ISA support for constant count loops

By: anon (a.delete@this.b.com), January 17, 2020 8:28 am
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.

An interesting implementation opportunity with more flexibility is that on Intel CPUs, short literal loads into registers get implemented strictly as register names. It works similarly to how xor r,r doesn't load a value into a register, but just sets the result value to point to a zero register (that gets interpreted as a zero value in an alu). I don't know how many of those "fixed-value" registers there are, but at least 16. In principle, the CPU is capable of doing any math where the result fits into this set in the in-order frontend.

So you could zero out a register with xor, test against a value in register loaded as immediate, and increment the loop counter every time with inc, and it would be possible for all of this to happen early enough in the in-order front-end that it could be used for perfect branch prediction, without adding any new user-visible mechanisms.
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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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