[PATCH] D17625: Do not select EhPad BB in MachineBlockPlacement when there is regular BB to schedule

[Xinliang David Li via llvm-commits]

On Wed, Mar 2, 2016 at 10:50 PM, Chandler Carruth <chandlerc at gmail.com>
wrote:

> chandlerc added inline comments.
>
> ================
> Comment at: lib/CodeGen/MachineBlockPlacement.cpp:545
> @@ -543,1 +544,3 @@
>
> +    // We prefer non ehpad over ehpads.
> +    bool IsCandidateEHPad = MBB->isEHPad();
> ----------------
> davidxl wrote:
> > The new code really lowers the readability. Can you describe the
> algorithm here? Is it
> > 1) if the best non eh block exists, return it; otherwise
> > 2) return the least likely eh pad?
> >
> > if that is the case, why not simply skip eh blocks in the original
> worklist loop. If the best bb is found return it, otherwise have another
> helper loop to find the least likely eh pad?
> I agree that this makes it essentially unreadable. =/
>
> I'd suggest instead to maintain two worklists, one of non-EH-pads, and
> another of EH-pads. Then you can write the direct loop over each, and you
> can even put the algorithm David suggested in a comment on the second loop.
> =]
>
> Note that looping over the worklist twice could be quite expensive, which
> is why I'd suggest two lists.
>

I assume that in majority of the cases, the first loop can find a
candidate, thus the second loop should be rarely executed.

David


>
>
> http://reviews.llvm.org/D17625
>
>
>
>
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