On Fri, Mar 31, 2017 at 09:39:27AM -0600, Jeff Law wrote: > >I am curious as to why (SP + offset) dereferencing is so problematic, > >as compared to the general registers; could you elaborate? > It's potentially problematic because you don't generally have reg+offset > addressing modes. Whether or not it actually causes a problem could > well depend on other aspects of your target and things like reload's > assumptions about how it can manipulate an insn. (I hope you are using LRA instead of reload, for a new port!) To expand on Jeff's comments a bit: If reload/LRA needs to spill a register it ends up on the stack, and things there are naturally accessed using sp+off addressing. If you do not support such addressing (or your arch doesn't, even), then reload will need to spill more, to free up another register (or two) for reg+reg addressing (where one reg is sp, the other is that offset), and you get pretty nasty code as a result. Segher
