Hi! On Mon, Apr 20, 2020 at 09:56:34AM +0200, Richard Biener wrote: > On Fri, Apr 17, 2020 at 10:51 PM Segher Boessenkool > <seg...@kernel.crashing.org> wrote: > > > Yeah well, but RTL is not in SSA form > > > > "Webs" are not the *same* as SSA, in a few crucial ways; but they serve > > similar purposes: they both make code transformations easier to do. > > Both do this by having more different (independent) names. > > > > > and there's no RTL IL verification > > > in place to track degradation. > > > > Adding this isn't hard in principle. But currently it is violated all > > over the place, including in backend code. > > It's a cheap excuse and clearly a lot of work. But we've done it for GIMPLE > and it was clearly worth it. If only for documentation purposes.
Yes, I very much agree :-) But how will we do this? Make it warnings only, and have those opt-in (for a port) even? > > > And we even work in the opposite way > > > when expanding to RTL from SSA, coalescing as much as we can ... > > > > Which often is bad. A lot of what expand does is premature optimisation > > that a) is much too complicated, and b) results in worse code in the end. > > Expand should not try to do *anything* "interesting", it should "merely" > > translate to RTL. > > > > (And in a way that can be debugged sanely at all, if possible ;-) ) > > Sure. I've tried to turn off TER with only moderate success. Another > approach > would be to not do any coalescing but for PHIs (and maybe there only for edges > where we cannot insert copies on w/o splitting it). Yeah, that would be much simpler to do as well. > But clearly all the magic expand does now should materialize itself on > GIMPLE (see the patch series Martin posted for vcond* / vec_cmp* > "expansion" on GIMPLE). Probably most, yeah, but some things that expand does should be done much *later* in RTL even :-) > It's the magic that requires TER and TER clearly > introduces the most noise in the GIMPLE -> RTL translation step (as to > code motion and initial instruction selection if it "fails"). Expand does a lot of things that are very much historic, too. Segher
