[llvm-dev] LLD default page size for arm32

[James Y Knight via llvm-dev]

On Thu, Apr 2, 2020 at 4:19 AM Peter Smith via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Hello,
>
> > Is 4k the right default for lld on Arm32? Does anyone know if there is a
> strong reason to not change this?
> >
>
> At the time the Arm support for LLD was added, 4k page size was the
> overwhelming default for the major platforms that wanted to use LLD
> (largely those migrating from Gold which also uses 4k page size), including
> several that were quite size conscious and had reduced the page-size to 4k
> from the 64k default on AArch64 due to size concerns.
>
> So a reason, whether it counts as strong or not, is that moving to 64k
> page size would mean a lot of users having to switch back to 4k pages.
>

I believe the issues with binary size stemmed from the lack of support for
common-page-size -- which is now implemented -- so this may not be an issue
anymore.


> > Since we have a workaround I am not in a hurry to change this - but if
> nothing else I hope this email will help someone else finding the solution
> to the problem I had.
>
> Thanks for reporting it. I think it will have to be a community decision.
> If the number of people with > 4k page size systems using LLD is very small
> and the number using 4k and care about the size is high, then it is may be
> more disruptive to change. However if there is a significant minority that
> do then it is worth changing to 64k for compatibility.
>
> At the moment, based on the platforms that have adopted LLD so far, I
> think most have used 4k as this is the first problem report citing it. If
> the user base of LLD grows, with more platforms using a greater than 4k
> page size then it makes sense to aim for compatibility and raise the page
> size.
>
> If you've got access would you be able to raise a PR with a title of
> something like Consider increasing LLD Arm default pagesize? We can collect
> opinions there.
>
> Peter
>
> ________________________________________
> From: llvm-dev <llvm-dev-bounces at lists.llvm.org> on behalf of Tobias
> Hieta via llvm-dev <llvm-dev at lists.llvm.org>
> Sent: 01 April 2020 20:33
> To: llvm-dev
> Subject: [llvm-dev] LLD default page size for arm32
>
> Hello,
>
> In the recent days we have been debugging a really thorny issue where
> binaries build with clang and linked with lld was just "Killed" when
> started on a specific armv7 device we ship on.
>
> After quite a bit of head scratching it turns out that the kernel on this
> device ships with a 32k default page size (getconf PAGESIZE) and lld uses
> 4k default page size.
>
> We fixed this by passing -zmax-page-size=0x10000 to lld.
>
> The default page size in GNU ld for arm is 64k so binaries linked with ld
> just worked on this device.
>
> I put the question in the discord lld channel and after a bit back and
> forth I think it's better to discuss it here.
>
> Is 4k the right default for lld on Arm32? Does anyone know if there is a
> strong reason to not change this?
>
> Since we have a workaround I am not in a hurry to change this - but if
> nothing else I hope this email will help someone else finding the solution
> to the problem I had.
>
> Thanks,
> Tobias
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