Re: [PATCH v9 04/10] x86: refcount: prevent gcc distortions

Nadav Amit
2018-10-04T08:56:44Z

at 1:40 AM, hpa@zytor.com wrote:

> On October 4, 2018 1:33:33 AM PDT, Ingo Molnar <mingo@kernel.org> wrote:
>> * Ingo Molnar <mingo@kernel.org> wrote:
>>
>>> I'm also somewhat annoyed at the fact that this series carries a
>> boatload
>>> of reviewed-by's and acked-by's, yet none of those reviewers found it
>>> important to point out the large chasm that is gaping between
>> description
>>> and reality.
>>
>> Another problem I just realized is that we now include
>> arch/x86/kernel/macros.S in every
>> translation pass when building the kernel, right?
>>
>> But arch/x86/kernel/macros.S expands to a pretty large hiearchy of
>> header files:
>>
>> $ make arch/x86/kernel/macros.s
>>
>> $ cat $(grep include arch/x86/kernel/macros.s | cut -d\" -f2 | sort |
>> uniq) | wc -l
>> 4128
>>
>> That's 4,100 extra lines of code to be preprocessed for every
>> translation unit, of
>> which there are tens of thousands. More if other pieces of code get
>> macrofied in
>> this fasion in the future.
>>
>> If we assume that a typical distribution kernel build has ~20,000
>> translation units
>> then this change adds 82,560,000 more lines to be preprocessed, just to
>> work around
>> a stupid GCC bug?
>>
>> I'm totally unhappy about that. Can we do this without adding macros.S?
>>
>> It's also a pretty stupidly central file anyway that moves source code
>> away
>> from where it's used.
>>
>> Thanks,
>>
>> Ingo
>
> It's not just for working around a stupid GCC bug, but it also has a huge
> potential for cleaning up the inline asm in general.
>
> I would like to know if there is an actual number for the build overhead
> (an actual benchmark); I have asked for that once already.

I can run some tests. (@hpa: I thought you asked about the -pipe overhead;
perhaps I misunderstood).

I guess you regard to the preprocessing of the assembler. Note that the C
preprocessing of macros.S obviously happens only once. That’s the reason
I assumed it’s not that expensive.

Anyhow, I remember that we discussed at some point doing something like
‘asm(“.include XXX.s”)’ and somebody said it is not good, but I don’t
remember why and don’t see any reason it is so. Unless I am missing
something, I think it is possible to take each individual header and
preprocess the assembly part of into a separate .s file. Then we can put in
the C part of the header ‘asm(".include XXX.s”)’.

What do you think?