[PATCH] D26037: Add LoopSink pass for PGO.

[Xinliang David Li via llvm-commits]

On Fri, Oct 28, 2016 at 10:20 AM, Hal Finkel <hfinkel at anl.gov> wrote:

>
> ------------------------------
>
> *From: *"Xinliang David Li" <davidxl at google.com>
> *To: *reviews+D26037+public+89d2f1349c56fe36 at reviews.llvm.org
> *Cc: *"陈德颢" <danielcdh at gmail.com>, "Hal Finkel" <hfinkel at anl.gov>,
> "Junbum Lim" <junbuml at codeaurora.org>, "llvm-commits" <
> llvm-commits at lists.llvm.org>, anna at azul.com
> *Sent: *Friday, October 28, 2016 12:11:04 PM
> *Subject: *Re: [PATCH] D26037: Add LoopSink pass for PGO.
>
>
>
> On Fri, Oct 28, 2016 at 9:40 AM, Dehao Chen <danielcdh at gmail.com> wrote:
>
>> danielcdh added a comment.
>>
>> We need this pass to remove LCSSA nodes because this will be the last
>> loop pass.
>>
>> Another question is: shall we pin this pass to PGO only? If not, we can
>> simply add it to lib/Transforms/IPO/PassManagerBuilder.cpp
>>
>
> Static branch prediction is conservative in predicting loop iterations
> (loop scale). This means it is likely some blocks (if wrapped under
> __builtin_expect) inside the loop will be wrongly marked as colder than
> preheader  which can trigger loop sinking wrongly.
>
> Can you please elaborate? If we statically predict a modest number of
> iterations, which seems like a reasonable trade off, and we have a block
> which is expected to not execute (because of __builtin_expect), then is it
> obviously wrong that it should be colder than the preheader? If the
> opposite of __builtin_expect is "almost never",
>


My observation is that the interpretation and use  of '__builtin_expect'
varies a lot from user to user. Some consider 80% or even as low as ~70%
probability as expected, while some considers >99%. Even with 99% expected
probability, it still can get wrong. For instance in a loop with 1 million
iterations, the cold block will execute 10k times -- still much hotter than
the preheader.

David



> which is true for error paths and the like, then that seems not
> unreasonable. Why would loop sinking hurt there?
>
>




> Thanks again,
> Hal
>
>   Making this pass depending on PGO is a safer thing to do.
>
> David
>
>
>>
>> https://reviews.llvm.org/D26037
>>
>>
>>
>>
>
>
>
> --
> Hal Finkel
> Lead, Compiler Technology and Programming Languages
> Leadership Computing Facility
> Argonne National Laboratory
>
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