Vladimir Makarov wrote: > On 07/19/2011 04:13 AM, Georg-Johann Lay wrote: >> Vladimir Makarov wrote: >>> On 07/18/2011 04:14 PM, Richard Henderson wrote: >>>> On 07/18/2011 12:15 PM, Georg-Johann Lay wrote: >>>>>> However, what you've done is try very hard to work around reload >>>>>> doing the Right Thing with constant spilling, namely re-generate >>>>>> the constant rather than spill and restore it. I cannot believe >>>>>> that's the right way to proceed. >>>>> You mean that with a clobber reload will rematerialize the constant >>>>> if there are not enough registers instead of spilling it? >>>>> And with an ordinary move like >>>>> (set (reg) (const_int)) >>>>> reload will not see that it can reaterialize it and spill it to >>>>> the stack? >>>> Well, it certainly didn't use to. >>>> >>>> Vlad, do you know what the current state of the register allocator >>>> is wrt this sort of rematerialization? >>>> >>> Rematerialization is done in reload. So it is the same as it was before >>> IRA. >>> >>> Reload should definitely rematerialize constants and I saw it many times >>> when I analyzed the code generated by IRA+reload. >>> >>> It will also do rematerialization of 'a hard reg + const'. >> If I understand you correctly, a pre-reload split has no disadvantage >> compared to a post-reload split? As oulined in >> http://gcc.gnu.org/ml/gcc-patches/2011-07/msg01422.html >> > It is hard to me to say it. Only the experiment on several targets can > show it and efforts of achieving this. > > GCC is an old compiler. More generally speaking, I believe in classic > approach that at this stage (1st insn scheduling, RA) RTL insns should > be as close as possible to real target insns.
That's what I tried with the pre-reload split: as close as possible to real instructions. > The splitting mechanism is mostly a consequence of an outdated method of > partial code selection (combiner). Moving to the classical approach is > a huge job and might be too difficult to implement it. >> The first split is pre-reload and generates a pseudo to load the >> constant. >> The second split is post-reload and uses a clobber reg. >> >> The question is what's best if the constant is used more than once, >> e.g. used >> again in similar insn. > Again it is hard to say definitely for me. To remove second constant > load, reload has inheritance mechanism. In post-reload passes, there is > gcse for this. Inside reloas it cannot work because the alternative to pre-reload split is a clobber. And as the reg is inside a clobber, it cannot be reused. It could reused after post-reload split, but as Richard mentioned post-reload CSE runs before post-reload split. So that cannot work either. > reload pass can combine such reloads for one insn. But again i think > you should try if you wan to know, reload is too complicated to > definitely affirm something. At least it cannot work for clobbers as explained above.
