Branch/jump target prediction -8

By: anon (anon.delete@this.ymous.org), August 16, 2016 9:09 am
Room: Moderated Discussions
⚛ (0xe2.0x9a.0x9b.delete@this.gmail.com) on August 16, 2016 9:23 am wrote:
> Ricardo B (ricardo.b.delete@this.xxxxx.xx) on August 16, 2016 7:39 am wrote:
> > ⚛ (0xe2.0x9a.0x9b.delete@this.gmail.com) on August 16, 2016 6:40 am wrote:
> > >
> > > The outcome of a conditional branch depends on data (not on
> > > code). An instruction prefix can help branch prediction
> > > only if the prefix is mentioning a register or a memory location containing the prediction(s).
> >
> > You're suggesting to have a register/memory access in the path of the branch prediction logic?
> > That will just stall the fetch/decode pipeline.
> > That's like operating without branch prediction and just stalling
> > the pipeline until the branch instruction is retired.
>
> What would be an alternative option to improving branch prediction over Skylake's other
> than computing the prediction and somehow feeding it to the parts of CPU which need it?

What I understand : "Why not just compute the branch at Fetch?".

>
> If a CPU needs to know the value of the register used by the conditional branch 8 cycles before the
> branch opcode enters the pipeline, then the compiler/programmer will emit branch-with-prediction-in-register
> only in case the register can be filled with information 8 cycles before the branch.

Programmer can't be expected to do that.
Also, the number of cycles needed depends on the uarch. This means that you will need to compile your program differnetly for every uarch if you want to benefit from this.

You are also considering that the frontend has access to register file ports somehow, which I think is problematic (floorplan amongst other things), and that you know which physical register corresponds to the arch reg that the branch should read.

>
> An alternative option that comes to my mind is: Conditional jump after 8 instructions. That is, the conditional
> branch instruction enters the pipeline, the CPU postpones its execution, starts counting from 1 to 8, and after
> it executes/retires the 8th instruction it executes the postponed conditional branch instruction.
>

So, a conditionnal branch? What is the difference with a regular branch? This is exactly what the CPU is doing for branches when their operands are not ready yet. Either I am missing something or you are basically proposing to execute branches at Fetch.

> Question: How many conditional branches in real-world code are predictable
> 8 cycles before the branch instruction enters the CPU pipeline?

This doesn't mean anything. 8 cycles may be 8 or 40+ instructions dependending on your ISA and fetch-width. It feels natural to say that a branch is more predictable with information that is 8-instruction old, but less so with information that is 40-instruction old.

>
> Linus believes (why?) that such a question is totally out of the question.

< Previous Post in ThreadNext Post in Thread >
TopicPosted ByDate
Branch/jump target predictionTravis2016/08/09 09:44 AM
  Early decode of unconditional jumpsPeter Cordes2016/08/09 11:35 AM
    Early decode of unconditional jumpsExophase2016/08/09 12:29 PM
  pipelines are too long, noHeikki Kultala2016/08/09 11:37 AM
    pipelines are too long, nono name2016/08/09 06:17 PM
      pipelines are too long, noWilco2016/08/10 01:43 AM
        pipelines are too long, noPaul A. Clayton2016/08/10 07:44 PM
    Converged BTB/IcachePaul A. Clayton2016/08/10 07:44 PM
  Branch/jump target predictionsylt2016/08/10 02:27 AM
    Branch/jump target predictionPeter Cordes2016/08/12 03:23 PM
      Branch/jump target predictionsylt2016/08/12 10:35 PM
  Branch/jump target predictionMr. Camel2016/08/10 09:43 AM
    Branch/jump target predictionLinus Torvalds2016/08/10 11:46 AM
      Branch/jump target predictionMegol2016/08/10 02:25 PM
        Branch/jump target predictionLinus Torvalds2016/08/10 04:14 PM
          Branch/jump target predictionDavid Kanter2016/08/11 11:09 PM
            Branch/jump target predictionLinus Torvalds2016/08/12 11:25 AM
          Branch/jump target prediction2016/08/14 04:24 AM
            Branch/jump target predictionMaynard Handley2016/08/14 06:47 AM
              Branch/jump target predictionDavid Kanter2016/08/14 07:13 AM
              Branch/jump target prediction2016/08/16 05:19 AM
            Branch/jump target predictionTim McCaffrey2016/08/14 07:12 AM
              Branch/jump target predictionDavid Kanter2016/08/14 07:18 AM
                Branch/jump target predictionGabriele Svelto2016/08/14 01:09 PM
            Just a thoughtAnon2016/08/14 09:40 AM
              Just a thought2016/08/16 05:58 AM
                Just a thoughtAnon2016/08/16 07:45 AM
                  Just a thought2016/08/16 08:36 AM
            Branch/jump target predictionLinus Torvalds2016/08/14 09:40 AM
              Branch/jump target prediction2016/08/16 05:40 AM
                Branch/jump target predictionRicardo B2016/08/16 06:39 AM
                  Branch/jump target prediction -82016/08/16 08:23 AM
                    Branch/jump target prediction -8anon2016/08/16 09:09 AM
                    Branch/jump target prediction -8Ricardo B2016/08/16 09:33 AM
                      Branch/jump target prediction -8Exophase2016/08/16 10:02 AM
                        Branch/jump target prediction -8Ricardo B2016/08/16 10:31 AM
                        SPU hbr instruction (hint for branch)vvid2016/08/16 11:31 AM
                        Branch/jump target prediction -8no name2016/08/17 07:16 AM
                    Branch/jump target prediction -8Gabriele Svelto2016/08/16 10:46 AM
                      Branch/jump target prediction -8Etienne2016/08/17 12:27 AM
                        Branch/jump target prediction -8Gabriele Svelto2016/08/17 02:52 AM
                    Branch/jump target prediction -8Maynard Handley2016/08/18 09:02 AM
                      Branch/jump target prediction -82016/08/18 05:21 PM
                        Branch/jump target prediction -8Maynard Handley2016/08/18 06:27 PM
                          Branch/jump target prediction -8Megol2016/08/19 03:29 AM
                          Part 1/N - CPU-internal JIT2016/08/19 03:44 AM
                        Atom, you're such a comedian.Jim Trent2016/08/18 09:39 PM
                          Atom, you're such a comedian.2016/08/19 02:23 AM
                      Branch/jump target prediction -8Etienne2016/08/19 12:25 AM
                        Branch/jump target prediction -8Simon Farnsworth2016/08/19 03:17 AM
                          Branch/jump target prediction -8Michael S2016/08/19 05:39 AM
                          Branch/jump target prediction -8anon2016/08/19 06:29 AM
                            Branch/jump target prediction -8Simon Farnsworth2016/08/19 07:34 AM
                              Branch/jump target prediction -8anon2016/08/19 07:48 AM
                                Branch/jump target prediction -8Exophase2016/08/19 10:03 AM
                                Branch/jump target prediction -8Maynard Handley2016/08/19 10:34 AM
                            Branch/jump target prediction -8David Kanter2016/08/19 11:23 PM
                        Branch/jump target prediction -8Ricardo B2016/08/19 06:18 AM
                          Branch/jump target prediction -8Maynard Handley2016/08/19 07:41 AM
                            Branch/jump target prediction -8Michael S2016/08/19 08:26 AM
                              Branch/jump target prediction -8Maynard Handley2016/08/19 12:47 PM
                                Branch/jump target prediction -8Michael S2016/08/21 12:53 AM
                                  Branch/jump target prediction -8Ricardo B2016/08/22 04:17 AM
                                    Branch/jump target prediction -8Michael S2016/08/22 04:58 AM
                                      Branch/jump target prediction -8Ricardo B2016/08/22 06:50 AM
                            Branch/jump target prediction -8Simon Farnsworth2016/08/19 08:28 AM
                              Branch/jump target prediction -8Simon Farnsworth2016/08/19 08:40 AM
                            Branch/jump target prediction -8David Kanter2016/08/22 11:05 PM
                              Branch/jump target prediction -8Maynard Handley2016/08/23 06:49 AM
                      Branch/jump target prediction -8anon2016/08/26 07:00 AM
                        Branch/jump target prediction -8anon2016/08/26 07:14 AM
                Branch/jump target predictionMegol2016/08/19 03:23 AM
          Branch/jump target predictionMegol2016/08/19 06:42 AM
            Branch/jump target predictionMaynard Handley2016/08/19 10:46 AM
              Branch/jump target predictionDavid Kanter2016/08/19 11:34 PM
                Branch/jump target predictionMaynard Handley2016/08/20 06:07 AM
            Branch/jump target predictionsylt2016/08/19 10:48 AM
              Branch/jump target predictionsylt2016/08/19 11:00 AM
              Branch/jump target predictionMegol2016/08/21 09:27 AM
                The (apparent) state of trace caches on modern CPUsMaynard Handley2016/08/22 02:10 PM
                  The (apparent) state of trace caches on modern CPUsExophase2016/08/22 07:55 PM
                    The (apparent) state of trace caches on modern CPUsanon2016/08/22 11:36 PM
                      The (apparent) state of trace caches on modern CPUsExophase2016/08/23 04:08 AM
                        The (apparent) state of trace caches on modern CPUsanon2016/08/23 08:51 PM
                          The (apparent) state of trace caches on modern CPUsExophase2016/08/23 10:12 PM
                          The (apparent) state of trace caches on modern CPUsMaynard Handley2016/08/24 06:38 AM
                            The (apparent) state of trace caches on modern CPUsanon2016/08/24 07:26 PM
                    The (apparent) state of trace caches on modern CPUsMaynard Handley2016/08/23 06:48 AM
                      That's not trueDavid Kanter2016/08/23 08:39 AM
                        That's not trueMaynard Handley2016/08/23 08:56 AM
                      The (apparent) state of trace caches on modern CPUsanon2016/08/23 08:54 PM
                  The (wrong) state of trace caches on modern CPUsEric Bron2016/08/25 01:38 AM
                    The (wrong) state of trace caches on modern CPUsMichael S2016/08/25 02:28 AM
                      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
                        The (wrong) state of trace caches on modern CPUsEric Bron2016/08/25 10:12 AM
                          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
                              The (wrong) state of trace caches on modern CPUsMaynard Handley2016/08/25 12:34 PM
        Branch/jump target predictionGabriele Svelto2016/08/11 12:15 PM
  Branch/jump target predictionGabriele Svelto2016/08/20 06:21 AM
Reply to this Topic
Name:
Email:
Topic:
Body: No Text
How do you spell tangerine? 🍊