[patch] Improve detection of widening multiplication in the vectorizer

[Richard Guenther]

On Mon, Jun 6, 2011 at 3:04 PM, Richard Sandiford
<richard.sandiford@linaro.org> wrote:
> Richard Guenther <richard.guenther@gmail.com> writes:
>> Thanks.  I would hope that we eventually can get rid of the
>> pattern recognizer ... at least for SSE there is also always
>> a scalar variant instruction for each vectorized one.
>
> AFAIK, that isn't true for ARM and NEON.  E.g. I don't know of a single
> instruction that does the scalar equivalent of things like VADDHN
> (add values and narrow to high half), VSUBL.U32 (subtract two values
> and extend the result), etc.
>
> FWIW, I think MIPS only has minimum and maximum operations for paired
> floats, not for single floats or doubles.  I don't have the manuals to
> hand to check though.
>
> It's probably OK for the particular case of widening multiplications.
> It sounded like you were making a more general statement though.
> If so, I think we should try to avoid assuming that every vectorisable
> operation has an equivalent scalar machine instruction.

Hmm, too bad ;)  Yes, I was suggesting that we assume that.  I guess
for now we can go with the vectorizer pattern matching enhancement
and re-visit re-ordering the passes later (I don't have time right now to
look into the reported issue).

Thanks,
Richard.

> Richard
>