Re: Replace REDUC_*_EXPRs with internal functions.

[Richard Biener <richard dot guenther at gmail dot com>]

On Wed, Nov 22, 2017 at 12:15 PM, Richard Sandiford
<richard.sandiford@linaro.org> wrote:
> Jakub Jelinek <jakub@redhat.com> writes:
>> On Wed, Nov 22, 2017 at 10:09:08AM +0000, Richard Sandiford wrote:
>>> This patch replaces the REDUC_*_EXPR tree codes with internal functions.
>>> This is needed so that the support for in-order reductions can also use
>>> internal functions without too much complication.
>>>
>>> This came out of the review for:
>>>   https://gcc.gnu.org/ml/gcc-patches/2017-11/msg01516.html
>>>
>>> Tested on aarch64-linux-gnu, x86_64-linux-gnu and powerpc64le-linux-gnu.
>>> OK to install?
>>
>> Wouldn't it be better to just have IFN_REDUC that takes as an additional
>> argument INTEGER_CST with tree_code of the operation (so REDUC_MAX_EXPR
>> would be transformed into REDUC (MAX_EXPR, ...) etc.)?
>> That way we wouldn't need to add further internal fns if we want say
>> multiplication reduction, or some other.
>
> I think it depends how we use them.  The functions added here map
> directly to optabs, so we'd only add a new one if we also added a
> new optab.  If there's no optab, or if there is an optab but the
> target doesn't support it, then we open-code the reduction during
> vectorisation.  (That open-coding already happens for MULT, AND, IOR
> and XOR, which have no optabs, although one of the SVE patches does
> add optabs for the last three.)
>
> I think having separate functions makes sense in that case, since it
> makes the mapping to optabs easier, and makes it easier to probe
> for target support.  Maybe an IFN_REDUC would be useful if we wanted
> to defer the open-coding of other reductions past vectorisation,
> but I'm not sure off-hand how useful that would be.  E.g. we'd still
> need to try to cost the eventual expansion when deciding profitability.

I also agree that separate IFNs map better to what we do (encode a target
specific insn).

Thus the patch is ok.

Thanks,
Richard.

> Thanks,
> Richard