Re: [PATCH] objtool: prefer memory clobber & %= to volatile & __COUNTER__

[Segher Boessenkool]

Hi!

On Mon, Jan 24, 2022 at 03:26:36PM -0800, Nick Desaulniers wrote:
> I noticed in that report and
> https://lore.kernel.org/lkml/202201190702.XNSXrMTK-lkp@xxxxxxxxx/
> that gcc-9 was used.  I wonder if %= has been fixed in gcc-10+? Have
> there been other reports with gcc-10+ for my patch?

None of the %= code (which is trivial) has been changed since 1992.

> If this is fixed in gcc-10, then we can probably add a comment with a
> FIXME link to the issue or commit to replace __COUNTER__ with %= one
> day.  If not, then we can probably come up with a reduced test case
> for the GCC devs to take a look at, then add the FIXME comment to
> kernel sources.

Please open a PR?

> I'm more confident that we can remove the `volatile` keyword (I was
> thinking about adding a new diagnostic to clang to warn that volatile
> is redundate+implied for asm goto or inline asm that doesn't have
> outputs) though that's not the problem here and will probably generate
> some kernel wide cleanup before we could enable such a flag.

Its main value is that it would discourage users from thinking volatile
is magic.  Seriously worth some pain!

> Perhaps
> there are known compiler versions that still require the keyword for
> those cases for some reason.

It was removed from compiler-gcc.h in 3347acc6fcd4 (which changed the
minimum required GCC version to GCC 5).


Segher