On Tue, 28 Aug 2007, Kenneth Zadeck wrote: > >>>> And finally at the stage of rtl unrolling it looks like this: > >>>> [6] r186 = r2 + C; > >>>> r318 = r186 + 160; > >>>> loop: > >>>> r186 = r186 + 16 > >>>> if (r186 != r318) then goto loop else exit > >>>> > > > > regardless of the set of blocks considered, there is no way how both the > > definition outside of the loop and the definition inside the loop could > > both be reaching definitions at the same point in the program. > > > sure it can. if the def is not a killing def, i.e. the def is a partial > or conditional, then the prior defs reach right in.
I don't think that's the case above for r186. Apart from that I agree that in the case of conditional defs multiple ones might reach a use. I disagree for partial defs, if one dominates the other. For most cases (except register allocation) a partial def is just a complete killing def, which happens to leave some contents unchanged (by conceptually copying that part from what was in there before, hence the usual read-mod-write dependency from a partial def which generates also a use). But the use doesn't care for this, for all it knows that def is the one changing the contents, hence a use-def chain should not contain a def dominating other defs (partial or not). The relationship between both defs will be described by the use-def chain hanging off the implicit use generated by the partial def. E.g. in this example (using my old hopefully understandable notation), if p1 is a DImode pseudo: 1 p1+0:SI <- bla 2 p1+4:SI <- blubb 3 use <- p1:DI insn 3 should only see def 2 in it's chain. That it also is influenced by the def 1 will be captured by the implicit use of p1 in insn 2. I.e. not different to 1 p1 <- bla 2 p1 <- p1 + blubb 3 use <- p1 Exact partial defs should only (and do) matter for register allocation. That's one of the reasons why (some years ago for new-ra) I exposed real partial defs of subregs from the df framework only for the register allocator, but not for general code. Ciao, Michael.
