[llvm-dev] SimplifyCFG, llvm.loop and latch blocks

[Hiroshi Yamauchi via llvm-dev]

Yeah, certainly not ideal. Maybe worth filing a bug regardless?

On Thu, Apr 8, 2021 at 9:55 AM Jeroen Dobbelaere <
Jeroen.Dobbelaere at synopsys.com> wrote:

> Hi Hiroshi,
>
> thank you for the reply.
>
> I am aware of the general rule that Metadata can be dropped.
> I am wondering if that should still hold for (at least) some of the loop
> annotations.


> In this particular case, there is also a 'real bug' where loop metadata
> indicating that a loop
> that is 'progressing but should not be unrolled' is, after some
> transformations, suddenly attached to
> a different, infinite loop (aka not progressing).
>
> Greetings,
>
> Jeroen Dobbelaere
>
> > From: Hiroshi Yamauchi
> >
> > I think that metadata / annotation preservation isn't perfect and is on
> a best-effort basis in general,
> > unfortunately, as the IR spec may say that >it's okay to drop them. But
> contributions would be welcome to improve it.
> >
> >> On Wed, Apr 7, 2021 at 10:17 AM Jeroen Dobbelaere via llvm-dev <mailto:
> llvm-dev at lists.llvm.org> wrote:
> >> Hi all,
> >>
> >> https://llvm.org/docs/LangRef.html#llvm-loop  states that a llvm.loop
> is put on the 'branch instruction of the loop's latch block'.
> >>
> >> With 'SimplifyCFG', I have come in a situation where a !llvm.loop is
> associated to
> >> a basicblock that is found to be a latch block for two different loops.
> >>
> >> The original annotation of one of those loops disappeared in the
> process.
> >> (See  https://www.godbolt.org/z/5r1T5e9fs and look for the
> disappearing !llvm.loop !10  in BB 'do.cond')
> >>
> >> Then later on,  'Canonicalize natural loops' splits the latch block up,
> so that it now is only
> >> the latch block for a single loop, but the original annotation is moved
> to the wrong loop.
> >>
> >> Questions:
> >> - The branch in 'do.cond' is simplified, but I have the impression that
> at the same time, the loop annotation is omitted.
> >>   This sounds like a bug ?
> >> - Is it a correct assumption that we should not merge blocks if both
> have a branch instruction with a !llvm.loop annotation set ?
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20210409/37a04149/attachment.html>