On Wed, 2005-04-20 at 18:51 +0200, Eric Botcazou wrote: > > Yes, I meant SET_DEST. Do you see how if a SET_DEST is a pseudo > > which did not get a hard register and is equivalent to a readonly > > memory location that the insn is useless? > > Yes, I think so: being equivalenced implies that there was a REG_EQUIV note, > so the insn cannot do anything else than setting the pseudo to the value it > is already equivalenced to. Right. So, in theory all we have to do is delete these insns.
The problem is detection and elimination of these insns. It's relatively easy to scan the register state to find pseudos which were not assigned a hard reg and which are equivalenced to a readonly memory location. If you find any, then you scan the IL and kill the offending insns. Seems trivial. Unfortunately, it's not that simple. The relatively unstructured nature of RTL means the offending assignment to the pseudo could appear in a multitude of contexts, including simple assignments or part of an arbitrarily complex parallel. It's probably not safe to simply remove the assignment if it appears in a PARALLEL as doing so may create an unrecognizable insn. Ugh. If we find such cases we could fall back to not recording the equivalence. But what worries me even more is spilling. Say a pseudo has a hard reg assigned and is also equivalent to a readonly memory location. Reload then decides to spill the pseudo out of the hard reg because the hard reg was needed for something else. When that occurs we'd have to scan affected live range of the pseudo for assignments into the pseudo and remove them like I've suggested above. But that brings another set of bookkeeping problems -- we're twiddling reg_equiv_XXX significantly later in the reload process, and I'm not sure how safe that's going to be. Given how rare this situation occurs, I'm leaning towards simply ignoring the problematical equivalences. jeff
