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

[Roman Lebedev via llvm-dev]

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

> 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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