Benefit of dynamic hammock predication

By: Wilco (Wilco.Dijkstra.delete@this.ntlworld.com), July 6, 2015 4:35 pm
Room: Moderated Discussions
Linus Torvalds (torvalds.delete@this.linux-foundation.org) on July 6, 2015 3:59 pm wrote:
> x86 doesn't have those insane memory ordering semantics. Loads are done in order (as far as software
> could tell - they do get re-ordered, but the semantics are guaranteed to be the same as if they were done
> in order), so it doesn't matter if the two accessed had a data or control dependency between them.

Wrong - of course it does matter! If there is just a control dependency then the compiler may reorder the loads, lift them, CSE them etc (it can't if there is a data dependency). So if you didn't tell the compiler about the dependency, the code would run incorrectly on any CPU irrespectively of what memory model it supports.

Nobody writes multithreading code in pure assembler - if you did then you might have had a point.

Wilco

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                                                Example of memory ordering causing excitement in the real worldanon2015/07/21 08:25 AM
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                                                            Example of memory ordering causing excitement in the real worlddmcq2015/07/24 01:08 AM
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          Benefit of dynamic hammock predicationsomeotherdude2015/07/06 05:12 PM
            Benefit of dynamic hammock predicationMichael S2015/07/07 01:19 AM
          Benefit of dynamic hammock predicationPatrick Chase2015/07/06 10:20 PM
            SIMD predication is also helpful, of course (NT)Paul A. Clayton2015/07/07 07:45 AM
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          isel works for storesPaul A. Clayton2015/07/07 07:43 AM
            isel works for storesPatrick Chase2015/07/07 11:08 AM
            isel DOES NOT work for storessomeotherdude2015/07/07 04:01 PM
              isel DOES NOT work for storesWilco2015/07/07 04:40 PM
              Yes it doesPatrick Chase2015/07/07 11:14 PM
                Yes it doesvvid2015/07/08 09:51 AM
            You don't need isel eitherGabriele Svelto2015/07/08 02:01 AM
              You don't need isel eitherPaul A. Clayton2015/07/08 04:46 AM
                You don't need isel eitherGabriele Svelto2015/07/08 05:22 AM
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