On Fri, Sep 10, 2021 at 7:49 AM Segher Boessenkool <seg...@kernel.crashing.org> wrote: > > On Thu, Sep 09, 2021 at 08:16:16AM +0200, Richard Biener wrote: > > > I think we should (longer term) get rid of the overloaded meanings and > > > uses of subregs. One fairly simple thing is to make a new rtx code > > > "bit_cast" (or is there a nice short more traditional name for it?) > > > > But subreg _is_ bit_cast. > > It is not. (subreg:M (reg:N) O) for O>0, little-endian, is not a > bit_cast. It is taking a part of a register, or a single register from > a multi-register thing. Paradoxicals are not bit-casts either. > > Subregs from or to (but not both) integer modes are generally bit_cast, > yeah. > > > What is odd to me is that a "disallowed" subreg > > like (subreg:SF (reg:TI ..) 0) magically becomes valid (in terms of > > validate_subreg) if you rewrite it as (subreg:SF (subreg:SI (reg:TI ..) 0) > > 0). > > Of course that's nested and invalid but just push the inner subreg to a > > new pseudo and the thing becomes valid. > > Bingo. > > And many targets have strange rules for bit-strings in which modes can > be used as bit-strings in which other modes, and at what offsets in > which registers. Now perhaps none of that is optimal (I bet it isn't), > but changing this without a transition plan simply does not work. > > > > But that is not the core problem we had here. The behaviour of the > > > generic parts of the compiler was changed, without testing if that > > > works on other targets but x86. That is an understandable mistake, it > > > takes some experience to know where the morasses are. But this change > > > should have been accompanied by testcases exercising the changed code. > > > We would have clearly seen there are issues then, simply by watching > > > gcc-testresults@ (and/or maintainers are on top of the test results > > > anyway). Also, if there were testcases for this, we could have some > > > confidence that a change in this area is robust. > > > > Well, that only works if some maintainers that are familiar enough > > with all this chime in ;) > > Not really. It works always. And it works way better than the > pandemonium we now have with broken targets left and right. > > With testcases anyone can see if any specific target is broken here. > > > It's stage1 so it's understandable that some > > people (like me ...) are tyring to help people making progress even > > if that involves trying to decipher 30 years of GCC history in this > > area (without much success in the end as we see) ;) > > Yeah :-) And my thanks to you and everyone involved for tackling this > problematic part of GCC, which has been neglected and patched over for > way too long. But from that same history it follows that anything you > do not super carefully (with testing everywhere) will cause some serious Frankly, testing everywhere is too heavy a burden for developers, after all, everyone has a limited variety of machines, and may not be familiar with using other targets' simulators. And back to the problem we were trying to solve at the beginning (subreg:HF(reg:SI)), I guess this is not just a problem in x86 backend, any backend can encounter similar problems, that's why we remove all the weird cases in validate_subreg. > problems. And nonse of these are easy to fix at all -- there is a > *reason* targets did this nastiness. > > > > p.s. Very unrelated... Should we have __builtin_bit_cast for C as well? > > > Is there any reason this could not work? > > Still interested in this btw :-) (And still very unrelated.) > > > Segher
-- BR, Hongtao
