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Re: [Qemu-devel] [PATCH] QEMU_BUILD_BUG_ON: use __COUNTER__

From: Eric Blake
Subject: Re: [Qemu-devel] [PATCH] QEMU_BUILD_BUG_ON: use __COUNTER__
Date: Tue, 31 Jan 2017 09:03:10 -0600
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.6.0

On 01/31/2017 08:43 AM, Michael S. Tsirkin wrote:
> Some headers use QEMU_BUILD_BUG_ON. This causes a problem
> if the C file including that header happens to have
> QEMU_BUILD_BUG_ON at the same line number.
> Fix using a widely available extension: __COUNTER__.

gcc 4.3 and later; clang documents it but doesn't say when it was added.

> If unavailable, provide a stub.

What fails in our build farm if we don't provide the stub? Can we just
require that our compiler is new enough to have __COUNTER__, or will
that break some of our existing targets?

> Signed-off-by: Michael S. Tsirkin <address@hidden>
> ---
>  include/qemu/compiler.h | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
> diff --git a/include/qemu/compiler.h b/include/qemu/compiler.h
> index e0fb18b..bad25a9 100644
> --- a/include/qemu/compiler.h
> +++ b/include/qemu/compiler.h
> @@ -89,8 +89,12 @@
>      struct { \
>          int:(x) ? -1 : 1; \
>      }
> +#ifdef __COUNTER__

Better would be:

#if defined __COUNTER__ && __COUNTER__ != __COUNTER__

to ensure that it has the semantics we need (that's what gnulib's
LGPLv2+ verify.h uses).

>  #define QEMU_BUILD_BUG_ON(x) typedef QEMU_BUILD_BUG_ON_STRUCT(x) \
> -    glue(qemu_build_bug_on__, __LINE__) __attribute__((unused))
> +    glue(qemu_build_bug_on__, __COUNTER__) __attribute__((unused))

But I agree that this is the right change to make.

> +#else
> +#define QEMU_BUILD_BUG_ON(x)

This means we lose sanity checks on older compilers.  Gnulib documents
that on older compilers, you can use:

extern int (*dummy (void)) [sizeof (struct {...})];

coupled with -Wno-redundant-decls, which lets you redeclare the same
thing as many times as you want without any use of __LINE__ or
__COUNTER__.  On the other hand, I don't think we have to bend over
backwards for compilers that old (RHEL 6 is at gcc 4.4.7, but 4.3 has

Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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