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From: Peter Maydell <peter.maydell@linaro.org>
Date: Tue, 17 Nov 2015 18:24:58 +0000
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Subject: Re: [Qemu-devel] [PATCH v2 for 2.5] QEMU does not care about left
 shifts of signed negative values
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To: Paolo Bonzini <pbonzini@redhat.com>
Cc: Laszlo Ersek <lersek@redhat.com>, Markus Armbruster <armbru@redhat.com>, QEMU Developers <qemu-devel@nongnu.org>

On 17 November 2015 at 18:21, Paolo Bonzini <pbonzini@redhat.com> wrote:
>
>
> On 17/11/2015 19:19, Peter Maydell wrote:
>> That doesn't seem like more than half-good news to me. In particular,
>> if ubsan is still diagnosing these cases and they're still a
>> problem in some constant expressions
>
> Constant expressions are standardese for e.g.
>
> static int x = 1 << 31;
>
> It doesn't mean _all_ constants, and the warning only triggers with
> -pedantic.

But if "-fwrapv" means "this dialect of C makes shifts of
negative numbers well defined and OK" then "-1 << 31"
should be fine and should not provoke a warning (whether in
a constant expression or not). If that's not what -fwrapv means,
then we shouldn't be using it as if it did.

thanks
-- PMM