[llvm-dev] LLD default page size for arm32

[Fangrui Song via llvm-dev]

On 2020-04-02, Tobias Hieta via llvm-dev wrote:
>Thanks for the reply Peter,
>
>I have uploaded a diff to phab: https://reviews.llvm.org/D77330
>
>-- Tobias

I asked Android, ChromeOS and FreeBSD arm folks for comments.

There should be no size impact for non-linker-script cases
after https://reviews.llvm.org/D66749 "[ELF][ARM] Allow PT_LOAD to have overlapping p_offset ranges on EM_ARM".

>On Thu, Apr 2, 2020 at 10:18 AM Peter Smith <Peter.Smith at arm.com> 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.
>>
>> > 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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