Adam Nemet <ane...@caviumnetworks.com> writes: > In order for my CSE const anchor patch to work I needed to drastically lower > the cost of immediate addition in the MIPS backend. This was acceptable as a > proof of concept but not in general of course. > > The problem is with "single-insn"/simple constants. We would also like these > to use const anchors in the hope that the resulting expression would get > propagated into MEM expressions. I was hoping that fwprop would do this > propagation for me. However, since a single-insn constant load is cheaper > than an immediate addition (make sense), fwprop is free to propagate either > (1) into (2) or (2) into (3) here: > > (1) a <- C > | > +--> (2) b <- a + D > | | > | +--> (3) mem(b) > | > +--> (4) use(a) > > Which one it does depends on which one it tries first. Right now we go in > insn order so we'd do (1) to (2) preventing to do (2) to (3) later.
Probably a dumb question, sorry, but is this only a problem for MIPS when C + D is in the range [0x8000, 0xFFFF]? Or is the (1)->(2) substitution somehow succeeding in other cases? Richard
