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

[Philip Reames via llvm-dev]

Take a look at the InductiveRangeCheckElimination pass in tree.  It's 
not on by default, but if you're wiling to use a custom pass pipeline 
you can get this case.

Philip

On 02/22/2018 01:15 PM, Jun Lim via llvm-dev 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();
>    }
>
> 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
>
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