[llvm-dev] Difficulty in Porting cloneLoop in LoopUnswitch

[Arthur Eubanks via llvm-dev]

The new PM's equivalent of LoopUnswitch is the (not super well named)
SimpleLoopUnswitch. Is there something specific that SimpleLoopUnswitch
doesn't handle?

On Wed, Apr 7, 2021 at 11:36 PM raghesh via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Hi All,
>
> Though the LoopUnswitch pass was not ported to the new pass manager by
> community,  we internally at AMD are in need to do that. While doing this
> we are facing issues to have a counterpart for the cloneLoop function, the
> body of which is given below.
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> *Loop *llvm::cloneLoop(Loop *L, Loop *PL, ValueToValueMapTy &VM,
>             LoopInfo *LI, LPPassManager *LPM) {  Loop &New =
> *LI->AllocateLoop();  if (PL)    PL->addChildLoop(&New);  else
> LI->addTopLevelLoop(&New);  if (LPM)    LPM->addLoop(New);  // Add all of
> the blocks in L to the new loop.  for (Loop::block_iterator I =
> L->block_begin(), E = L->block_end();       I != E; ++I)    if
> (LI->getLoopFor(*I) == L)
> New.addBasicBlockToLoop(cast<BasicBlock>(VM[*I]), *LI);  // Add all of the
> subloops to the new loop.  for (Loop *I : *L)    cloneLoop(I, &New, VM, LI,
> LPM);  return &New;}*
>
> In the new pass manager flow, we are trying to use
> LPMUpdater's addSiblingLoops in place of addLoop, which will be invoked
> only for the top level loop (loop to be unswitched). According to the
> comments given in AddSiblingLoops the loops nested inside will be traversed
> by the pass manager anyway. In that assumption we are not adding the
> subloops. With this we are facing assertion failures further.
>
> We also tried capturing all the newly created loops and adding it towards
> the end of the LoopUnswitch pass, as done in the SimpleLoopUnswitch pass.
> That also did not help.
>
> Another approach we tried is not to add the cloned loops at all to the
> pass manager.. This worked  but not sure about its implications. We might
> miss further optimization opportunities on these loops.
>
> It would be a great help if someone could point out the right way to go
> about it.
>
> Regards,
> Raghesh
> ------------------------------
> Raghesh Aloor
> AMD India Pvt. Ltd.
> Bengaluru.
> ------------------------------
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