than Mark Roulo2022/08/25 09:57 AM
            OT: You wanted "pre" rather than "code" rather than Carlie Coats2022/08/25 11:43 AM
    Do we need branch prediction?Kevin G2022/08/25 08:47 AM
      Do we need branch prediction?rwessel2022/08/25 11:10 AM
          Do we need branch prediction?rwessel2022/08/26 10:37 AM
  Do we need branch prediction?Geoff Langdale2022/08/24 07:18 PM
  Do we need branch prediction?Adrian2022/08/25 12:58 AM
  Do we need branch prediction?Brendan2022/08/25 07:54 AM
    Do we need branch prediction?Anon42022/08/25 11:45 AM
      Do we need branch prediction?anon22022/08/25 09:08 PM
  Predicable branching patterns (aykm; )Kara2022/08/27 01:00 PM
    Predicable branching patterns (aykm; )Jukka Larja2022/08/27 11:38 PM
    Predicable branching patterns (aykm; )Eric Fink2022/08/28 01:22 AM
      Predicable branching patterns (aykm; )Doug S2022/08/28 10:17 AM
    Predicable branching patterns (aykm; )---2022/08/28 02:16 PM
      Predicable branching patterns (aykm; )Anon2022/08/28 02:35 PM
  Q: Do we need branch prediction? A: We don't need it, but....2022/08/29 02:17 PM
    Quantum stuff.Kara2022/08/29 03:01 PM
      Q: Do we need branch prediction? A: We don't need it, but....2022/08/30 11:48 AM
    "Quantum Woo" (hint; that's NOT how it works) (NT)anonymous22022/08/29 04:19 PM
      Q: Do we need branch prediction? A: We don't need it, but....2022/08/30 12:14 PM
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