llvm dev - Feb 2018 - CallSiteSplitting and musttail calls

Hello!

I've discovered that `CallSiteSplitting` optimization doesn't support
musttail calls. The easiest fix as it stands is disabling it for such call
sites: https://reviews.llvm.org/D43729 . However, I'm not happy with such
contribution.

My more sophisticated attempt has failed due to my poor understanding of
llvm internals. Here is the attempted patch:
https://gist.github.com/indutny/240c33522563ebd633613a903479d5e6

I'd greatly appreciate any help with it.

Just in case, there're few questions that I'm trying to find answers
for:

* Why replacing `removeFromParent()` with `eraseFromParent` breaks the code?
* How to properly remove predecessors from the resulting Tail block?
* How to remove the Tail block itself when we're done?

Thank you,
Fedor.
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Update:

I was able to make progress on it today ( See
https://reviews.llvm.org/D43729 ). Apparently my problems were:

* Iterating through the instruction/block list after erasing
block/instruction
* Trying to split block after removing one predecessor

Regarding the latter, it appears that semantics of
`DuplicateInstructionsInSplitBetween` change significantly in such case,
and it starts to loop indefinitely. The `SplitEdge` function that it calls
places all of the instructions into the split block, so that the original
block becomes empty.

Is it expected behavior, or am I doing something wrong?

Thanks,
Fedor.

On Sat, Feb 24, 2018 at 2:16 AM, Fedor Indutny <fedor at indutny.com>
wrote:
> Hello!
>
> I've discovered that `CallSiteSplitting` optimization doesn't
support
> musttail calls. The easiest fix as it stands is disabling it for such call
> sites: https://reviews.llvm.org/D43729 . However, I'm not happy with
such
> contribution.
>
> My more sophisticated attempt has failed due to my poor understanding of
> llvm internals. Here is the attempted patch: https://gist.github.
> com/indutny/240c33522563ebd633613a903479d5e6
>
> I'd greatly appreciate any help with it.
>
> Just in case, there're few questions that I'm trying to find
answers for:
>
> * Why replacing `removeFromParent()` with `eraseFromParent` breaks the
> code?
> * How to properly remove predecessors from the resulting Tail block?
> * How to remove the Tail block itself when we're done?
>
> Thank you,
> Fedor.
>
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I think you realized this now, but to be clear:
More likely, you've found some bugs.
Unfortunately, not all of these utilities have good unit tests (though they
should!).

This would not be the first set of bugs people have found wrt to very
start/end of blocks, or bb == predbb issues.


On Sat, Feb 24, 2018 at 12:58 PM, Fedor Indutny via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
> Update:
>
> I was able to make progress on it today ( See https://reviews.llvm.org/
> D43729 ). Apparently my problems were:
>
> * Iterating through the instruction/block list after erasing
> block/instruction
> * Trying to split block after removing one predecessor
>
> Regarding the latter, it appears that semantics of `
> DuplicateInstructionsInSplitBetween` change significantly in such case,
> and it starts to loop indefinitely. The `SplitEdge` function that it calls
> places all of the instructions into the split block, so that the original
> block becomes empty.
>
> Is it expected behavior, or am I doing something wrong?
>
> Thanks,
> Fedor.
>
> On Sat, Feb 24, 2018 at 2:16 AM, Fedor Indutny <fedor at indutny.com>
wrote:
>
>> Hello!
>>
>> I've discovered that `CallSiteSplitting` optimization doesn't
support
>> musttail calls. The easiest fix as it stands is disabling it for such
call
>> sites: https://reviews.llvm.org/D43729 . However, I'm not happy
with
>> such contribution.
>>
>> My more sophisticated attempt has failed due to my poor understanding
of
>> llvm internals. Here is the attempted patch: https://gist.github.com
>> /indutny/240c33522563ebd633613a903479d5e6
>>
>> I'd greatly appreciate any help with it.
>>
>> Just in case, there're few questions that I'm trying to find
answers for:
>>
>> * Why replacing `removeFromParent()` with `eraseFromParent` breaks the
>> code?
>> * How to properly remove predecessors from the resulting Tail block?
>> * How to remove the Tail block itself when we're done?
>>
>> Thank you,
>> Fedor.
>>
>
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
>
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