You don't need isel either

By: Paul A. Clayton (paaronclayton.delete@this.gmail.com), July 8, 2015 3:46 am
Room: Moderated Discussions
Gabriele Svelto (gabriele.svelto.delete@this.gmail.com) on July 8, 2015 2:01 am wrote:
> Paul A. Clayton (paaronclayton.delete@this.gmail.com) on July 7, 2015 7:43 am wrote:
> > To "predicate" a store, one selects the address (either the ordinary address or a
> > safe but unused address ) and performs the store. A store is more expensive than just
> > an ALU operation, but it could still be cheaper than a branch misprediction.
>
> An instruction such as isel can be easily reproduced using a sequence of other instructions (see
> Hacker's Delight for a zillion short sequences suitable for the task) but that's not the point.
> It just costs more than using an isel (both in terms of execution time and resources spent in
> implementing it) and the same applies to making a store output to a dummy address. You need to
> setup the appropriate address (unless your processor already has one), load it to a dedicated
> register or synthesize it, etc... In the end it's more work and more cycles/power spent than just
> jumping over a store that effectively causes it to be dismissed if the jump is taken.

Yes, using a select instruction to avoid a store is more work, but if the processor does not support dynamic hammock branch predication (or predicated stores) then doing this extra work may still be less expensive than the alternative (frequent branch mispredictions flushing many instructions). When a mispredicted branch can easily discard tens of operations, sometimes even expensive alternatives make sense.

As Michael S commented earlier in this thread:
Sometimes, when direction of branch is really unpredictable and pipeline is really long and data dependencies are either non-problematic or "true" and inevitable and the processor is wide and it (processor) does not have anything better to do these people are not morons at all even when each side of branch is close to 10 instructions. There are no hard rules except "Don't trust your intuition. Measure!".


The question of what interface to provide to the compiler/programmer is not simple either. The RISC V developers specifically chose not to include conditional move/select (page 17, The RISC-V Instruction Set Manual, Volume I: User-Level ISA, Version 2.0; PDF):

We considered but did not include conditional moves or predicated instructions, which can effectively replace unpredictable short forward branches. Conditional moves are the simpler of the two, but are difficult to use with conditional code that might cause exceptions (memory accesses and floating-point operations). Predication adds additional flag state to a system, additional instructions to set and clear flags, and additional encoding overhead on every instruction. Both conditional move and predicated instructions add complexity to out-of-order microarchitectures, adding an implicit third source operand due to the need to copy the original value of the destination architectural register into the renamed destination physical register if the predicate is false. Also, static compile-time decisions to use predication instead of branches can result in lower performance on inputs not included in the compiler training set, especially given that unpredictable branches are rare, and becoming rarer as branch prediction techniques improve.


We note that various microarchitectural techniques exist to dynamically convert unpredictable short forward branches into internally predicated code to avoid the cost of flushing pipelines on a branch mispredict [7, 11, 10] and have been implemented in commercial processors [20]. The simplest techniques just reduce the penalty of recovering from a mispredicted short forward branch by only flushing instructions in the branch shadow instead of the entire fetch pipeline, or by fetching instructions from both sides using wide instruction fetch or idle instruction fetch slots. More complex techniques for out-of-order cores add internal predicates on instructions in the branch shadow, with the internal predicate value written by the branch instruction, allowing the branch and following instructions to be executed speculatively and out-of-order with respect to other code [20].



I am inclined to disagree, but I also know that I am inordinately fond of software communicating information to hardware.
< Previous Post in ThreadNext Post in Thread >
TopicPosted ByDate
Cyclone Macro-op fusionMaynard Handley2015/07/05 05:01 PM
  Cyclone Macro-op fusionGabriele Svelto2015/07/06 01:11 AM
    Cyclone Macro-op fusionMaynard Handley2015/07/06 09:25 AM
      Cyclone Macro-op fusionWilco2015/07/06 10:17 AM
        Cyclone Macro-op fusionMaynard Handley2015/07/06 11:07 AM
          Cyclone Macro-op fusionMaynard Handley2015/07/06 11:55 AM
            Cyclone Macro-op fusionGabriele Svelto2015/07/07 01:49 AM
      Power8 branch conversionSHK2015/07/06 11:41 AM
        Power8 branch conversionMaynard Handley2015/07/06 12:01 PM
          Power8 branch conversionMaynard Handley2015/07/06 12:09 PM
        Benefit of dynamic hammock predicationPaul A. Clayton2015/07/06 02:02 PM
          Benefit of dynamic hammock predicationMaynard Handley2015/07/06 02:34 PM
            Benefit of dynamic hammock predicationEduardoS2015/07/06 06:56 PM
            Benefit of dynamic hammock predicationPatrick Chase2015/07/06 09:30 PM
          Benefit of dynamic hammock predicationLinus Torvalds2015/07/06 02:59 PM
            Benefit of dynamic hammock predicationWilco2015/07/06 03:35 PM
              Benefit of dynamic hammock predicationLinus Torvalds2015/07/06 04:25 PM
                Benefit of dynamic hammock predicationanon2015/07/06 08:02 PM
            Benefit of dynamic hammock predicationPatrick Chase2015/07/06 09:31 PM
              Benefit of dynamic hammock predicationanon2015/07/06 10:02 PM
              Benefit of dynamic hammock predicationMichael S2015/07/07 12:07 AM
              Benefit of dynamic hammock predicationLinus Torvalds2015/07/07 08:23 AM
                Benefit of dynamic hammock predicationPatrick Chase2015/07/07 09:49 AM
                  Benefit of dynamic hammock predicationSHK2015/07/07 10:34 AM
                  Benefit of dynamic hammock predicationMaynard Handley2015/07/07 11:00 AM
                    Benefit of dynamic hammock predicationExophase2015/07/07 12:26 PM
                      Benefit of dynamic hammock predicationWilco2015/07/07 03:54 PM
                      Benefit of dynamic hammock predicationMaynard Handley2015/07/07 04:25 PM
                    Benefit of dynamic hammock predicationPatrick Chase2015/07/07 01:32 PM
                    Benefit of dynamic hammock predicationMichael S2015/07/08 12:57 AM
                  Benefit of dynamic hammock predicationSylvain Collange2015/07/08 05:30 AM
                    Benefit of dynamic hammock predicationMaynard Handley2015/07/08 08:46 AM
                      Benefit of dynamic hammock predicationSylvain Collange2015/07/08 09:32 AM
                        Benefit of dynamic hammock predicationMaynard Handley2015/07/08 09:57 AM
                        Sadly, skewed associativity has not been broadly adoptedPaul A. Clayton2015/07/08 01:00 PM
                        Prediction quality of A8 and Intel (was Benefit of dynamic hammock predication)Maynard Handley2015/07/08 07:23 PM
                          Prediction quality of A8 and Intel (was Benefit of dynamic hammock predication)David Kanter2015/07/09 06:20 AM
                            Prediction quality of A8 and Intel (was Benefit of dynamic hammock predication)Maynard Handley2015/07/09 09:04 AM
                              Prediction quality of A8 and Intel (was Benefit of dynamic hammock predication)Tim McCaffrey2015/07/09 09:41 AM
                                Prediction quality of A8 and Intel (was Benefit of dynamic hammock predication)Patrick Chase2015/07/09 12:08 PM
                                  Prediction quality of A8 and Intel (was Benefit of dynamic hammock predication)Sylvain Collange2015/07/10 01:03 AM
                              Prediction quality of A8 and Intel (was Benefit of dynamic hammock predication)David Kanter2015/07/09 03:15 PM
                                Prediction quality of A8 and Intel (was Benefit of dynamic hammock predication)Maynard Handley2015/07/09 04:21 PM
                            Prediction quality of A8 and Intel (was Benefit of dynamic hammock predication)Patrick Chase2015/07/09 11:44 AM
                          Prediction quality of A8 and Intel (was Benefit of dynamic hammock predication)Patrick Chase2015/07/09 08:43 AM
                          Prediction quality of A8 and Intel (was Benefit of dynamic hammock predication)someone2015/07/10 04:43 AM
            Benefit of dynamic hammock predicationKonrad Schwarz2015/07/07 06:54 AM
            Benefit of dynamic hammock predicationdmcq2015/07/07 01:38 PM
              Benefit of dynamic hammock predicationLinus Torvalds2015/07/08 07:23 AM
                Benefit of dynamic hammock predicationdmcq2015/07/08 09:11 AM
                  Benefit of dynamic hammock predicationLinus Torvalds2015/07/08 12:38 PM
                    Example of memory ordering causing excitement in the real worldMark Roulo2015/07/08 03:54 PM
                      Example of memory ordering causing excitement in the real worlddmcq2015/07/09 03:37 AM
                        Example of memory ordering causing excitement in the real worldMark Roulo2015/07/09 07:24 AM
                          Example of memory ordering causing excitement in the real worlddmcq2015/07/09 09:09 AM
                          Example of memory ordering causing excitement in the real worldMaynard Handley2015/07/09 09:12 AM
                            Example of memory ordering causing excitement in the real worldLinus Torvalds2015/07/09 11:14 AM
                              Example of memory ordering causing excitement in the real worlddmcq2015/07/09 03:24 PM
                                Example of memory ordering causing excitement in the real worldSimon Farnsworth2015/07/10 12:07 PM
                                  Example of memory ordering causing excitement in the real worlddmcq2015/07/10 02:49 PM
                                    Example of memory ordering causing excitement in the real worldSimon Farnsworth2015/07/12 12:02 PM
                                      Example of memory ordering causing excitement in the real worlddmcq2015/07/13 04:03 AM
                                        Example of memory ordering causing excitement in the real worldMichael S2015/07/13 04:19 AM
                                        Example of memory ordering causing excitement in the real worldSimon Farnsworth2015/07/14 12:21 PM
                                          Example of memory ordering causing excitement in the real worlddmcq2015/07/14 01:54 PM
                                            Example of memory ordering causing excitement in the real worldSimon Farnsworth2015/07/15 12:13 PM
                                              Example of memory ordering causing excitement in the real worlddmcq2015/07/15 02:43 PM
                                                Example of memory ordering causing excitement in the real worldSimon Farnsworth2015/07/16 11:41 AM
                                                  Example of memory ordering causing excitement in the real worldWilco2015/07/17 12:57 PM
                                                    Example of memory ordering causing excitement in the real worldLinus Torvalds2015/07/17 03:29 PM
                                                      Example of memory ordering causing excitement in the real worlddmcq2015/07/18 03:07 AM
                                          Example of memory ordering causing excitement in the real worldJouni Osmala2015/07/16 02:52 AM
                                            Example of memory ordering causing excitement in the real worldSimon Farnsworth2015/07/16 12:05 PM
                                              Example of memory ordering causing excitement in the real worldJouni Osmala2015/07/17 12:18 AM
                                                ISA support for bound checkingMichael S2015/07/17 03:51 AM
                                                  ISA support for bound checkingEtienne2015/07/17 05:39 AM
                                                  MPX on SkylakeMark Roulo2015/07/17 07:19 AM
                                                    MPX on SkylakeMichael S2015/07/17 07:44 AM
                                                      MPX on SkylakeMark Roulo2015/07/17 08:03 AM
                                  Example of memory ordering causing excitement in the real worldanon2015/07/12 02:42 AM
                                    Example of memory ordering causing excitement in the real worlddmcq2015/07/12 09:27 AM
                                    Example of memory ordering causing excitement in the real worldLinus Torvalds2015/07/12 10:24 AM
                                      Example of memory ordering causing excitement in the real worldEduardoS2015/07/12 10:53 AM
                                        Example of memory ordering causing excitement in the real worldLinus Torvalds2015/07/12 12:07 PM
                                          Example of memory ordering causing excitement in the real worldanon2015/07/12 06:52 PM
                                            Example of memory ordering causing excitement in the real worldLinus Torvalds2015/07/13 10:05 AM
                                              Example of memory ordering causing excitement in the real worlddmcq2015/07/13 11:20 AM
                                                Example of memory ordering causing excitement in the real worldLinus Torvalds2015/07/13 12:46 PM
                                                  Example of memory ordering causing excitement in the real worldMaynard Handley2015/07/13 01:10 PM
                                                    Example of memory ordering causing excitement in the real worldanon2015/07/13 07:51 PM
                                                    Simple cores justifying weak ordering?Paul A. Clayton2015/07/14 08:01 AM
                                                      David's Paul A. Clayton2015/07/14 08:37 AM
                                                      David's 2015-07-12 10:34 post confirmed some of that (NT)Paul A. Clayton2015/07/14 08:39 AM
                                                    Example of memory ordering causing excitement in the real worldEtienne2015/07/17 12:15 AM
                                                  Example of memory ordering causing excitement in the real worlddmcq2015/07/13 01:14 PM
                                                  Reality is even worseTerry Gray2015/07/13 05:05 PM
                                                    Reality is even worseKonrad Schwarz2015/07/20 11:13 PM
                                                  A simple CAS/MCAS semantics2015/07/15 11:54 AM
                                                    A simple CAS/MCAS semanticsLinus Torvalds2015/07/15 12:55 PM
                                                    A simple CAS/MCAS semanticsEduardoS2015/07/15 01:31 PM
                                                Example of memory ordering causing excitement in the real worldsylt2015/07/13 01:19 PM
                                                  Example of memory ordering causing excitement in the real worlddmcq2015/07/13 02:24 PM
                                                    Example of memory ordering causing excitement in the real worldEduardoS2015/07/13 03:48 PM
                                                      Example of memory ordering causing excitement in the real worldMaynard Handley2015/07/13 06:18 PM
                                                        Example of memory ordering causing excitement in the real worldEduardoS2015/07/13 06:57 PM
                                                      Example of memory ordering causing excitement in the real worldanon2015/07/13 08:01 PM
                                                    What high level languages do you have in mind?Mark Roulo2015/07/13 04:00 PM
                                                      What high level languages do you have in mind?dmcq2015/07/14 05:54 AM
                                                  Example of memory ordering causing excitement in the real worldWilco2015/07/13 03:36 PM
                                                    Example of memory ordering causing excitement in the real worldEduardoS2015/07/13 03:56 PM
                                                      Example of memory ordering causing excitement in the real worldanon2015/07/13 05:43 PM
                                                        Example of memory ordering causing excitement in the real worldEduardoS2015/07/13 07:11 PM
                                                          Example of memory ordering causing excitement in the real worldanon2015/07/13 08:07 PM
                                      Example of memory ordering causing excitement in the real worldMaynard Handley2015/07/12 11:32 AM
                                        Example of memory ordering causing excitement in the real worldEduardoS2015/07/12 12:35 PM
                                        Example of memory ordering causing excitement in the real worldLinus Torvalds2015/07/12 12:38 PM
                                          Example of memory ordering causing excitement in the real worldEduardoS2015/07/12 05:47 PM
                                            Example of memory ordering causing excitement in the real worldNoSpammer2015/07/13 11:34 AM
                                              Example of memory ordering causing excitement in the real worldWilco2015/07/13 03:50 PM
                                                Example of memory ordering causing excitement in the real worldNoSpammer2015/07/14 01:02 PM
                                                  Example of memory ordering causing excitement in the real worldanon2015/07/14 08:37 PM
                                                    Example of memory ordering causing excitement in the real worldMichael S2015/07/15 12:19 AM
                                                      Example of memory ordering causing excitement in the real worldanon2015/07/15 01:26 AM
                                                        Example of memory ordering causing excitement in the real worldNoSpammer2015/07/15 06:05 AM
                                                          Example of memory ordering causing excitement in the real worlddmcq2015/07/15 07:04 AM
                                                            Example of memory ordering causing excitement in the real worldMichael S2015/07/15 07:29 AM
                                                              Example of memory ordering causing excitement in the real worlddmcq2015/07/15 09:09 AM
                                                              Can string ops use microarchitectural state?Paul A. Clayton2015/07/16 08:33 AM
                                                                Can string ops use microarchitectural state?rwessel2015/07/16 12:00 PM
                                                                  Can string ops use microarchitectural state?Paul A. Clayton2015/07/16 05:16 PM
                                                                    Can string ops use microarchitectural state?rwessel2015/07/16 08:34 PM
                                                                      Can string ops use microarchitectural state?Michael S2015/07/17 04:39 AM
                                                                        Can string ops use microarchitectural state?rwessel2015/07/17 08:02 AM
                                                                Consider interrupt followed by task preemptionMichael S2015/07/16 03:31 PM
                                                                  Consider interrupt followed by task preemptionPaul A. Clayton2015/07/16 05:23 PM
                                                            Example of memory ordering causing excitement in the real worldNoSpammer2015/07/15 11:50 AM
                                                              Example of memory ordering causing excitement in the real worldLinus Torvalds2015/07/15 01:27 PM
                                                                Example of memory ordering causing excitement in the real worldNoSpammer2015/07/15 02:14 PM
                                                                  Example of memory ordering causing excitement in the real worldLinus Torvalds2015/07/15 04:42 PM
                                                      Example of memory ordering causing excitement in the real worldNoSpammer2015/07/15 01:44 AM
                                                    Example of memory ordering causing excitement in the real worldLinus Torvalds2015/07/15 01:17 PM
                                                      Example of memory ordering causing excitement in the real worldWilco2015/07/15 01:43 PM
                                                        Example of memory ordering causing excitement in the real worldanon2015/07/15 04:45 PM
                                                        Example of memory ordering causing excitement in the real worldLinus Torvalds2015/07/15 05:35 PM
                                                          Example of memory ordering causing excitement in the real worlddmcq2015/07/16 01:46 AM
                                                            Example of memory ordering causing excitement in the real worlddmcq2015/07/16 01:49 AM
                                        Example of memory ordering causing excitement in the real worldKonrad Schwarz2015/07/21 06:15 AM
                                      Example of memory ordering causing excitement in the real worldanon2015/07/12 06:42 PM
                                        Example of memory ordering causing excitement in the real worldanon2015/07/12 08:48 PM
                                        Example of memory ordering causing excitement in the real worldDavid Kanter2015/07/12 09:34 PM
                                          Example of memory ordering causing excitement in the real worldMichael S2015/07/13 12:51 AM
                                            Example of memory ordering causing excitement in the real worldDavid Kanter2015/07/13 07:31 AM
                                          Example of memory ordering causing excitement in the real worldanon2015/07/13 05:05 AM
                                            Example of memory ordering causing excitement in the real worldGabriele Svelto2015/07/17 04:40 AM
                                          Example of memory ordering causing excitement in the real worlddmcq2015/07/13 07:09 AM
                                        Example of memory ordering causing excitement in the real worldanon2015/07/13 06:46 AM
                                          Example of memory ordering causing excitement in the real worldanon2015/07/14 03:12 AM
                                        Example of memory ordering causing excitement in the real worlddmcq2015/07/15 02:51 AM
                                          Example of memory ordering causing excitement in the real worldanon2015/07/15 03:38 AM
                                            Example of memory ordering causing excitement in the real worlddmcq2015/07/15 05:21 AM
                                      Store queues in POWER/PowerPC processorsGabriele Svelto2015/07/16 12:55 AM
                                        Not sure about that conclusionDavid Kanter2015/07/16 06:34 AM
                                          The POWER4 had the same structureGabriele Svelto2015/07/17 04:20 AM
                                        Committing stores in program orderPaul A. Clayton2015/07/16 12:35 PM
                                          Committing stores in program orderMaynard Handley2015/07/17 11:52 AM
                                      Example of memory ordering causing excitement in the real worldKonrad Schwarz2015/07/20 03:44 AM
                                        Example of memory ordering causing excitement in the real worldanon2015/07/20 06:29 AM
                                          Example of memory ordering causing excitement in the real worldKonrad Schwarz2015/07/20 11:08 PM
                                            Example of memory ordering causing excitement in the real worlddmcq2015/07/21 12:27 AM
                                            Example of memory ordering causing excitement in the real worldMichael S2015/07/21 02:11 AM
                                              Example of memory ordering causing excitement in the real worldKonrad Schwarz2015/07/21 04:24 AM
                                            Example of memory ordering causing excitement in the real worldanon2015/07/21 05:22 AM
                                              Example of memory ordering causing excitement in the real worldKonrad Schwarz2015/07/21 06:36 AM
                                                Example of memory ordering causing excitement in the real worldanon2015/07/21 07:25 AM
                                              Example of memory ordering causing excitement in the real worldanon2015/07/21 07:17 AM
                                                Example of memory ordering causing excitement in the real worldanon2015/07/21 08:50 AM
                                                Example of memory ordering causing excitement in the real worldGabriele Svelto2015/07/21 11:46 AM
                                                Example of memory ordering causing excitement in the real worldKonrad Schwarz2015/07/21 11:35 PM
                                                  Example of memory ordering causing excitement in the real worldKonrad Schwarz2015/07/22 12:37 AM
                                                  Example of memory ordering causing excitement in the real worldanon2015/07/23 02:17 AM
                                                    Example of memory ordering causing excitement in the real worldKonrad Schwarz2015/07/23 03:59 AM
                                                      Example of memory ordering causing excitement in the real worldMichael S2015/07/23 06:31 AM
                                                        Example of memory ordering causing excitement in the real worldKonrad Schwarz2015/07/23 02:38 PM
                                                          Example of memory ordering causing excitement in the real worldKonrad Schwarz2015/07/23 10:44 PM
                                                            Example of memory ordering causing excitement in the real worlddmcq2015/07/24 12:08 AM
                                    Example of memory ordering causing excitement in the real worlddmcq2015/07/13 06:27 AM
                          Example of memory ordering causing excitement in the real worldKonrad Schwarz2015/07/16 04:11 AM
                      Example of memory ordering causing excitement in the real worldAnonymousse2015/07/16 04:07 AM
          Benefit of dynamic hammock predicationsomeotherdude2015/07/06 04:12 PM
            Benefit of dynamic hammock predicationMichael S2015/07/07 12:19 AM
          Benefit of dynamic hammock predicationPatrick Chase2015/07/06 09:20 PM
            SIMD predication is also helpful, of course (NT)Paul A. Clayton2015/07/07 06:45 AM
        Power8 branch conversionGabriele Svelto2015/07/07 01:50 AM
          isel works for storesPaul A. Clayton2015/07/07 06:43 AM
            isel works for storesPatrick Chase2015/07/07 10:08 AM
            isel DOES NOT work for storessomeotherdude2015/07/07 03:01 PM
              isel DOES NOT work for storesWilco2015/07/07 03:40 PM
              Yes it doesPatrick Chase2015/07/07 10:14 PM
                Yes it doesvvid2015/07/08 08:51 AM
            You don't need isel eitherGabriele Svelto2015/07/08 01:01 AM
              You don't need isel eitherPaul A. Clayton2015/07/08 03:46 AM
                You don't need isel eitherGabriele Svelto2015/07/08 04:22 AM
Reply to this Topic
Name:
Email:
Topic:
Body: No Text
How do you spell green?