[foreign-memaccess] on value kinds

Fri Jul 19 11:45:31 UTC 2019

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
>
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