[PATCH, middle-end]: Introduce TARGET_REJECT_COMBINED_INSN target hook

[Uros Bizjak]

On Thu, Aug 23, 2012 at 7:59 PM, Andrew Pinski <pinskia@gmail.com> wrote:

>>> This patch introduces TARGET_REJECT_COMBINED_INSN target hook, so
>>> targets are able to reject combinations of two or more insns. The hook
>>> is called from recog_for_combine, so it is the target that has the
>>> final say on the combined insn.
>>
>> Hi,
>>
>> great place for a hook, it was missing, IMO.
>>
>> Just a note:  Wouldn't it be good to have a hook that may transform
>> a pattern to a new one and return that to combine?
>>
>> Your reject_combined_insn would be a special case, e.g. return
>> NULL_RTX.
>>
>> Sometimes recog_for_combine fails (resp. recog fails) because
>> recog_for_combine does not try all possible transformations and
>> therefore recog then fails because there is no combine pattern.
>
> Or just better yet improve recog_for_combine.  I was planing on doing
> that for some cases dealing with zero_extract.

We have tried that (see previous discussions on recog_for_combine
change. But there is no way to satisfy all targets: some works better
when reload fixes invalid hard regs (sh), others (x86) require totally
different strategy. Even if recog_for_combine will be improved for all
targets, we will still need something target specific.

Uros.