On Sat, Feb 11, 2012 at 1:12 AM, Richard Henderson <r...@redhat.com> wrote: > Seeing as how Uros is starting to go down the path of cleaning up the > flags handling for x86, I thought I'd go ahead and knock up the idea > that I've been tossing around to help automate the process of building > patterns that match both clobbering the flags and setting the flags to > a comparison. > > This is far from complete, but it at least shows the direction. > > What I know is missing off the top of my head are: > > (0) Documentation in some .texi file; atm there's only what's in rtl.def. > > (1) Generate (clobber (reg flags)) from genemit, should this construct > be used in a named insn pattern. > > (2) Can't be usefully used with define_insn_and_split, and no way to tell. > This problem should simply be documented in the .texi file as user error. > > (3) Can't be used for x86 add patterns, as the "clobber" version wants the > freedom to use "lea" and the "set flags" version cannot. And there are > different sets of constraints if lea may be used or not. > > What would be nice, however, is exposing the targetm.cc_modes_compatible > thing in such a way that the x86 add patterns could use that, for the > separate insn that does do the set flags. > > Exposing the targetm.cc_modes_compatible thing separately might also > clean up some of the evil magic in genrecog.c too. > > Comments?
To see if I understand what a cc0 port conversion would look like with "match_flags", I tried to apply this magic to convert one of the pet ports, the mighty pdp11. The pdp11 port doesn't have any define_insn_and_splits, so I didn't run into the problem you mentioned in (2). What is the problem here? I suppose it has to do with finding out what the flags setter is after the split? If so, then couldn't that be resolved with some rules about how the post-split patterns should be constructed? Other than that: To convert a port, there is still a lot of work to be done to define and handle the various CC modes properly (well, not for the pdp11, because it writes out >1 insn for most define_insns), but it is great not having to define all the pairs of clobber-flags and set-flags insns. At least, I didn't end up rewriting the complete .md file. It was relatively easy. Less book-keeping involved, etc. Hope this goes in for GCC 4.8. Ciao! Steven
