[llvm-dev] Compiling linux with `clang -fbasic-block-sections=labels`

[Akshith Gunasekaran via llvm-dev]

Hi,

Here is a public gist with the repro steps:
https://gist.github.com/akshithg/e5a58fc7012a65e6ec2e94794d66569f
It should have 1) the config i'm using 2) the toolchain version 3) build
steps

Feel free to shoot more questions if you need more details :)


On Tue, Jun 8, 2021 at 7:24 AM Sriraman Tallam <tmsriram at google.com> wrote:

>
>
> On Mon, Jun 7, 2021 at 10:39 PM David Blaikie <dblaikie at gmail.com> wrote:
>
>> BB sections has only been tested/implemented with a fairly narrow set of
>> flags so far as I know, and is sufficiently novel that various features
>> won't interact correctly with it without some work to support the
>> combination of bb sections and those features.
>>
>> I'd /guess/ BPF, for instance, probably doesn't work with BB sections out
>> of the box & may require some work (like some work was needed to support BB
>> sections with DWARF) - I believe BPF is mandatory/used by default when
>> building the Linux kernel? So maybe that's what's being tripped over here.
>> But could something else entirely - that's just my first guess.
>>
>
> Right, we have not tried to build the linux kernel with these flags but it
> is something we would like to test and fix.
>
>
>>
>>
>> On Mon, Jun 7, 2021 at 9:15 PM A G via llvm-dev <llvm-dev at lists.llvm.org>
>> wrote:
>>
>>> Hi,
>>> I'm trying to build the linux kernel with *`clang
>>> -fbasic-block-sections=labels`.*
>>>
>>
> Labels does not actually generate sections and it must be a simpler step
> as it only adds metadata to the binary.
>
>
>>
>>> I ran into this error:
>>>
>>> /tmp/main-81886d.s: Assembler messages:
>>> /tmp/main-81886d.s: Error: .size expression for .BB.set_reset_devices
>>> does not evaluate to a constant
>>> /tmp/main-81886d.s: Error: .size expression for .BB.debug_kernel does
>>> not evaluate to a constant
>>> /tmp/main-81886d.s: Error: .size expression for .BB.quiet_kernel does
>>> not evaluate to a constant
>>> /tmp/main-81886d.s: Error: .size expression for .BB.loglevel does not
>>> evaluate to a constant
>>>
>>
>
> Would you be able to give us a smaller repro for this, we can take a
> look.  Basic block labels uses size expressions when generating metadata
> and it looks like we are not doing this right under some circumstances.
>
>
>
>>
>>> make kernelversion
>>> 5.10.0
>>>
>>> clang version 11.1.0 (g... at github.com:llvm/llvm-project.git
>>> 1fdec59bffc11ae37eb51a1b9869f0696bfd5312)
>>> Target: x86_64-unknown-linux-gnu
>>>
>>>
>>> 1. Can someone help me understand what this error means?
>>> 2. Is *clang-built-linux* a better place to ask this question?
>>> 3. Is there a workaround for this?
>>>
>>> Thanks
>>> Akshith
>>>
>>> _______________________________________________
>>> LLVM Developers mailing list
>>> llvm-dev at lists.llvm.org
>>> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>
>>
>
> --
>
> Sri    Tallam |
>
> Software Engineer |
>
> tmsriram at google.com |
>
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