ISA support for constant count loops

By: Paul A. Clayton (paaronclayton.delete@this.gmail.com), January 16, 2020 11:28 am
Room: Moderated Discussions
Linus Torvalds (torvalds.delete@this.linux-foundation.org) on January 16, 2020 9:12 am wrote:
[snip]
> Which was definitely a huge loss. Yes, the direct copies were larger, but because it was a constant
> size, there were no mispredict issues or "jump out-of-line" I$ behavior issues with the direct
> copies. So rep movs lost by a mile, and it really stuck out like a sore thumb on profiles.

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.

For small copies, individual stores (full unrolling for general loops) might still be more efficient. This factor then discourages hardware overhead for low overhead small count loops.
< Previous Post in ThreadNext Post in Thread >
TopicPosted ByDate
LLVM comments on mem*Maynard Handley2020/01/14 01:51 PM
  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
Reply to this Topic
Name:
Email:
Topic:
Body: No Text
How do you spell purple?