llvm dev - Jun 2019 - [RFC] Expressing preserved-relations between passes from different modules (was: Re: Linker issue)

Some background: We have an issue with in loop values being correctly
marked uniform but the out of loop uses can be non-uniform. Currently the
out of loop users are not marked as divergent because the in loop value is
uniform inside the loop. We have gotten around this problem for the moment
by applying LCSSA which inserts a PHI in the loop exit for the in loop
uniform value that allows the divergent info to be passed onto isel.

The LCSSA is being inserted into XXXDAGToDAGISel class but this causes a
pass scheduling conflict with StackProtector. So what we've done is try to
preserve StackProtector in LCSSA, the issue is that the linker fails in
Debug build (but not Release build).

StackProtector lies in CodeGen while LCSSA lies in Tranforms/Utils.

Matt had mentioned to me that you can't refer to a preserved pass if
it's
not on the same library and that it might make sense to move the transform
IDs into a separate library. We need a way to mark transforms/analyses as
preserved without depending on linking the transform itself.

Thanks,

Ryan
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Hi all,

re-upping and renaming this thread to get some attention, as we'd like
some feedback on a change that affects the future design direction.

The concrete problem: We would like to mark the LCSSA pass as
preserving the StackProtector pass in order to be able to properly
express all dependencies we have. Unfortunately, those passes are
defined in different modules that do not (and should not) depend on
each other, which causes linking to fail.

Our proposal is to introduce a collection of cross-module pass IDs in
the core module (lib/IR) to work around such issues. This is done by
the RFC patch at https://reviews.llvm.org/D62802.

The only potential alternative I can see would be to generate the IDs
using some kind of template magic and then rely on (runtime) linker
magic to common the generated symbols. That feels like a bit too much
magic for a fundamentally simple problem to me, hence the more
low-tech proposal in the linked Phabricator review.

Please let us know your thoughts!

Thanks,
Nicolai

On Tue, May 14, 2019 at 3:55 PM Ryan Taylor via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
>
> Some background: We have an issue with in loop values being correctly
marked uniform but the out of loop uses can be non-uniform. Currently the out of
loop users are not marked as divergent because the in loop value is uniform
inside the loop. We have gotten around this problem for the moment by applying
LCSSA which inserts a PHI in the loop exit for the in loop uniform value that
allows the divergent info to be passed onto isel.
>
> The LCSSA is being inserted into XXXDAGToDAGISel class but this causes a
pass scheduling conflict with StackProtector. So what we've done is try to
preserve StackProtector in LCSSA, the issue is that the linker fails in Debug
build (but not Release build).
>
> StackProtector lies in CodeGen while LCSSA lies in Tranforms/Utils.
>
> Matt had mentioned to me that you can't refer to a preserved pass if
it's not on the same library and that it might make sense to move the
transform IDs into a separate library. We need a way to mark transforms/analyses
as preserved without depending on linking the transform itself.
>
> Thanks,
>
> Ryan
>
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev


-- 
Lerne, wie die Welt wirklich ist,
aber vergiss niemals, wie sie sein sollte.

Any comments at all on this? Chandler perhaps?

I've since dug a bit further, and it seems like the template-based
solution wouldn't work anyway because DLL loading on Windows can't do
the required commoning. So the general approach taken in
https://reviews.llvm.org/D62802 seems to be the only technically
viable path forward, though it would still be good to get an outside
look at the concrete details.

Thanks,
Nicolai

On Mon, Jun 3, 2019 at 10:35 AM Nicolai Hähnle-Montoro
<nhaehnle at gmail.com> wrote:
>
> Hi all,
>
> re-upping and renaming this thread to get some attention, as we'd like
> some feedback on a change that affects the future design direction.
>
> The concrete problem: We would like to mark the LCSSA pass as
> preserving the StackProtector pass in order to be able to properly
> express all dependencies we have. Unfortunately, those passes are
> defined in different modules that do not (and should not) depend on
> each other, which causes linking to fail.
>
> Our proposal is to introduce a collection of cross-module pass IDs in
> the core module (lib/IR) to work around such issues. This is done by
> the RFC patch at https://reviews.llvm.org/D62802.
>
> The only potential alternative I can see would be to generate the IDs
> using some kind of template magic and then rely on (runtime) linker
> magic to common the generated symbols. That feels like a bit too much
> magic for a fundamentally simple problem to me, hence the more
> low-tech proposal in the linked Phabricator review.
>
> Please let us know your thoughts!
>
> Thanks,
> Nicolai
>
> On Tue, May 14, 2019 at 3:55 PM Ryan Taylor via llvm-dev
> <llvm-dev at lists.llvm.org> wrote:
> >
> > Some background: We have an issue with in loop values being correctly
marked uniform but the out of loop uses can be non-uniform. Currently the out of
loop users are not marked as divergent because the in loop value is uniform
inside the loop. We have gotten around this problem for the moment by applying
LCSSA which inserts a PHI in the loop exit for the in loop uniform value that
allows the divergent info to be passed onto isel.
> >
> > The LCSSA is being inserted into XXXDAGToDAGISel class but this causes
a pass scheduling conflict with StackProtector. So what we've done is try to
preserve StackProtector in LCSSA, the issue is that the linker fails in Debug
build (but not Release build).
> >
> > StackProtector lies in CodeGen while LCSSA lies in Tranforms/Utils.
> >
> > Matt had mentioned to me that you can't refer to a preserved pass
if it's not on the same library and that it might make sense to move the
transform IDs into a separate library. We need a way to mark transforms/analyses
as preserved without depending on linking the transform itself.
> >
> > Thanks,
> >
> > Ryan
> >
> >
> > _______________________________________________
> > LLVM Developers mailing list
> > llvm-dev at lists.llvm.org
> > https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
>
>
> --
> Lerne, wie die Welt wirklich ist,
> aber vergiss niemals, wie sie sein sollte.


-- 
Lerne, wie die Welt wirklich ist,
aber vergiss niemals, wie sie sein sollte.