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Re: enabling libgcc for 64-bit divisions, was Re: PROBLEM: XFS on ARM corruption 'Structure needs cleaning'

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On 08/12/2015 08:49 AM, Linus Torvalds wrote:
> On Tue, Aug 11, 2015 at 11:24 PM, Christoph Hellwig <hch@infradead.org> wrote:
>>
>> Maybe it's time to rely on gcc to handle 64 bit divisions now?
>
> Ugh. gcc still does a pretty horrible job at it. While gcc knows that
> a widening 32x32->64 multiplication can be simplified, it doesn't do
> the same thing for a 64/32->64 division, and always calls __udivdi3
> for it.
>
> Now, __udivdi3 does avoid the general nasty case by then testing the
> upper 32 bits of the divisor against zero, so it's not entirely
> disastrous. It's just ugly.
>
> But perhaps more importantly, I'm not at all sure libgcc is
> kernel-safe. In particular, I'm not at all sure it *remains*
> kernel-safe. Just as an example: can you guarantee that libgcc doesn't
> implement integer division on some architecture by using the FP
> hardware?
>
> There's been a few cases where not having libgcc saved us headaches. I
> forget the exact details, but it was something like several years ago
> that we had gcc start to generate some insane crap exception handling
> for C code generation, and the fact that we didn't include libgcc was
> what made us catch it because of the resulting link error.
>
> libgcc just isn't reliable in kernel space. I'm not opposed to some
> random architecture using it (arch/tile does include "-lgcc" for
> example), but I _do_ object to the notion that we say "let's use
> libgcc in general".
>
> So no. I do not believe that the occasional pain of a few people who
> do 64-bit divides incorrectly is a good enough argument to start using
> libgcc.
>

Does your objection still apply if we supplied our own implementations 
of a handful of libgcc helpers?

--Andy

>                   Linus
>

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enabling libgcc for 64-bit divisions, was Re: PROBLEM: XFS on ARM  corruption 'Structure needs cleaning' Christoph Hellwig <hch@infradead.org> - 2015-08-12 08:30 +0200
  Re: enabling libgcc for 64-bit divisions, was Re: PROBLEM: XFS on ARM  corruption 'Structure needs cleaning' Linus Torvalds <torvalds@linux-foundation.org> - 2015-08-12 17:50 +0200
    Re: enabling libgcc for 64-bit divisions, was Re: PROBLEM: XFS on ARM  corruption 'Structure needs cleaning' Andy Lutomirski <luto@kernel.org> - 2015-08-13 00:30 +0200
      Re: enabling libgcc for 64-bit divisions, was Re: PROBLEM: XFS on ARM  corruption 'Structure needs cleaning' Andy Lutomirski <luto@amacapital.net> - 2015-08-13 00:40 +0200
      Re: enabling libgcc for 64-bit divisions, was Re: PROBLEM: XFS on ARM  corruption 'Structure needs cleaning' Linus Torvalds <torvalds@linux-foundation.org> - 2015-08-13 00:40 +0200
      Re: enabling libgcc for 64-bit divisions, was Re: PROBLEM: XFS on  ARM corruption 'Structure needs cleaning' Andrew Morton <akpm@linux-foundation.org> - 2015-08-13 05:30 +0200

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