On Tue, Feb 2, 2021 at 4:03 PM Richard Sandiford <richard.sandif...@arm.com> wrote: > > Richard Biener <richard.guent...@gmail.com> writes: > > On Mon, Feb 1, 2021 at 6:54 PM Joel Hutton <joel.hut...@arm.com> wrote: > >> > >> Hi Richard(s), > >> > >> I'm just looking to see if I'm going about this the right way, based on > >> the discussion we had on IRC. I've managed to hack something together, > >> I've attached a (very) WIP patch which gives the correct codegen for the > >> testcase in question (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98772). > >> It would obviously need to support other widening patterns and > >> differentiate between big/little endian among other things. > >> > >> I added a backend pattern because I wasn't quite clear which changes to > >> make in order to allow the existing backend patterns to be used with a > >> V8QI, or how to represent V16QI where we don't care about the top/bottom > >> 8. I made some attempt in optabs.c, which is in the patch commented out, > >> but I'm not sure if I'm going about this the right way. > > > > Hmm, as said, I'd try to arrange like illustrated in the attachment, > > confined to vectorizable_conversion. The > > only complication might be sub-optimal code-gen for the vector-vector > > CTOR compensating for the input > > vector (on RTL that would be a paradoxical subreg from say V4HI to V8HI) > > Yeah. I don't really like this because it means that it'll be > impossible to remove the redundant work in gimple. The extra elements > are just a crutch to satisfy the type system.
We can certainly devise a more clever way to represent a paradoxical subreg, but at least the actual operation (WIDEN_MINUS_LOW) would match what the hardware can do. OTOH we could simply accept half of a vector for the _LOW (little-endial) or _HIGH (big-endian) op and have the expander deal with subreg frobbing? Not that I'd like that very much though, even a VIEW_CONVERT <v8hi> (v4hi-reg) would be cleaner IMHO (not sure how to go about endianess here ... the _LOW/_HIGH paints us into some corner here) A new IFN (direct optab?) means targets with existing support for _LO/HI do not automatically benefit which is a shame. > As far as Joel's patch goes, I was imagining that the new operation > would be an internal function rather than a tree code. However, > if we don't want that, maybe we should just emit separate conversions > and a normal subtraction, like we would for (signed) x - (unsigned) y. > > Thanks, > Richard
