Re: [Qemu-devel] [PATCH v3-a 26/27] target/arm: Extend vec_reg_offset to larger sizes

[Peter Maydell]

On 17 May 2018 at 17:51, Richard Henderson <address@hidden> wrote:
> On 05/17/2018 08:57 AM, Peter Maydell wrote:
>> This looks right for element sizes up to 8, but I don't understand
>> how it handles 16 byte elements as the commit message says -- the
>> d[0] and d[1] are the wrong way round and don't form a single
>> 16-byte big-endian value, so they must need special casing somewhere
>> else ?
>
> You're right that it's not a proper int128 for host arithmetic.  Fortunately,
> the only operation we have on 128-bit values, at present, is move.
>
> How better might you word the commit message?

You could add in the comment in the function something like:
"For 32 byte elements, we return an offset of zero. The two halves will
not form a host int128 if the host is bigendian, since they're in
the wrong order. However the only 32 byte operation we have is a move,
so we can ignore this for the moment. More complicated 32 byte operations
would have to special case loading and storing from the zregs array."

thanks
-- PMM