[foreign-memaccess] on value kinds

Fri Jul 19 13:24:46 UTC 2019

> Comments?

It seems that the need for embedding structural carrier info is specific 
to implementing ABI bahaviour. So, part of the choice seems to be 
whether we want to use an ABI agnostic API to encode the ABI specific 
stuff into (like layout annotations), or if we want to create ABI 
specific abstractions (e.g. have a CStruct type).

Maybe we ultimately need a mix of both (note that we already have a 
pretty C specific AddressLayout added), but for now I think layout 
annotations are the way to go, also since inventing something like a 
CStruct would almost make GroupLayouts redundant it seems.

Looking at memaccess, and the fact that we never actually use the 'kind' 
of a value, I think morally (all) the value kinds should be moved to 
annotations.

Jorn

On 2019-07-19 13:45, Maurizio Cimadamore wrote:
> Picking up this again.
> 
> I've been doing more thinking on this topic, and it seems to me that
> attaching a notion of 'kind' on a layout seems suboptimal for a number
> of reasons:
> 
> * layouts should be about sizes, alignments and endianness - not 
> semantics
> 
> * by attaching kinds to layouts we end up replicating some of the
> information that is already available in carrier types
> 
> * a kind-based system feels 'arbitrary' and it is difficult to extend
> - unless we resort to plain strings/annotations
> 
> * some kinds are just plain useless - e.g. no difference between
> signed vs. unsigned
> 
> Now, I'd very much like to propose to just get rid of ValueLayout
> kinds. After all, when dereferencing memory the _carrier_ will drive
> the process and make sure that the right semantics is applied; so,
> carriers are for _semantics_, layouts are, well, for layouts! Another
> advantage is that, if we do this, padding layouts just disappears -
> just some unnamed value layout.
> 
> But, if you pull on this string, while things are still perfectly fine
> for the memory access API (we always have a carrier when we need to
> dereference), we run into an issue with ABI classification. Let's
> simplify things a bit, and assume there are three main categories of
> values that a foreign function has to deal with:
> 
> 1) scalars (e.g. int, float, long double)
> 2) pointers (function pointers, object pointers)
> 3) composites (structs/unions)
> 
> Now, eliminating kinds from ValueLayout will have zero consequences on
> (1) and (2). After all, the layout + carrier info is always enough to
> do a basic classification - e.g. (using the SysV terminology)
> 
> byte.class, char.class, short.class, integer.class, long.class ->
> INTEGER or MEMORY (if no register available)
> float.class, double.class -> SSE or MEMORY (if no register available)
> MemoryAddress -> POINTER or MEMORY (if no register available)
> 
> (we can also have extra carriers for exotic types such as x87).
> 
> This is all good and well - maybe we'll have to tweak the
> classification routines a little to work not just on layouts, but
> layout + carrier, but it's all doable.
> 
> But what about (3) ? The classification routines for structs/unions
> are mind-bogglingly complex (at least in SysV), and you need _full_
> knowledge of the ins and outs of a struct in order to classify it
> ABI-wise. That is, you have to know whether the struct fields belong
> in (1), (2) or (3), recursively.
> 
> And here's the issue - if we use MemorySegment as a carrier for _all_
> structs/unions, that carrier is just not powerful enough to allow us
> to do that kind of recursive classification! Our answer to this has
> been: don't use carriers for recursive stuff - just use layouts, which
> is why we have the FP vs. INT distinction in the ValueLayout. So, in a
> way, while there are many arguments for pushing kind info outside
> layouts, there are also strong arguments in favor of keeping it in.
> 
> If we were to completely drop kinds from layouts, I see only two 
> options:
> 
> 1) Put the client in charge of recursive classification - that is,
> essentially, eliminate (3) from the picture - and lower all struct
> arguments to a sequence of (1) and (2).
> 
> 2) Invent a new carrier type that is powerful enough to embed
> structural carrier info:
> 
> e.g.
> 
> static Struct of(MemorySegment segment, Class<?>... fieldCarriers)
> 
> Now, as much as I see the appealing simplicity of (1), I can't help
> but feeling that it pushes the problem around, it doesn't completely
> address it. I imagine that no user will really want to 'decompose' a
> memory segment into multiple chunks by hand before passing it down to
> a native function. Which brings up a (big) question:
> 
> "On how on earth are we going to infer a MH adaptation from the
> signature the user expects and the signature the ABI expects?"
> 
> I don't see any way to address that question, without, again, doing
> some hacks to layouts --- or inventing a completely new kind of
> description for functions that is expressive enough to capture all the
> moving parts, but in that case we can just use that new description to
> solve (3) ?
> 
> Some of these problems, as it appears are not 100% new, and have been
> discussed in the context of the LLVM project:
> 
> http://lists.llvm.org/pipermail/llvm-dev/2019-January/129137.html
> 
> The thread is _very_ interesting - although some of the specifics of
> the solution are different, there is a strong correlation with what's
> being discussed here, and with the fact that declarative ways to
> specify calling convention work pretty well for calls which only takes
> scalar values, but kind of break apart for composite (as stated
> above).
> 
> 
> All things considered, I think kind-less layouts with abi-specific
> annotations still features the best bang for bucks ratio than any of
> the alternatives we've considered. And, if we wanted to support some
> minimal subset of kinds - I think adding distinction between floating
> point and integral is probably the most crucial and ABI-agnostic one
> we can possibly come up with, something that will basically reduce the
> need for ABI annotations in 90% (more?) of cases. Signed vs. unsigned
> distinction, on the other hand, should just go away - it doesn't add
> any material difference to how arguments get classified by ABIs, and
> it mostly represent a user-level distinction (and layouts should not
> be in the business of capture that degree of distinctions).
> 
> Comments?
> 
> Maurizio
> 
> 
> On 15/07/2019 22:18, Maurizio Cimadamore wrote:
>> Hi,
>> as I was (re)starting the works on the second step of the Panama 
>> pipeline (foreign function access), it occurred to me that one piece 
>> of the design for ValueLayout is not 100% flushed out. I'm referring 
>> to the different 'kinds' of value layouts available in the API:
>> 
>> * signed int
>> * unsigned int
>> * floating point
>> 
>> We made this distinction long ago - the intention was to capture 
>> important distinctions between different layouts in an explicit 
>> fashion. For instance, system ABI typically pass integer values via 
>> general register, while they pass floating point values via floating 
>> point or vector registers. So it seemed an important distinction to 
>> capture.
>> 
>> When I later started to work on support for x87 types, I realized that 
>> it wasn't all that simple - a "long double" in SysV ABI is typically 
>> encoded as a 128 bit floating point using the x87 extended precision 
>> format [1], but so is a "binary128" which instead uses the quad 
>> precision format [2]. In other words, the kind/size pair does not 
>> unambiguously denote a specific type semantics. Moreover, x87 types 
>> only really make sense when it comes to the SysV ABI, and the 
>> implementation of that ABI will have to ask whether a certain layout 
>> is that of an x87 floating point value - which brings up the question 
>> on how are these special, platform-dependent layouts denoted in the 
>> first place?
>> 
>> Since Panama layouts support annotations, we always had the annotation 
>> route available to us to distinguish between these different types - 
>> that is:
>> 
>> f128[abi=x87]
>> 
>> could denote an extended precision x87 value, whereas:
>> 
>> f128[abi=quadfloat]
>> 
>> could denote a 128 floating point value using the 'quad' float format 
>> (binary128).
>> 
>> This is of course still a viable option - yes, the memory access API 
>> no longer have general purpose annotations, but it's easy enough to 
>> add them back in as part of the System ABI support, and then retrofit 
>> layout 'names' as a special kind of annotation - that's a move we have 
>> pulled in the past and we know it works.
>> 
>> But looking at this problem with fresh eyes, I'm noting an asymmetry, 
>> one that John pointed out in the past: the set of kinds supported by 
>> ValueLayout seem somewhat arbitrary, fixed and non-extensible in ways 
>> other than using annotations. What is the advantage of being able to 
>> tell an 'int' from a 'float' if we can't tell a 'x87' double from a 
>> 'quad float' ? Why is the former distinction supported _natively_ by 
>> the layout descriptions, whereas the latter is only supported 
>> indirectly, via annotations?
>> 
>> Of course, we know that former proposals, such as LDL [3], precisely 
>> for this reason, decided not to embed any semantics in their 'kinds'. 
>> That is, LDL really has only bits and group of bits - all the 
>> semantics is specified via annotations. This is a more symmetric 
>> approach - there are no 'blessed' kinds, everything happens through 
>> annotations. This is certainly a fine decision when designing a layout 
>> language with a given fixed grammar.
>> 
>> But, using annotations inside layouts is also a very indirect 
>> approach. Can we do better?After all, it seems that, if we leverage 
>> the fact that layouts are API elements, or objects, we can formulate 
>> an alternate solution where:
>> 
>> * value layouts _cannot be created_ you have to use one of the 
>> pre-baked constants - we have already discussed introducing layout 
>> constants in [4] anyway
>> * among the constants, users will find some that are ABI-specific 
>> (e.g. there will be one constant for x87 values, one for quadfloat, 
>> and so forth).
>> * testing 'is this layout a x87 layout?' reduces to an equality test 
>> (e.g. "layout == SYSV_X87")
>> 
>> Something like this would allow us to have layouts which are 
>> _internally_ general enough to express system ABI specific types - but 
>> at the level of the public API, a layout for a 128-bit x87 value would 
>> be the same as the one for a quad float - there would be no way for 
>> the user to tell them apart, other than noting that the two layouts 
>> correspond to different pre-baked constants. And this is, perhaps, a 
>> good outcome - after all, the distinction between these two layouts is 
>> a _semantic_ distinction, not (strictly speaking) a layout one (in 
>> fact the layout is, in terms of size and alignment, indeed the same in 
>> both cases). Therefore, it is very likely that this semantic 
>> distinction will only be of interest to very critical component of the 
>> Panama runtime - and that most of the clients will not care much about 
>> the distinction (other than maybe occasionally testing for "is this an 
>> x87 layout").
>> 
>> Thoughts?
>> 
>> Maurizio
>> 
>> [1] - 
>> https://en.wikipedia.org/wiki/Extended_precision#IEEE_754_extended_precision_formats
>> [2] - 
>> https://en.wikipedia.org/wiki/Quadruple-precision_floating-point_format
>> [3] - http://cr.openjdk.java.net/~jrose/panama/minimal-ldl.html
>> [4] - 
>> https://mail.openjdk.java.net/pipermail/panama-dev/2019-July/005908.html
>> 
>> 
>> 
>> 
>> 
>> 
>> 