[PATCH] D31528: [ELF][MIPS] Multi-GOT implementation

Fri Apr 14 14:12:02 PDT 2017

Simon Atanasyan <simon at atanasyan.com> writes:
>  On Fri, Apr 14, 2017 at 3:56 AM, Rui Ueyama <ruiu at google.com> wrote:
> > On Thu, Apr 13, 2017 at 8:52 AM, Simon Atanasyan <simon at atanasyan.com>
> > wrote:
> >>
> >> On Tue, Apr 11, 2017 at 3:42 AM, Rui Ueyama <ruiu at google.com> wrote:
> >> > On Mon, Apr 10, 2017 at 5:13 PM, Davide Italiano <davide at freebsd.org>
> >> > wrote:
> >> >>
> >> >> On Mon, Apr 10, 2017 at 4:58 PM, Rui Ueyama via llvm-commits
> >> >> <llvm-commits at lists.llvm.org> wrote:
> >> >> > On Sat, Apr 8, 2017 at 12:08 AM, Simon Atanasyan
> >> >> > <simon at atanasyan.com>
> >> >> > wrote:
> >> >> >>
> >> >> >> On Tue, Apr 4, 2017 at 9:09 PM, Rui Ueyama <ruiu at google.com> wrote:
> >> >> >> > On Tue, Apr 4, 2017 at 5:46 AM, Simon Atanasyan
> >> >> >> > <simon at atanasyan.com>
> >> >> >> > wrote:
> >> >> >> >>
> >> >> >> >> On Sat, Apr 1, 2017 at 4:20 AM, Rui Ueyama via Phabricator
> >> >> >> >> <reviews at reviews.llvm.org> wrote:
> >> >> >> >> >
> >> >> >> >> > This is not your fault, but I have to say that this MIPS GOT
> >> >> >> >> > layout
> >> >> >> >> > is
> >> >> >> >> > very odd,
> >> >> >> >> > too different from other architectures, and too complicated. I
> >> >> >> >> > want
> >> >> >> >> > to
> >> >> >> >> > avoid supporting
> >> >> >> >> > this unless I'm convinced that it is absolutely necessary. It
> >> >> >> >> > seems
> >> >> >> >> > to
> >> >> >> >> > me that MIPS
> >> >> >> >> > needs a clean, common new ABI. Only the MIPS ABI imposes a lot
> >> >> >> >> > of
> >> >> >> >> > restrictions
> >> >> >> >> > on the size of GOT sections and the order of GOT section
> >> >> >> >> > members,
> >> >> >> >> > even
> >> >> >> >> > though MIPS
> >> >> >> >> > as a processor is an ordinary RISC ISA. This change would
> >> >> >> >> > really
> >> >> >> >> > hurt
> >> >> >> >> > maintainability
> >> >> >> >> > of LLD which I already found some MIPS-specific behavior is
> >> >> >> >> > hard
> >> >> >> >> > to
> >> >> >> >> > keep
> >> >> >> >> > it right
> >> >> >> >> > when editing code for all the other architectures.
> >> >> >> >>
> >> >> >> >> MIPS will not always use old, obsoleted ABIs. It will switch to
> >> >> >> >> new
> >> >> >> >> one. But it does not
> >> >> >> >> happen this year or so. Besides other obstacles, there is a
> >> >> >> >> hardware
> >> >> >> >> problem prevents from
> >> >> >> >> fast switching and common acceptance of the new ABI. Historically
> >> >> >> >> many
> >> >> >> >> MIPS instructions
> >> >> >> >> are partitioned as 16 bit for opcode and 16 bit bit for
> >> >> >> >> address/index.
> >> >> >> >> That is one of
> >> >> >> >> the source of GOT size limitation and reason of multi-GOT
> >> >> >> >> invention.
> >> >> >> >>
> >> >> >> >> The biggest part of the patch isolated in the MipsGotSection
> >> >> >> >> class.
> >> >> >> >> It
> >> >> >> >> adds some new
> >> >> >> >> MIPS specific code like new constructor of the DynamicReloc
> >> >> >> >> class.
> >> >> >> >> But
> >> >> >> >> at the same
> >> >> >> >> time it removes some `if (Config->EMachine == EM_MIPS)`
> >> >> >> >> statements
> >> >> >> >> and
> >> >> >> >> MIPS specific
> >> >> >> >> fields from the `SymbolBody` class.
> >> >> >> >
> >> >> >> >
> >> >> >> > It is isolated as a separate class, but we still need to
> >> >> >> > understand
> >> >> >> > and
> >> >> >> > modify it when we need to do something for relocation processing.
> >> >> >> > I'm
> >> >> >> > actually trying to change the design of relocation processing, to
> >> >> >> > increase
> >> >> >> > parallelism of relocation processing. We can't parallelize it
> >> >> >> > entirely,
> >> >> >> > but
> >> >> >> > some part (such as making a decision whether a symbol needs a GOT
> >> >> >> > slot
> >> >> >> > or
> >> >> >> > not) can be processed per-file or per-relocation basis.
> >> >> >> >
> >> >> >> > Then I found that this part of code is very complex and has grown
> >> >> >> > organically. I tried to reduce its complexity and found that
> >> >> >> > keeping
> >> >> >> > everything right for MIPS is hard. I'm really don't want to
> >> >> >> > increase
> >> >> >> > complexity of this code. If you increase the complexity, I won't
> >> >> >> > be
> >> >> >> > able
> >> >> >> > to
> >> >> >> > refactor it anymore because I'm struggling to do that even for the
> >> >> >> > current
> >> >> >> > code.
> >> >> >> >
> >> >> >> > In addition to that, the MIPS multi-GOT ABI doesn't seem a right
> >> >> >> > design
> >> >> >> > to
> >> >> >> > me. If multi-GOT is in use, only the first GOT is recognized as a
> >> >> >> > real
> >> >> >> > GOT
> >> >> >> > by the dynamic linker, and secondary GOTs are just some sections
> >> >> >> > that
> >> >> >> > simulates GOT. It's too hacky, isn't it?
> >> >> >> >
> >> >> >> >> > I wonder what is the performance penalty you would have to pay
> >> >> >> >> > when
> >> >> >> >> > you
> >> >> >> >> > use the -mxgot
> >> >> >> >> > option. With the option, you'll need three instructions as
> >> >> >> >> > opposed
> >> >> >> >> > to
> >> >> >> >> > a
> >> >> >> >> > single instruction
> >> >> >> >> > to access an GOT entry. Does that actually make observable
> >> >> >> >> > difference
> >> >> >> >> > in
> >> >> >> >> > performance?
> >> >> >> >>
> >> >> >> >> Regular (without -mxgot) access to GOT requires a single
> >> >> >> >> instruction:
> >> >> >> >>
> >> >> >> >> lw  t9,0(gp)
> >> >> >> >>
> >> >> >> >> I was wrong when say about two instructions. With -mxgot option
> >> >> >> >> we
> >> >> >> >> get
> >> >> >> >> three instructions.
> >> >> >> >>
> >> >> >> >> lui     at,0x0
> >> >> >> >> addu    at,at,gp
> >> >> >> >> lw      t9,0(at)
> >> >> >> >>
> >> >> >> >> In case of MIPS global offset table is used not only to call
> >> >> >> >> external
> >> >> >> >> functions / access
> >> >> >> >> external data but for local calls / access under some conditions.
> >> >> >> >> So
> >> >> >> >> using -mxgot we can
> >> >> >> >> easily grow the code size and reduce performance.
> >> >> >> >
> >> >> >> >
> >> >> >> > How much is the actual performance hit?
> >> >> >>
> >> >> >> Multi-GOT is an attempt to bypass say limitation of MIPS
> >> >> >> architecture.
> >> >> >> It's not my invention, this feature was implemented in GNU linker
> >> >> >> more
> >> >> >> than ten years ago. Every time when GOT exceeds ~64KB limit BFD and
> >> >> >> gold linkers create multi-GOT layout.
> >> >> >>
> >> >> >> I do not think that my implementation of multi-GOT makes LLD much
> >> >> >> more
> >> >> >> complicated. General idea remains the same - collect information
> >> >> >> about
> >> >> >> various type of required GOT entries, layout GOT entries, write this
> >> >> >> layout. Merging multiple GOT created for each file into larger GOT
> >> >> >> is
> >> >> >> rather complicated routine though. From another side, creating a
> >> >> >> separate GOT for each input file makes possible to parallelize this
> >> >> >> process. Current implementation, where MipsGotSection maintains a
> >> >> >> single `GotEntries` vector for all files, does not allow to process
> >> >> >> multiple input files at the same time without some sort of "locks".
> >> >> >
> >> >> >
> >> >> > I understand that you are just trying to implement a MIPS ABI, and I
> >> >> > also
> >> >> > understand that you made your effort to write good code. Your code
> >> >> > seems
> >> >> > to
> >> >> > be a straightforward implementation of the ABI if I understand it
> >> >> > correctly.
> >> >> > But still new code inevitably adds complexity, and that's
> >> >> > particularly
> >> >> > true
> >> >> > for this patch that introduces a new notion of "multi-GOT" only for
> >> >> > MIPS.
> >> >> > Also, it is not my fault to say that this feature is too odd, because
> >> >> > I
> >> >> > think it's a consequence of MIPS ABI's peculiarities. I believe many
> >> >> > peculiarities in the MIPS ABI could have been fixed by now since they
> >> >> > were
> >> >> > implemented more than 10 years ago.
> >> >> >
> >> >> > I really do not want to add this much complexity to our relocation
> >> >> > processing code which is already too complicated. Even I don't
> >> >> > understand
> >> >> > the exact behavior of the current code, and I'm am trying to refactor
> >> >> > that
> >> >> > code now. This patch could make my refactoring impossible.
> >> >> >
> >> >> >> Performance degradation in case of using -mxgot depends on
> >> >> >> application. My tests show that application use -mxgot slower on
> >> >> >> 1%-4%. But it's more important that there are large applications
> >> >> >> which
> >> >> >> cannot be linked without multi-GOT at all even if they built with
> >> >> >> -mxgot option. Because there are some relocations which operate by
> >> >> >> 16-bit GOT index only.
> >> >> >>
> >> >>
> >> >> I don't necessarily disagree, but what you're saying is true for a
> >> >> bunch of features in MIPS, which is the in-tree lld backend with more,
> >> >> let's say, "peculiarities". I'm under the impression that as a project
> >> >> we should make a call and decide whether we want to support the MIPS
> >> >> ABI entirely or just don't support it at all (unless there's an OK
> >> >> reason to drop some features). In this case, unless I'm reading the
> >> >> patch incorrectly, this is needed to link/self-host clang/lld, so this
> >> >> feature seems needed (in some form). What do you think?
> >> >
> >> >
> >> > We should support MIPS, but my point is that 1) the current code is
> >> > already
> >> > complicated that even I do not understand, and 2) this patch will likely
> >> > to
> >> > make it impossible to hack anymore because of an additional complexity.
> >> > So
> >> > adding this code right now is not a good idea. I want to explore other
> >> > way
> >> > to circumvent this ABI, and if it is impossible to do, I want to
> >> > implement
> >> > it in a really good way, probably after cleaning up the current code
> >> > with
> >> > the multi-GOT in mind.
> >>
> >> As to me, "cleaning up the current code with the multi-GOT in mind" while
> >> "even I do not understand" is not the best strategy. Various parts of LLVM
> >> code are successfully refactored constantly and it does not prevent
> >> implementation
> >> of new features, targets, ABIs.
> >
> >
> > If it is designed cleanly, it might be true, but this is not the case. I
> > found that in order to refactor the code for relocation processing, I had to
> > understand the every detail of all architectures that LLD currently
> > supports, and editing it without breaking the existing implementation is
> > incredibly hard. And again, since MIPS is pretty different from other ABIs,
> > it is always the largest roadblock.
> >
> >> One more point is your desire "to implement it in a really good way".
> >> Could
> >> you please describe shortcomings of the current implementation? Probably
> >> if I know them, I will be able to make multi-GOT better.
> >
> >
> > The ABI is just too complicated. We already have probably too much code for
> > MIPS, while other ABIs are straightforward to support. If I knew that the
> > current MIPS ABI is this much different from other ELF ABIs from the
> > beginning, we could have this discussion before start working on MIPS. But
> > the real situation is that MIPS-specific features were added one at a time
> > without presenting the whole picture and grows to the point that I start
> > thinking if we really wanted this situation.
> 
> You say that MIPS ABI is always the largest roadblock. Why I have
> never listened from you or somebody else something like "I try to
> refactor this code but MIPS specific part prevents me to do this" or
> "I reorganized this code and made it cleaner but now some MIPS tests
> failed"?
> 
> LLD code contains ~20 "if (Config->EMachine == EM_MIPS)" statements
> for ~20000 lines of LLD code. Some of this "if" are just for adding
> MIPS specific dynamic table entries and for definition of MIPS
> specific symbols. Am I the only who do not think that it's
> unacceptable price for supporting one more widely used architecture by
> LLD? In fact, more than one architecture because MIPS32 / MIPS64 can
> be considered as a separate targets.
> 
> > Is "multi-GOT" the last feature you want for MIPS? I wonder if there any
> > more peculiarities in the MIPS ABI.
> 
> Now I do not know an answer on this question. For example LLD does not
> support microMIPS ABI. It is something like ARM Thumb. microMIPS
> implementation probably adds 3-5 new "if (MIPS)" expressions in the
> LLD code. A few week ago I though it is not a problem. Now I'm not
> sure.
> 
> >> And last but not least, what about at least rough time estimations? I
> >> would
> >> like to include multi-GOT in the next (4.1 ?) release of LLD because now
> >> LLD
> >> on MIPS is just partly usable. When do you plan to finish the LLD
> >> refactoring
> >> so it will be ready to include multi-GOT implementation?
> >
> >
> > I cannot promise, but I personally want to finish it in a few months.
> > However, finishing refactoring doesn't immediately mean that we want MIPS
> > multi-GOT support, as I wish to reduce complexity that MIPS brings in.
> >
> >> > Also, the MIPS ABI needs updating. Only MIPS is very weird among ELF
> >> > ABIs.
> >> > Someone really needs to make an effort to streamline it. I'm ok to
> >> > accept
> >> > MIPS peculiarities if it is transient ("transient" can be like 10
> >> > years),
> >> > but if no effort is being made to make it compatible with other ELF
> >> > ABIs, I
> >> > think I can say "no, this really needs fixing."
> >>
> >> New ABI is under development / discussions but transient period will be
> >> large
> >> because part of the MIPS ABI peculiarities have a hardware nature.
> >
> >
> > Where is it discussed?
> 
> I'm sorry but I cannot provide any details.
> 
> > I could imagine that if the new ABI will greatly simplified the MIPS ABI,
> > we'd be able to have a choice to support only the new ABI in LLD and
> > continue using the existing linker for the old ABI.
> >
> > I'm sorry to say this, but as you know I don't really like the current MIPS
> > ABI and the current situation that no improvements seem to have been made to
> > improve it, although it could have been fixed, say, 10 years ago. I don't
> > like to blindly support it just because it is the current ABI.
> 
> Do you suggest to drop MIPS support from LLD? Multi-GOT is essential
> feature and without it the linker is not completely useless but
> definitely cannot be named as a linker for MIPS targets. For example,
> guys who work on FreeBSD will never be able to make LLD a system
> linker for MIPS targets. Do you think it is a good price for keeping
> the LLD code cleaner?

Hi Rui,

I'd like to find out what your expectation would be about MIPS getting a new ABI?
I'm sure you will be aware of the great pains every architecture endures to
provide compatibility when extending either hardware architecture or software ABI
features. Changing an ABI in an incompatible way or simply inventing a new ABI
entirely means a complete binary break for all users. In that sense it means
Debian mipsel, mips, mips64el distributions would be dead, Android mipsel would
be dead, the BSDs could possibly survive better (as they are build-from-source
distros as I understand) but the porting effort would be large.

It is unfortunately us, as tools engineers, that bear the brunt of the effort to
keep things working and as much as I 100% agree about the pain of dragging
along weird MIPS history there are very few options for reducing its complexity
without breaking compatibility. The last ABI change for MIPS was to extend o32
to allow the use of 64-bit FPRs and the design and implementation of just that
tweak took nearly 18 months end-to-end.

Having been exposed to MIPS ABIs now for several years I have come to realise
that the oddities in MIPS are not half-hearted nor without merit as you can
usually find a performance/size/safety reason for the feature but the effort
to implement them is often high.

While I know that other architectures have added new ABIs I am not aware of any
major architecture creating a breaking change in its main ABI or dropping its
main ABI (at least not unless the architecture changed such that the ABI could
not be supported anymore).

None of this helps of course in trying to keep LLD's internals clean unless each
oddity is promoted to a first class feature. There is always the chance that this
would lead to other architectures using the feature... because it would be easy.
multi-GOT is quite difficult to understand and implement but the same can
be said about almost every aspect of dynamic linking; it's just that other
oddities are commonly known now and accepted. I would say TLS fits into the same
category as multi-GOT when it was first conceived.

Obviously each project will make decisions about what architectures to support
and also how much history to cope with but in this instance I believe we are
looking at a support MIPS or not kind of decision. In terms of adding MIPS
features to LLD I believe Simon's approach is the only possible way with fast
moving projects related to LLVM as the infrastructure changes so fast that you
have to design and submit small pieces. I'm not sure it would ever have been
possible to deep dive into every MIPS ABI feature up front in LLD and get them
accounted for in the design as it is almost always true that community maintained
projects are cautious about allowing features to be designed ahead of use in
fear of the rest of the work never getting finished.

I wish there was something we could do that would trivially reduce the complexity
of MIPS ABIs but just absorbing all the subtleties of the current ABI let alone
deciding both what is better and a compatibility story is easily years-worth of
dedicated design. The benefit of which is not likely to register to ordinary
users so the impetus is low.

Thanks,
Matthew