Hi! On Thu, Apr 23, 2020 at 08:29:34AM -0600, Jeff Law wrote: > On Thu, 2020-04-23 at 07:07 -0500, Segher Boessenkool wrote: > > Most of expand is *other stuff*. Expand does a *lot* of things that are > > actually changing the code. And much of that is not done anywhere else > > either yet, so this cannot be fixed by simply deleting the offending code. > A lot of what is done by the expansion pass is historical and dates back to > when > we never optimized more than a statement at a time for trees and the real > heavy > lifting was all done in RTL.
Yes. > THe introduction of tree-ssa meant that all the expansion code which wanted to > see a "nice" complex statement and generate good RTL code was useless and as a > result we saw significant regressions in the end code quality. > > That in turn brought in TER who's sole purpose was to reconstruct those more > complex trees for the purposes of improving initial expansion. I think > match.pd > has the potential to make TER go away as match.pd is a generic combining > framework. Ooh that would be great news as well! For moving stuff out of expand, it needs to *move*, not just be deleted. > WRT extending SSA deeper, I'm all for it and it's always been something I > wanted > to see happen. I've always believed we could do RTL SSA to the register > allocation phase and that doing so would be a positive. Well, since we have hard regs as well, it cannot really be SSA? And SSA doesn't fit RTL all that well anyway? But the "webs always" idea is the same of course. > If we start at the front > of the RTL pipeline and work towards the back we bump into CSE quickly, which > would be good. Our CSE is awful across multiple axis. RTL CSE does quite many things that aren't really CSE... Have to figure out for each whether it still is useful, and what to do with it then. > I suspect most RTL passes > would be reimplementations rather than bolting SSA on the side and I suspect > those reimplementations would be simpler than the existing stuff -- I'm a > strong > believer there's a lot of dead code in the RTL passes as well. Sure, but there also is a lot of stuff that perhaps is historical, and perhaps is badly architected, but still does beneficial things. If a reimplementation hugely improves things it's not bad in the end to lose all such little things, but otherwise? Segher
