On Thu, Apr 23, 2020 at 12:31 AM Jeff Law <l...@redhat.com> wrote: > > On Wed, 2020-04-22 at 15:50 -0500, Segher Boessenkool wrote: > > > > > > In some ways it feels like it would be easier to resurrect RTL SSA :-) > > > > Why was RTL SSA abandoned? > > > > It might well work to keep everything in SSA form all the way to RA. > > Hrm, that doesn't sound bad at all :-) > > > > (The PHIs need to be made explicit to something that resembles the > > machine code we will end up with, very early in the pipeline, but it > > could still also be some valid SSA form; and we can of course also > > have hard registers in all RTL, so that needs to be dealt with sanely > > some way as well Lots of details, I don't see a crucial problem though, > > probably means I need to look harder ;-) ) > Lack of time mostly. There's some complications like subregs, argument > registers > and the like. But you can restrict ssa based analysis & optimizations to just > the set of pseudos that are in SSA form and do something more conservative on > the > rest.
I guess time is better spent on trying to extend GIMPLE + SSA up to RA, thus make instruction selection on GIMPLE. Back in time Steven spent quite some time doing factored SSA but I don't remember either what went wrong or whether simply DF appeared first. Richard. > Jeff >
