Hi, On Thu, 10 Jul 2014, Jeff Law wrote:
> > The insight to note is, that undefined SSA names should really be > > coalesced with something (otherwise you lost an optimization opportunity), > > but it doesn't matter with _what_ each use of the undefined name is > > coalesced, you can even identify different uses of them with different SSA > > names (e.g. the LHS of each using stmt). Requires some change in the > > order things are done in out-of-ssa. > > The last part is what I hinted might be problematical. If some > undefined SSA_NAME appears on the RHS of two PHIs and we want to > coalesce that undefined SSA_NAME with the LHS of each of those PHIs, > then the LHS of those two PHIs must coalesce as well. At least that's > my recollection of how all that stuff worked. Only with the usual definition of coalescing (being transitive). For undefined SSA names the definition can be mended. > It was that realization that made me wonder if we should have a unique > SSA_NAME at each undefined use point. It's easier to implicitely regard every individual use of an undefined SSA name as a unique name in coalescing I think (instead of having it be a unique name explicitely). That is, given: bb1: x_1 = PHI <a_2, b_3(UND)> ... bb2: x_4 = PHI <y_5, b_3(UND)> ... There is no reason to not regard the two uses of b_3 as separate and identify the first with x_1 and the second with x_2, _without_ coalescing x_1 and x_2. But yes, this doesn't fit readily into the normal coalescing merge-find framework, but rather would have to be something after coalescing when writing out-of-ssa (whenever an undefined use is rewritten just take a random other fitting variable). Ciao, Michael.
