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Re: [Qemu-devel] [PATCH] QEMU_BUILD_BUG_ON: use __COUNTER__
From: |
Markus Armbruster |
Subject: |
Re: [Qemu-devel] [PATCH] QEMU_BUILD_BUG_ON: use __COUNTER__ |
Date: |
Tue, 31 Jan 2017 16:34:34 +0100 |
User-agent: |
Gnus/5.13 (Gnus v5.13) Emacs/25.1 (gnu/linux) |
Peter Maydell <address@hidden> writes:
> On 31 January 2017 at 15:03, Eric Blake <address@hidden> wrote:
>> 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?
>
> I think the problem is not so much our build farm (which is
> I think all recent enough to postdate introduction of
> __COUNTER__) but the probability of there being users
> out there who are still using older compilers to build
> QEMU releases. We don't need to enforce compile time
> asserts on those old compilers but given that it's
> trivial to avoid actually breaking the build for them,
> why not provide the do-nothing fallback?
I don't think we actually need __COUNTER__.
If we want it, providing a fallback is nice. However, the "expand to
nothing" fallback is syntactically unsound. A sound fallback expands to
the exact same syntactic construct as the real thing does, in this case
a declaration. When it doesn't, we risk parse errors.
Of course, we may decide to accept that risk.
Re: [Qemu-devel] [PATCH] QEMU_BUILD_BUG_ON: use __COUNTER__, Markus Armbruster, 2017/01/31
Re: [Qemu-devel] [PATCH] QEMU_BUILD_BUG_ON: use __COUNTER__, Peter Maydell, 2017/01/31
Re: [Qemu-devel] [PATCH] QEMU_BUILD_BUG_ON: use __COUNTER__, Daniel P. Berrange, 2017/01/31