Re: [Qemu-devel] target-i386: guest variable shift by 0 provokes shift by -1

[Peter Maydell]

On 18 March 2014 14:47, Peter Maydell <address@hidden> wrote:
> On 18 March 2014 14:39, Richard Henderson <address@hidden> wrote:
>> On 03/18/2014 07:25 AM, Peter Maydell wrote:
>>> Why do you think this? tcg/README says out of
>>> range shifts are undefined behaviour. That means we
>>> mustn't execute them, and this code doesn't attempt
>>> to branch around or otherwise avoid the shift by -1.
>>
>> Bah.  Stuff and nonsense.  None of our backends are so
>> stupid as to start WWIII with an out of range input.
>
> Then we should document that this case is an
> unspecified-result, not use the same term we
> do for division-by-zero or division-overflow (which
> really can cause things to blow up).

Mailing thread from last time around:
 http://lists.gnu.org/archive/html/qemu-devel/2012-09/msg02562.html

I don't particularly object to changing to "undefined
result" if you want to audit the backends and the
optimizer...

thanks
-- PMM