[PATCH] D74156: [llvm-exegesis] Exploring X86::OperandType::OPERAND_COND_CODE

[Guillaume Chatelet via Phabricator via llvm-commits]

gchatelet added a comment.

In D74156#1893156 <https://reviews.llvm.org/D74156#1893156>, @lebedev.ri wrote:

> Thank you for replying!


Sure, my apologies for the lag. I'm extra busy right now.

> 
> 
> In D74156#1893104 <https://reviews.llvm.org/D74156#1893104>, @gchatelet wrote:
> 
>> In D74156#1873657 <https://reviews.llvm.org/D74156#1873657>, @lebedev.ri wrote:
>>
>> > In D74156#1869502 <https://reviews.llvm.org/D74156#1869502>, @gchatelet wrote:
>> >
>> > > In D74156#1864292 <https://reviews.llvm.org/D74156#1864292>, @lebedev.ri wrote:
>> > >
>> > > > In D74156#1864096 <https://reviews.llvm.org/D74156#1864096>, @gchatelet wrote:
>> > > >
>> > > > > - repetition mode (to see the impact of the decoder)
>> > > >
>> > > >
>> > > > This //appears// to be currently handled via `-repetition-mode` switch (D68125 <https://reviews.llvm.org/D68125>).
>> > >
>> > >
>> > > yes
>> > >
>> > > > Does this really have to be accounted for (a dimension in) in the greedy approach?
>> > >
>> > > @courbet and I believe that having two measures are better than one and can demonstrate the impact of decoding the instruction.
>> >
>> >
>> > Oh, now i see what you mean. At least for `CMOV`, i'm seeing wildly different results
>> >
>> > |           | Latency | RThroughput |
>> > | duplicate | 1       | 0.8         |
>> > | loop      | 2       | 0.6         |
>> > |
>> >
>> > where latency=1 seems correct, and i'd expect the througput to be close to 1/2 (since there are two execution units).
>> >
>> > So i would personally guess that `--repetition-mode=` shouldn't even be an another measurement,
>> >  but instead much like what is already done by running the whole snippet a few times
>> >  and denoising the result (not averaging!), the same should be done with this - run both, take minimum
>> >  https://github.com/llvm/llvm-project/blob/c55cf4afa9161bb4413b7ca9933d553327f5f069/llvm/tools/llvm-exegesis/lib/LatencyBenchmarkRunner.cpp#L38-L39
>> >
>> > What are you thoughts on this?
>>
>>
>> Well that depends on how you see `llvm-exegesis`. It was originally designed to understand the behavior of the CPU.
>>  To me `--repetition-mode` is interesting to explore sensitivity to instruction decoding.
>>  If you mix everything and take the minimum then you lose information on the root cause and then you deduce that the minimum latency is X but it only happens if the instruction is already decoded and run in a tight loop.
>>  I think it's important to keep this information separate.
> 
> 
> I do agree in principle.
> 
>> You can always take the minimum between the two during analysis though.
> 
> I think this has the same-ish basic issue as merging results from different `--mode=`s - what if there is more than one result?
>  Currently, we form `PerInstructionStats`, which accumulates min/max/averages, which is nice for when results end up being noisy.
>  How would we aggregate (via taking `min`) results from different `--repetition-mode=`s, without sacrificing that?
>  Pick all `--repetition-mode=duplicate` and all `--repetition-mode=loop` and zipper-merge each pair?

Right now you can simply run the benchmark twice (with `loop` or `duplicate` repetition mode), concatenate the two outputs and run them through the analysis.
We currently don't consider the flag as part of the instruction so it would be reflected into the `PerInstructionStats`, increasing the confidence interval if there's a discrepancy between the two.


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