On 21/02/17 19:53, Matthew Fortune wrote: > Eric Botcazou <ebotca...@adacore.com> writes: >>> Agreed. I don't think things like WORD_MODE_OPERATIONS should change >>> rtl semantics, just optimisation decisions. > > Sorry, I did cover two topics in one email. My point was about whether > simplifying: > > (subreg:OUTER (mem:INNER ...)) > To: > (mem:OUTER ...) > > Should ever be a requirement for successful reloading rather than just an > optimisation that 'could' be applied. There are a few cases it seems that > require this simplification to happen which I find odd. >
If mem:INNER has side effects (volatile, pre/post addressing etc), then the mem must never be accessed in any mode other than INNER. So I agree that if there are places that assume otherwise that is odd; but they may have already dealt with the side effects problem earlier. R. >>> And using the smallest >>> possible spill size is often good even for RISCy targets. > > Yes, if (subreg(mem)) simplification only ever happened when it was reducing > the size of the load/store I would understand the code more too but actually > it will currently happily simplify to a wider mode. Using a wider mode may > well be beneficial in other situations where a further reload would be needed > due to insn constraints I guess. It all feels a bit like magic still. > >> I don't think that's correct, WORD_MODE_OPERATIONS does change RTL semantics >> for SUBREGs smaller than a word since it can make all bits defined, even if >> only the lower part is assigned. For example (SUBREG:SI (MEM:QI)) has the >> higher bits defined according to LOAD_EXTEND_OP if WORD_MODE_OPERATIONS. > > It would almost be simpler if we had another variant of subreg (like we have > strict_low_part) to explicitly represent the WORD_REGISTER_OPERATIONS > behaviour > with the mode change. I'll stop myself and hold this for later though! > >> LRA simply needs to preserve the semantics, just as reload does. > > I will keep working on this code post GCC 7 as the topic is bugging me now :-) > > Thanks, > Matthew >
