[llvm-dev] CallSiteSplitting and musttail calls

Daniel Berlin via llvm-dev
Tue, 27 Feb 2018 16:32:23 -0000

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.
>>
>
>
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