Static Data Re: llvm

22 Sep 2013

      agreed on most of those points

@john, no, that would be deeply wrong and dangerous (i think), the way to
have a global IORef is using unsafePerformIO, which is actually quite safe
if you have a good reason for introducing a globally visible shared
reference.

theres probably a bunch of sublties, but the notion of  compile time data
that the code generator can safely use has a number of awesome
applications. One other one would be the ability to have static lookup
arrays baked into the binary, rather than generated at runtime.

I'll be at icfp tomorrow mid afternoon through at least the first half of
the week, i'll pester the relevant folks for feedback and thoughts about
this (sub)topic.

having static data would tie into a number of features i want in ghc and
associated code gen, and I'm quite willing to pester and do the associated
engineering to make sure static data, or some more systematic feature thats
more general. Good thing i'm decent at pestering and ok at engineering.

If there are any technical objections to adding support for static data,
i'm all ears.
 I'm quite willing to do the leg work to make it happen, but its something
that will touch enough pieces of GHC's entire pipeline (lightly, but it
will), that some collaborative thought on making this feature work nicely
would be great!

cheers,
-Carter

On Fri, Sep 20, 2013 at 6:05 PM, John Lato  wrote:
...
@carter you're completely correct, typed template haskell would be very
nice for this.
I think something like asStatic would be fine. It would be slightly
annoying to not be able to use numeric literals, but it seems like a good
first step at any rate.
It would probably be possible to generate a Static value at runtime via
the ghc api. But perhaps not, I haven't played with the typed template
haskell enough to know.
As to constructions like Static (IORef Int), that seems to capture the
semantics of a global IORef, maybe it could be useful for that sort of
application?
On Sep 20, 2013 3:19 PM, "Carter Schonwald" 
wrote:
...
@John, thats actually close to the sort of near term proposal i've been
thinking of too!
We could probably make it nicer by using the new typed template haskell
(which geoff was/is also working on!).
http://ghc.haskell.org/trac/ghc/blog/Template%20Haskell%20Proposal
With typed template haskell, couldn't we have something like
asStatic :: alpha -> TExp (Static alpha),
where asStatic is the only way to introduce terms with a type (Static
alpha)? (ie: we could only introduce static values that are closed, at Th
time, which would also prevent us from construction a value of type (Static
alpha) at runtime?)
I think that would be enough to introduce the right compile time Static
Data property. That would also probably capture the right notion of static
for a number of other uses cases (I think. Probably would need a type class
constraint on alpha, to rule out crazy things like Static (IORef Int) or
the like ).
we'd probably need to also have a fromStatic :: Static a -> a, too.
But you're absolutely right, with a teeny bit of work, we could have at
least a near term work around like that. Theres probably some subtlties i'm
overlooking though
On Fri, Sep 20, 2013 at 2:09 PM, John Lato  wrote:
...
I think Geoffrey's suggestion is workable, but I wouldn't like it so
much.  The difficulty is that if you aren't aware of it, it's possible to
build up a large system that you discover is unworkable the first time you
try to compile it.
In that sense it's not too different from Template Haskell staging
restrictions, which IIRC exist to deal with exactly this problem.
As the necessity of compile-time constants has shown up at least twice,
a more principled solution is worth investigating.
In the meantime, a horrible hack would be something like:
newtype CStatic v = CStatic ExpQ
    instance Num a => Num ( CStatic a) where
      fromInteger x = [| x |]
and then the value could be spliced at the call site. Staging
restrictions would ensure it's available at compile time. I guess the
instance decl needs Lift too.
Downsides are general hideous-ness, misleading error messages, and the
necessity for compiling with template haskell. But you'd get an error if s
value isn't a compile-time constant. (Please don't put this in ghc, but
it's not so bad as a separate lib).
John L.
On Sep 19, 2013 3:44 PM, "Geoffrey Mainland" 
wrote:
...
If you pass a constant, unboxed value to a primop, I assume GHC won't
ever bind the argument to a value. So although we have no way to enforce
"static const argument" in the type system, if this is a valuable (and
experts-only?) operation, I'm not sure it matters that much if the user
gets an error at code-generation time complaining about non-const
arguments.
Another way to look at it: if we wait until someone enhances the type
system to support the notion of static arguments, we will likely never
have a bit shuffle primitive.
The other option would be to fall back on a different implementation if
we saw a non-constant argument. I think that would actually be worse
than erroring out, but I'm sure others would disagree.
Geoff
...
tldr; we can't express / expose the LLVM shuffleVector intrinsic in a
type safe way that will correctly interact with the static argument
requirement for associated code generation.
On Thu, Sep 19, 2013 at 12:40 AM, Carter Schonwald
mailto:carter.schonwald@gmail.com>
wrote:
yup, i hit a gap in what we can currently express in haskell
    types. We don't have a way of expressing static data! I actually
    put ticket on trac noting
    this. http://ghc.haskell.org/trac/ghc/ticket/8107
    (note that when i was initially writing the ticket, i incorrectly
    thought the int# arg to ghc's prefetch was the locality level
    rather than a byte offset)
Currently GHC has no way of expressing "this argument needs to be
    a static compile/codegen time constant" in surface haskell or
    core! This means we could at best provide a suite of special cased
    operations. (eg: we could provide the inter-lane shuffle for
    swapping halves of YMM registers, and the miniature analogue for
    XMM), but that would really be missing the point: being able to
    write complex algorithms that can work completely in registers!
the vast majority of the simd shuffle operations have certain
    arguments that need to be compile time static values that are used
    in the actual code generation. The llvm data model doesn't express
    this constraint. This invariant failure was also hit internally
    recently  via a bug in how GHC generated code  for llvm's
    memcopy! http://ghc.haskell.org/trac/ghc/ticket/8131
If we could express llvm'sshuffleVector
    http://llvm.org/docs/LangRef.html#shufflevector-instruction
    intrinsic in a type safe way, then we could express any of them. I
    would be over the moon if we could expose an operation like
    shuffleVector, but I dont' think GHC currently can express it in a
    type safe way that won't make LLVM vomit.
I want simd shuffle, but i don't see how to give the fully general
    shuffle operations in type safe ways with ghc currently. We need
    to add support for some notion of static data first! If theres a
    way, i'm all for it, but I don't see such a way.
I hope that answers your question. that seems to be a deep enough
    issue that theres no way to resolve it with simple engineering in
    the next few weeks.
-Carter
On Wed, Sep 18, 2013 at 9:41 PM, Geoffrey Mainland
    mailto:mainland@apeiron.net> wrote:
On 09/18/2013 04:49 PM, Carter Schonwald wrote:
        > I've some thoughts on how to have a better solution, but
        they are
        > feasible only on a time scale suitable for 7.10, and not for
        7.8.
        >
        > a hacky solution we could do for 7.8 perhaps is have a
        warning that
        > works as follows:
        >
        > either
        > a)
        > throw a warning on functions that use the SIMD primops, if
On 09/19/2013 11:42 AM, Carter Schonwald wrote:
that
...
> function is being exported by a module, and that function
        isn't marked
        > NOINLINE ? Theres probably a few subtleties to it, and this
        is just a
        > naive idea
        That wouldn't inform the consumers of a module. And for a
        library like
        vector, we definitely want to export unfoldings for code that
        contains
        SIMD primops. That's the only way to get good code out of the
        library!
        > b) somehow put both the -fllvm and -fasm core for inlineable
        functions
        > in the .hi file? (this one prevents the most problems, but
        is probably
        > the most complex work around we could do).
        The problem being that there *is* no -fasm code...because the
NCG
...
doesn't support SIMD operations. Unless we added a mechanism
        to have two
        completely different, but simultaneous, definitions for a
        function, one
        for -fasm and one for -fllvm. But that would be a lot of work
and
...
couldn't be done for 7.8.
        >
        >
        > its worth noting that the LLVM simd in 7.8, either way,
        won't support
        > simd shuffles, which will seriously curtail its general
utility,
...
> either way.
You told me you would send me example use cases, type
        signatures, etc.
        Did I miss an email? If this is very important to you, was
there a
...
particular difficulty you had implementing these primops?
> On Wed, Sep 18, 2013 at 4:22 PM, Simon Marlow
        mailto:marlowsd@gmail.com
        > mailto:marlowsd@gmail.com>>
wrote:
...
>
        >     On 18/09/13 20:01, Geoffrey Mainland wrote:
        >
        >         We did discuss this, but you may not have been
present.
...
>
        >         If LLVM-only primops show up in a non-LLVM codegen,
        a "sorry"
        >         error is
        >         reported telling the user that they need to compile
with
...
>         "-fllvm". Yes,
        >         this is not a fantastic solution. Options I see:
        >
        >         1) Live with the error message.
        >         2) Remove all SIMD support until the NCG catches up.
        >         3) Figure out a mechanism that avoids inlining any
        code containing
        >         LLVM-only primops when we're not using the LLVM back
        end.
        >
        >         Maybe you can think of another solution?
        >
        >
        >     Those are the three unsatisfactory solutions that I know
        of.  Even
        >     if we did (3), the user still wants to know when that is
        happening
        >     because they're getting less good code, so you'd want a
        warning.
        >
        >     One thing we might try to do is automatically enable
        -fllvm when
        >     the compilation would otherwise fail.  If LLVM isn't
        installed and
        >     the compilation still fails, it's no worse than failing
        to compile
        >     the module with the sorry error.
        >
        >     Simon
        >
        >
        >
        >         Geoff
        >
        >         On 09/18/2013 02:54 PM, Simon Marlow wrote:
        >
        >             This is slightly problematic.  What if we have a
        wonderful
        >             SIMD-enabled vector library that we compile with
        -fllvm,
        >             and then use
        >             it in a program that isn't compiled with -fllvm,
        and some
        >             of the
        >             wonderful SIMD-enabled functions get inlined?
         Presumably
        >             we get a
        >             panic in the NCG.
        >
        >             Did we discuss this before? I have vague
        memories, but
        >             don't remember
        >             what the outcome was.
        >
        >             Cheers,
        >                  Simon
        >
        >             On 12/09/13 03:10, Geoffrey Mainland wrote:
        >
        >                 We support compiling some code with -fllvm
        and some
        >                 not in the same
        >                 executable. Otherwise how could users of the
        Haskell
        >                 Platform link their
        >                 -fllvm-compiled code with
        native-codegen-compiled
        >                 libraries like
        >                 base, etc.?
        >
        >                 In other words, the LLVM and native back
        ends use the
        >                 same calling
        >                 convention. With my SIMD work, they still
        use the same
        >                 calling
        >                 conventions, but the native codegen can
        never generate
        >                 code that uses
        >                 SIMD instructions.
        >
        >                 Geoff
        >
        >                 On 09/11/2013 10:03 PM, Johan Tibell wrote:
        >
        >                     OK. But that doesn't create a problem
        for the code
        >                     we output with the
        >                     LLVM backend, no? Or do we support
        compiling some
        >                     code with -fllvm and
        >                     some not in the same executable?
        >
        >
        >                     On Wed, Sep 11, 2013 at 6:56 PM,
        Geoffrey Mainland
        >                     mailto:mainland@apeiron.net
        >                     mailto:mainland@apeiron.net>
        >                     mailto:mainland@apeiron.net
        >                     mailto:mainland@apeiron.net>>> wrote:
        >
        >                           We definitely have interop
between the
...
>                     native codegen and the LLVM
        >                           back
        >                           end now. Otherwise anyone who
        wanted to use
        >                     the LLVM back end
        >                           would have
        >                           to build GHC themselves. Interop
        means that
        >                     users can install the
        >                           Haskell Platform and still use
        -fllvm when
        >                     it makes a performance
        >                           difference.
        >
        >                           Geoff
        >
        >                           On 09/11/2013 07:59 PM, Johan
        Tibell wrote:
        >                           > Do nothing different than you're
        doing for
        >                     7.8, we can sort
        >                     it out
        >                           > later. Just put a comment on the
        primops
        >                     saying they're
        >                           LLVM-only. See
        >                           > e.g.
        >                           >
        >                           >
        >                           >
        >
        >
https://github.com/ghc/ghc/blob/master/compiler/prelude/primops.txt.pp#L181
...
>                           >
        >                           > for an example how to add docs
        to primops.
        >                           >
        >                           > I don't think we need interop
        between the
        >                     native and the LLVM
        >                           > backends. We don't have that now
        do we
        >                     (i.e. they use different
        >                           > calling conventions).
        >                           >
        >                           >
        >                           >
        >                           > On Wed, Sep 11, 2013 at 4:51 PM,
        Geoffrey
        >                     Mainland
        >                           > mailto:mainland@apeiron.net
        >                     mailto:mainland@apeiron.net>
        >                     mailto:mainland@apeiron.net
        >                     mailto:mainland@apeiron.net>>
        >                           mailto:mainland@apeiron.net
        >                     mailto:mainland@apeiron.net>
        >                     mailto:mainland@apeiron.net
        >                     mailto:mainland@apeiron.net>>>>
        >                     wrote:
        >                           >
        >                           >     On 09/11/2013 07:44 PM,
        Johan Tibell
        >                     wrote:
        >                           >     > On Wed, Sep 11, 2013 at
        4:40 PM,
        >                     Geoffrey Mainland
        >                           >     mailto:mainland@apeiron.net
        >                     mailto:mainland@apeiron.net>
        >                     mailto:mainland@apeiron.net
        >                     mailto:mainland@apeiron.net>>
        >                           mailto:mainland@apeiron.net
        >                     mailto:mainland@apeiron.net>
        >                     mailto:mainland@apeiron.net
        >                     mailto:mainland@apeiron.net>>>>
        >                     wrote:
        >                           >     > > Do you mean we need a
        reasonable
        >                     emulation of the SIMD
        >                           primops for
        >                           >     > > the native codegen?
        >                           >     >
        >                           >     > Yes. Reasonable in the
        sense that it
        >                     computes the right
        >                           result.
        >                           >     I can
        >                           >     > see that some code might
        still want
        >                     to #ifdef (if the
        >                           fallback isn't
        >                           >     > fast enough).
        >                           >
        >                           >     Two implications of this
        requirement:
        >                           >
        >                           >     1) There will not be SIMD in
        7.8. I
        >                     just don't have the
        >                           time. In fact,
        >                           >     what SIMD support is there
        already
        >                     will have to be
        >                     removed if we
        >                           >     cannot
        >                           >     live with LLVM-only SIMD
        primops.
        >                           >
        >                           >     2) If we also require
        interop between
        >                     the LLVM back-end and
        >                           the native
        >                           >     codegen, then we cannot pass
        any SIMD
        >                     vectors in
        >                           registers---they all
        >                           >     must be passed on the stack.
        >                           >
        >                           >     My plan, as discussed with
        Simon PJ,
        >                     is to not support SIMD
        >                           primops at
        >                           >     all with the native codegen.
        If there
        >                     is a strong feeling
        >                     that
        >                           >     this *is
        >                           >     not* the way to go, the I
        need to know
        >                     ASAP.
        >                           >
        >                           >     Geoff
        >                           >
        >                           >
        >                           >
        >
        >
        >
        >
        >
        >
        >
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