[llvm-dev] Loop splitting as a special case of unswitch

[Roman Lebedev via llvm-dev]

On Fri, Feb 23, 2018 at 1:31 AM,  <junbuml at codeaurora.org> wrote:
> On 2018-02-22 16:21, Roman Lebedev wrote:
>>
>> On Fri, Feb 23, 2018 at 12:15 AM, Jun Lim via llvm-dev
>> <llvm-dev at lists.llvm.org> wrote:
>>>
>>> For the example code below,
>>>   int L = M + 10;
>>>   for (k = 1 ; k <=L; k++) {
>>>     dummy();
>>>     if (k < M)
>>>       dummy2();
>>>   }
>>> , we can split the loop into two parts like :
>>>
>>>   for (k = 1 ; k != M; k++) {
>>>     dummy();
>>>     dummy2();
>>>   }
>>>   for (; k <=L; k++) {
>>>     dummy();
>>>   }
>>
>> I believe i have reported a similar case as
>> https://bugs.llvm.org/show_bug.cgi?id=34364
>
>
>
> What I meant here is little different from the loop peeling mentioned in
> PR34364. While loop splitting remove the conditional block in the loop by
> breaking the loop into two parts, loop peeling take the first or last few
> iterations out of the loop. I think the loop splitting can cover more
> dynamic cases where the iteration size of each portion is unknown.
Yes, absolutely, i'm not arguing against that at all.

Moreover, the bug i have reported is not currently being done by clang,
https://reviews.llvm.org/D31613#910343, so if the loop splitting would
be done by clang, (and would resolve that bugreport), i'd be very happy :)

>>
>>> By splitting the loop, we can remove the conditional block in the loop
>>> and indirectly increase vectorization opportunities. If we know that a loop
>>> has a conditional branch that is always true for one part of the iteration
>>> space and false for the other, then we can split such loop into two parts.
>>> As far as I check, GCC seems to handle this already (-fsplit-loops). For me,
>>> it seem possible to handle this as a special case of the loop-unswitch. Does
>>> anyone have any opinion about this transformation?
>>>
>>> Thanks,
>>> Jun
>>
>> Roman.

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>
>
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