ICL memory renaming?

By: Travis Downs (travis.downs.delete@this.gmail.com), August 4, 2019 2:55 pm
Room: Moderated Discussions
anonlitmus (anon.delete@this.litmus.org) on August 3, 2019 7:06 am wrote:
> 10-20 cycles seems too much for something that is a clear-cut Store-to-Load forwarding
> case. In steady state it should be more or less similar to L1D hit, shouldn't it (see
> some oldish data at http://blog.stuffedcow.net/2014/01/x86-memory-disambiguation/)?
>
> Also, any idea why the 16-bit version would be slower than the 8/32/64-bit version?
> I am guessing that it is the partial register semantics that makes it not memory
> rename-able but then the 8-bit case should have the same issue, right?

Browse through static assembly or a dynamic trace for a typical program and you'll probably find that 16-bit instructions are considerably less common than other sizes: 32-bit and 64-bit are obviously common and 8-bit is still very common too, due to the prevalence of char/byte use the setcc instructions operating on bytes, and because some types of larger width code ends up using byte operations.

If you scan the instruction latency you'll find a few cases where 16-bit operands are disadvantaged: an extra uop for most multiplies, movsx, lea and a few others. Other cases don't take any extra uop but end up much slower (at least in a tight loop) because of length changing prefixes which stall the front end.

All that to say that 16-bit operands are the least importan size, by far, and perhaps Intel was able to simply the implementation or dedicate more resources to the other sizes by leaving them out. Or perhaps the 16-bit code just didn't trigger the optimization for some reason.
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TopicPosted ByDate
ICL memory renaming?Travis Downs2019/08/02 06:43 PM
  ICL memory renaming?anonlitmus2019/08/03 07:06 AM
    ICL memory renaming?Travis Downs2019/08/04 02:55 PM
      ICL memory renaming?anonlitmus2019/08/04 03:20 PM
        ICL memory renaming?Travis Downs2019/08/04 04:51 PM
  ICL memory renaming?ll2019/08/03 08:37 PM
    ICL memory renaming?Montaray Jack2019/08/03 11:53 PM
      ICL memory renaming?Montaray Jack2019/08/04 12:44 AM
    ICL memory renaming?Travis Downs2019/08/04 03:00 PM
      ICL memory renaming?ll2019/08/05 06:05 AM
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