Re: [Qemu-devel] [PATCH] Drop QEMU_GNUC_PREREQ() checks for gcc older than 4.1

["Daniel P. Berrange" <berrange@redhat.com>]

On Tue, Jan 31, 2017 at 06:00:13PM +0000, Peter Maydell wrote:
> On 31 January 2017 at 17:40, Markus Armbruster <armbru@redhat.com> wrote:
> > Peter Maydell <peter.maydell@linaro.org> writes:
> >
> >> We already require gcc 4.1 or newer (for the atomic
> >> support), so the fallback codepaths for older gcc
> >> versions than that are now dead code and we can
> >> just delete them.
> >>
> >> NB: clang reports itself as gcc 4.2 (regardless of
> >> clang version), so clang won't be using the fallbacks
> >> either.
> >>
> >> Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
> >> ---
> >> For compatibility with clang we should probably try to avoid
> >> using QEMU_GNUC_PREREQ() and instead have something in
> >> compiler.h that abstracts away whether the test for "does
> >> the compiler support feature foo" is via a GCC version
> >> check or a clang __has_feature or whatever.
> >
> > Yes, testing for feature is better than testing a version.
> >
> > This patch reduces use of QEMU_GNUC_PREREQ roughly by half.  Good.
> >
> >>
> >>
> >>  include/qemu/compiler.h   |   8 ---
> >>  include/qemu/host-utils.h | 121 ----------------------------------------------
> >>  tcg/arm/tcg-target.h      |   7 ---
> >>  3 files changed, 136 deletions(-)
> >>
> >> diff --git a/include/qemu/compiler.h b/include/qemu/compiler.h
> >> index 157698b..fc12e49 100644
> >> --- a/include/qemu/compiler.h
> >> +++ b/include/qemu/compiler.h
> >> @@ -24,17 +24,9 @@
> >>
> >>  #define QEMU_NORETURN __attribute__ ((__noreturn__))
> >>
> >> -#if QEMU_GNUC_PREREQ(3, 4)
> >>  #define QEMU_WARN_UNUSED_RESULT __attribute__((warn_unused_result))
> >> -#else
> >> -#define QEMU_WARN_UNUSED_RESULT
> >> -#endif
> >
> > Should we inline this macro?
> 
> We have attributes which we wrap in QEMU_ macros already
> even though they always expand to the same thing:
> QEMU_NORETURN and QEMU_ALIGNED. I'm happy to leave these
> to follow that pattern. (If you wanted to send a patch
> series that uninlined all of those then I wouldn't hugely
> object to it, but I think it touches enough files that it's
> a separate thing from removing the #if guards that this
> patch does.)

The other option is just to replace QEMU_WARN_UNUSED_RESULT with

  #define QEMU_WARN_UNUSED_RESULT  G_GNUC_WARN_UNUSED_RESULT

and convert code to use G_GNUC_WARN_UNUSED_RESULT directly until we
can kill the QEMU specific define. There's no benefit to QEMU having
its own defines that duplicate stuff already covered by our min
required glib - G_GNUC_WARN_UNUSED_RESULT was added in 2.10 for
example.

Regards,
Daniel
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