ISA support for constant count loops

By: Gabriele Svelto (gabriele.svelto.delete@this.gmail.com), January 16, 2020 2:15 pm
Room: Moderated Discussions
Paul A. Clayton (paaronclayton.delete@this.gmail.com) on January 16, 2020 10:28 am wrote:
> In theory an ISA could provide a simple mechanism to load the loop count for constant iteration (inner) loops.
> A simple load constant instruction could trigger microarchitectural optimization of branch prediction (and
> other functions). For constant count loops there is no reason for branch misprediction other than difficulty
> of recognizing the count and the associated branch. Even much of the overhead of the branch instruction could
> be reduced (like some DSPs' special support for loops). For x86, this could just be idiom recognition for REP-prefixed
> instructions; loading a constant into the count register would prepare for such a loop.

POWER - and PowerPC before it - has a special register called the count register (CTR, section 2.3.3 in the PowerISA 3.0 doc) that can be used implicitly by branch instructions to loop. A field in a conditional branch instruction can be set to decrement the CTR and then branch if it's zero or if it's not zero.

In older processors using it guaranteed perfectly predictable loops but it came at a cost because the register was not renamed, so loading the count was a serializing instruction. I haven't seen it used much but it might have been.

Crazily enough you can also load an address in the CTR and branch to it. That's one way of doing indirect branches (the other is using the link register).

Naturally various DSPs have that too and I've seen at least two distinct RISC-V extensions which offer the same type of functionality.
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TopicPosted ByDate
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  LLVM comments on mem*Anon32020/01/15 06:28 AM
  Interesting comment about rep instructions & code sizeGabriele Svelto2020/01/15 07:12 AM
    Interesting comment about rep instructions & code sizenone2020/01/15 08:59 AM
      Interesting comment about rep instructions & code sizeGabriele Svelto2020/01/16 03:56 AM
        Interesting comment about rep instructions & code sizeLinus Torvalds2020/01/16 10:12 AM
          ISA support for constant count loopsPaul A. Clayton2020/01/16 11:28 AM
            ISA support for constant count loopsGabriele Svelto2020/01/16 02:15 PM
              PowerPC "front-end registers"Paul A. Clayton2020/01/16 03:34 PM
              ISA support for constant count loopsTravis Downs2020/01/16 05:21 PM
                ISA support for constant count loopsLinus Torvalds2020/01/16 08:41 PM
                  ISA support for constant count loopsTravis2020/01/16 09:48 PM
                    ISA support for constant count loopsBrett2020/01/17 01:28 AM
              Branch to CTRMaya2020/01/18 08:15 AM
                Branch to CTRGabriele Svelto2020/01/18 01:14 PM
            ISA support for constant count loopsanon2020/01/17 08:28 AM
              ISA support for constant count loopsTravis Downs2020/01/17 08:34 AM
            ISA support for constant count loops: ineffective compared to micro-threads2020/01/20 08:02 AM
              ISA support for constant count loops: ineffective compared to micro-threadssomeone2020/01/20 12:23 PM
                ISA support for constant count loops: ineffective compared to micro-threads2020/01/22 09:23 AM
              ISA support for too slow computersEtienne2020/01/21 02:42 AM
                ISA support for constant count loops: ineffective compared to micro-threads2020/01/22 09:18 AM
                  ISA support for constant count loops: ineffective compared to micro-threads2020/01/22 10:04 AM
                  ISA support for constant count loops: ineffective compared to micro-threadsHeikki Kultala2020/01/22 10:47 AM
                    ISA support for constant count loops: ineffective compared to micro-threadsdmcq2020/01/22 01:31 PM
                    ISA support for constant count loops: ineffective compared to micro-threads2020/01/22 03:28 PM
                      ISA support for constant count loops: ineffective compared to micro-threadsEtienne2020/01/22 04:35 PM
          Interesting comment about rep instructions & code sizeGabriele Svelto2020/01/16 02:00 PM
    Interesting comment about rep instructions & code sizeTravis Downs2020/01/15 03:40 PM
      Interesting comment about rep instructions & code sizeChester2020/01/15 05:16 PM
        Interesting comment about rep instructions & code sizeTravis Downs2020/01/15 05:50 PM
          Interesting comment about rep instructions & code sizeChester2020/01/15 07:24 PM
            Interesting comment about rep instructions & code sizeTravis Downs2020/01/16 02:26 PM
              Interesting comment about rep instructions & code sizeChester2020/01/17 01:16 PM
                Interesting comment about rep instructions & code sizeTravis Downs2020/01/17 03:41 PM
        Interesting comment about rep instructions & code sizeGabriele Svelto2020/01/16 03:53 AM
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