Replace REDUC_*_EXPRs with internal functions.

[Richard Sandiford]

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.

Thanks,
Richard