Re: [Qemu-devel] [PATCH 3/5] target-arm: convert shl and shr helpers to TCG

[Peter Maydell]

On 17 September 2012 14:36, Laurent Desnogues
<address@hidden> wrote:
> On Mon, Sep 17, 2012 at 11:43 AM, Peter Maydell
> <address@hidden> wrote:
>> The TCG shift operations are undefined behaviour (not merely
>> undefined result) if the shift is >= 32, so we must avoid
>> doing that even if we're going to throw away the answer.
>
> That's odd that it doesn't just state that the result is undefined.
> I wonder what "undefined behavior" means.  I understand
> what undefined behavior (as opposed tu undefined result)
> means for divisions by 0, but not for a shift larger than data
> type width.

Well, it means anything the implementation likes :-)
The only concrete example I could come up with was the old
pre-286 behaviour where the 8086 didn't mask the shift
count at all, so a variable shift by a large amount would
be implemented as that many one-bit-per-cycle shifts and
could take an extremely long time.

-- PMM