[LLVMdev] How to unroll loops in opposite loop nest order

[Martin Apel]

Hi Andy,

sorry, my mail wasn't precise enough: I am trying to do this in a
plugin, i.e. in a LLVM client. I try to do this without modifying
LLVM itself. In the meantime I have found a way to do it the way I need
it. I only consider top-level loops in my LoopPass
and add the child loops again using LPPassManager::insertLoopIntoQueue.
This seems to have the desired effect.

Martin

On 05/06/12 20:11, Andrew Trick wrote:
>
> On Jun 5, 2012, at 3:20 AM, Martin Apel <martin.apel at SIMPACK.de
> <mailto:martin.apel at SIMPACK.de>> wrote:
>
>> I am trying to implement loop unrolling in a context, where lots of
>> constant propagation has taken place.
>> Unrolling an outer loop might make an inner loop have constant
>> bounds, therefore I want to process the loops
>> outside in, i.e. from parent loops to nested loops.
>> Unfortunately the standard loop pass manager performs loop passes
>> inside out, i.e. from nested loops to parent loops,
>> thereby missing optimization opportunities in my case.
>>
>> I haven't found a way how to implement this on my own. I tried
>> implementing a function pass, where I could process the
>> loops outside in. Unfortunately I cannot use the UnrollLoop utility
>> function in this case, which crashes, when called with a
>> NULL LPPassManager object, despite opposite documentation.
>>
>> Is there any way how to process loops outside in still using the
>> UnrollLoop function?
>
> I noticed another case of referencing LPM without a null check and
> fixed it here r158007.
>
> You'll should be careful to check everything that's currently disabled
> when LPM is missing: domtree update, ScalarEvolution update,
> SimplifyIndvar. If you need any of these things I suggest changing the
> interface.
>
> Maybe it would work to pass in FunctionPass instead of LPM. In the one
> place we actually need an LPM (deleteLoopFromQueue), we could check
> FunctionPass's type and cast.
> Maybe getAsPMDataManager()->getPassManagerType() == PMT_LoopPassManager?
>
> -Andy

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