> On Sep 20, 2019, at 1:45 PM, Jeff Law <l...@redhat.com> wrote:
>
> On 9/20/19 11:22 AM, Richard Biener wrote:
>> ...
>> It seems to be that for the goal to keep a target alive a variant #2
>> conversion (according to the wiki) should be closely mirror CC0
>> behavior and thus should be easier to achieve and with less patterns
>> touched than the 'good' variant. Can you acknowledge that and would
>> such 'simple' conversions be OK?
> Unless the target has condition code preserving arithmetic a variant #2
> conversion is the only way to go.
Yes.
> Now if someone did a variant #2 without the optimization bits (ie,
> adding appropriate _set_flags pattern variants), I think that should be
> considered explicitly OK even though the code quality would potentially
> suffer. Essentially it's a choice between dropping the port or keeping
> the port, but with slightly less optimization. THe latter seems like a
> better choice to me.
True. Then again, the added work of creating the pattern pairs is modest.
With define_subst, much of the work can be automated.
For pdp11, I found this to be the case; the hard part was to learn what is
needed, and for that the Wiki ends up a big help. Also, pdp11 is harder than
most because it has two CC registers (one for float ops, one for the rest). I
don't know all the others, but for example VAX only has one, and I'm pretty
sure the same applies to m68k.
paul
