On Fri, Sep 10, 2021 at 10:08 PM David Edelsohn <dje....@gmail.com> wrote: > > On Thu, Sep 9, 2021 at 11:03 PM Hongtao Liu <crazy...@gmail.com> wrote: > > > > 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. > > Hongtao, > > This is why the GNU Toolchain community sponsors the GCC Compile Farm. > Not every architecture is provided, but a good subset. And there are > a few, powerful Linux on Power systems in the cluster, such as gcc135. > I see. > And the comments in the code warned of port-specific problems. Even > if you cannot test the patch yourself, you should have publicly > requested that other port maintainers test the patch to shake out > problems. Yes, sorry for the inconvenience. > > The Tree-SSA passes are mostly target-independent, but RTL is > target-dependent, which can elicit widely different behavior in the > RTL passes. When you make changes to RTL passes in the common parts > of the compiler, you must consider the impact on all targets. GCC > explicitly supports a wide variety of architectures, ABIs and OSes. > All of the developers strive to ensure that changes don't adversely > affect any targets. > > Thanks, David
-- BR, Hongtao
