On May 16, 2007, at 10:09 AM, Chris Lattner wrote: > On May 16, 2007, at 1:18 AM, Evan Cheng wrote: >>> But even then I am not sure if that eliminate the need to have a >>> virtual function isPredicatable. Conditional branches probably would >>> require special handling. Also, while it isn't possible now, it's >>> conceivable some instruction predicate field may not be 'always' >>> coming into the if-conversion pass. In that case, it would require a >>> target hook to examine the field anyway. >>> >> >> cmov is another class of M_PREDICATED ops that need to be dealt with >> differently. We'll need a hook to check if something is already >> predicated with non-always predicate. > > Yes, I think this should be the distinction between M_PREDICATED (the > instr already has a predicate field, but may be set to 'always') vs > M_PREDICABLE (the instr may or may not have a predicate field, but > PredicateInstruction can change it into one that does).
I dunno if that's the right solution. To me, whether something is "predicated" is a dynamic property. It's dependent on the value of its predicated field. > > Another thing that occurred to me is that predicates have > implications for livevar analysis (specifically, the scavenger). > Consider this code: > > R3 = add [lt] ... > R3 = sub [ge] ... > > Note that the add is not dead. :) Yeah, this is a problematic issue. For now, I think the if-converter has to be a pre-emit pass. That means it will have to do some basic CFG xform (i.e. remove dead blocks) unless we can move branch folding past it. :-( > >> Come to think of it, perhaps we should not have selects if the target >> uses if-conversion. > > That would be very nice. Alternatively, we could have the isel > "know" about predicated moves and generate them? I think the right solution is to transform select's back to control flow. However, this requires doing the ugly hack to do the lowering at scheduling time, right? Evan > > The isel does not know anything about moves at this point. > > -Chris > _______________________________________________ > llvm-commits mailing list > llvm-commits@cs.uiuc.edu > http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits _______________________________________________ llvm-commits mailing list llvm-commits@cs.uiuc.edu http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits
