Re: [PATCH] x86: work around clang IAS bug referencing __force_order

[Arnd Bergmann]

On Sat, Aug 22, 2020 at 12:26 PM Segher Boessenkool
<segher@xxxxxxxxxxxxxxxxxxx> wrote:
> [ There is GCC 4.9.4, no one should use an older 4.9. ]
>
> I mentioned 5 for a reason: the whole function this patch is to did not
> exist before then!  That does not mean the bug existed or did not exist
> before GCC 5, but it does for example mean that a backport to 4.9 or
> older isn't trivial at all.
>
> > I am asking myself who is using such ancient compilers?
>
> Some distros have a GCC 4.8 as system compiler.  We allow building GCC
> itself with a compiler that far back, for various reasons as well (and
> this is very sharp already, the last mainline GCC 4.8 release is from
> June 2015, not all that long ago at all).
>
> But, one reason this works is because people actually test it.  Does
> anyone actually test the kernel with old compilers?  It isn't hard to
> build a new compiler (because we make sure building a newer compiler
> works with older compilers, etc. :-) ), and as you say, most distros
> have newer compilers available nowadays.

We only recently changed the minimum from 4.6 to 4.8, and
subsequently to 4.9. Most people have fairly recent compilers,
but there are a number of notable kernel developers that
intentionally stick to old versions because of compile speed.

Each major compiler release adds about 4% overhead in total time
to compile a kernel, so between gcc-4.6 and gcc-11 you add over
50% in build time.

      Arnd