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

[Sriraman Tallam via llvm-dev]

On Tue, Jun 8, 2021 at 1:16 PM Akshith Gunasekaran <akshith.573 at gmail.com>
wrote:

> 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 :)
>

Thanks, in the interests of simplifying this further,  would it be possible
to give me the preprocessed source and the failing command?


>
>
> 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 |
>>
>

-- 

Sri    Tallam |

Software Engineer |

tmsriram at google.com |
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