Hi Richard, > But even if the costs are too high, the patch seems to be overcompensating. > It doesn't make logical sense for an ADRP+LDR to be cheaper than an LDR.
An LDR is not a replacement for ADRP+LDR, you need a store in addition the original ADRP+LDR. Basically a simple spill would be comparing these 2 sequences: ADRP x0, ... LDR x0, [x0, ...] STR x0, [SP, ...] ... LDR x0, [SP, ...] ADRP x0, ... LDR x0, [x0, ...] ... ADRP x0, ... LDR x0, [x0, ...] Obviously it's far cheaper to do the latter than the former. > Giving X zero cost means that a sequence like: > > (set (reg x0) X) > (set (reg x1) X) > > should stay as-is rather than be changed to: > > (set (reg x0) X) > (set (reg x1) (reg x0)) > > I don't think we want that for multi-instruction constants when > optimising for size. I don't believe this is a real problem. The cost queries for address constants come from register allocation, I don't see them affect other optimizations. > Yeah, I wasn't suggesting that we increase the spill costs. I'm saying I'm saying that because we've set the spill costs low on purpose to work around register allocation bugs. There have been some fixes since, so increasing the spill costs may now be feasible (but not trivial). > that we should look at whether the target-independent RA heuristics need > to change, whether new target hooks are needed, etc. We shouldn't go > into this with the assumption that the target-independent code is > invariant and that any fix must be in existing aarch64 hooks (rtx costs > or spill costs). But what bug do you think exists in target independent code? It behaves correctly once we supply more accurate costs. If there was no rematerialization irrespectively of the cost settings then you could claim there was a bug. > Maybe it would help to turn the question around for a minute. Can we > describe the cases in which it's *better* for the RA to spill a constant > address to the stack and reload it, rather than rematerialise on demand? Rematerialization is almost always better than spilling and reloading from the stack. If the constant requires multiple instructions and there are more than 2 references it would be better for codesize to spill, but for performance it is better to rematerialize unless there are many references. You also want to prefer rematerialization over spilling a different liferange when other aspects are comparable. Cheers, Wilco
