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SubjectRe: [PATCH] arm64: fix unreachable code issue with cmpxchg
On Tue, Sep 10, 2019 at 9:46 AM Will Deacon <will@kernel.org> wrote:
>
> On Mon, Sep 09, 2019 at 10:21:35PM +0200, Arnd Bergmann wrote:
> > On arm64 build with clang, sometimes the __cmpxchg_mb is not inlined
> > when CONFIG_OPTIMIZE_INLINING is set.
>
> Hmm. Given that CONFIG_OPTIMIZE_INLINING has also been shown to break
> assignment of local 'register' variables on GCC, perhaps we should just
> disable that option for arm64 (at least) since we don't have any toolchains
> that seem to like it very much! I'd certainly prefer that over playing
> whack-a-mole with __always_inline.

Right, but I can also see good reasons to keep going:

- In theory, CONFIG_OPTIMIZE_INLINING is the right thing to do -- the compilers
also make some particularly bad decisions around inlining when each inline
turns into an __always_inline, as has been the case in Linux for a long time.
I think in most cases, we get better object code with CONFIG_OPTIMIZE_INLINING
and in the cases where this is worse, it may be better to fix the compiler.
The new "asm_inline" macro should also help with that.

- The x86 folks have apparently whacked most of the moles already, see this
commit from 2008

commit 3f9b5cc018566ad9562df0648395649aebdbc5e0
Author: Ingo Molnar <mingo@elte.hu>
Date: Fri Jul 18 16:30:05 2008 +0200

x86: re-enable OPTIMIZE_INLINING

re-enable OPTIMIZE_INLINING more widely. Jeff Dike fixed the remaining
outstanding issue in this commit:

| commit 4f81c5350b44bcc501ab6f8a089b16d064b4d2f6
| Author: Jeff Dike <jdike@addtoit.com>
| Date: Mon Jul 7 13:36:56 2008 -0400
|
| [UML] fix gcc ICEs and unresolved externs
[...]
| This patch reintroduces unit-at-a-time for gcc >= 4.0,
bringing back the
| possibility of Uli's crash. If that happens, we'll debug it.

it's still default-off and thus opt-in.

- The inlining decisions of gcc and clang are already very different, and
the bugs we are finding around that are much more common than
the difference between CONFIG_OPTIMIZE_INLINING=y/n on a
given compiler.

Arnd

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