Re: [PATCH v2 02/13] qemu/atomic: Drop special case for unsupported compiler

[Daniel P . Berrangé]

On Thu, Dec 10, 2020 at 01:42:51PM +0000, Daniel P. Berrangé wrote:
> On Thu, Dec 10, 2020 at 05:17:05PM +0400, Marc-André Lureau wrote:
> > Hi
> > 
> > On Thu, Nov 26, 2020 at 4:23 PM Peter Maydell <peter.maydell@linaro.org>
> > wrote:
> > 
> > > On Thu, 26 Nov 2020 at 12:06, Daniel P. Berrangé <berrange@redhat.com>
> > > wrote:
> > > >
> > > > On Thu, Nov 26, 2020 at 11:49:28AM +0000, Peter Maydell wrote:
> > > > > On Thu, 26 Nov 2020 at 11:29, <marcandre.lureau@redhat.com> wrote:
> > > > > >
> > > > > > From: Philippe Mathieu-Daudé <philmd@redhat.com>
> > > > > >
> > > > > > Since commit efc6c070aca ("configure: Add a test for the
> > > > > > minimum compiler version") the minimum compiler version
> > > > > > required for GCC is 4.8, which has the GCC BZ#36793 bug fixed.
> > > > > >
> > > > > > We can safely remove the special case introduced in commit
> > > > > > a281ebc11a6 ("virtio: add missing mb() on notification").
> > > > > >
> > > > > > With clang 3.8 (xenial amd64) __ATOMIC_RELAXED is defined, so the
> > > chunk
> > > > > > to remove (which is x86-specific), isn't reached.
> > > > >
> > > > > The minimum clang version enforced by configure is 3.4, not 3.8.
> > > > > (Or Apple XCode clang 5.1 -- they use a different versioning scheme!)
> > > >
> > > > We picked clang 3.4 based on fact that is what ships in EPEL7, and
> > > > Debian Jessie 3.5.  We then picked the XCode version to match.
> > > >
> > > > Based on our platform support matrix we no longer support Debian
> > > > Jessie, and IMHO we also don't really need to consider 3rd party
> > > > add-on repos shipping non-default toolschains. So IMHO we could
> > > > entirely ignore clang in EPEL7 when picking min versions.
> > > >
> > > > IOW, we are likely justified in picking a new clang version if
> > > > someone wants to research what is a suitable min version across
> > > > our intended supported distros.
> > >
> > > Sure, but if we do that then the series should start with the
> > > "bump the minimum clang version" patch with accompanying
> > > justification.
> > >
> > 
> > 
> > With clang-3.4.2-9.el7.x86_64 (epel7), __ATOMIC_RELAXED is defined. I'll
> > update the commit message.
> > 
> > Some research on google suggests that it might be true also with XCode
> > clang 5.1, I could use some help to verify that:
> > clang -dM -E - < /dev/null | grep __ATOMIC_RELAXED
> 
> The oldest xcode that travis has support for is 7.3. I'd really just
> suggest bumping min clang to something more modern, than trying to
> find xcode 5.1

Actually, since you've validated main clang 3.4 code, you can assume
this applies equally to xcode 5.1, as that is using the clang 3.4 codebase.

Basically we can assume features match across the regular and xcode
clangs, with the mapped versions. IOW changing either is sufficient,
no need to check both IMHO.

  https://en.wikipedia.org/wiki/Xcode#Xcode_5.0_-_6.x_(since_arm64_support)_2

Regards,
Daniel
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