llvm calling convention matters
Carter Schonwald
carter.schonwald at gmail.com
Fri Sep 20 20:19:01 UTC 2013
@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 <jwlato at gmail.com> 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" <mainland at apeiron.net> 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
>>
>> On 09/19/2013 11:42 AM, Carter Schonwald wrote:
>> > 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
>> > <carter.schonwald at gmail.com <mailto:carter.schonwald at 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
>> > <mainland at apeiron.net <mailto:mainland at 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
>> 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
>> > <marlowsd at gmail.com <mailto:marlowsd at gmail.com>
>> > > <mailto:marlowsd at gmail.com <mailto:marlowsd at 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
>> > > <mainland at apeiron.net
>> > <mailto:mainland at apeiron.net>
>> > > <mailto:mainland at apeiron.net
>> > <mailto:mainland at apeiron.net>>
>> > > <mailto:mainland at apeiron.net
>> > <mailto:mainland at apeiron.net>
>> > > <mailto:mainland at apeiron.net
>> > <mailto:mainland at 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
>> > > > <mainland at apeiron.net
>> > <mailto:mainland at apeiron.net>
>> > > <mailto:mainland at apeiron.net
>> > <mailto:mainland at apeiron.net>>
>> > > <mailto:mainland at apeiron.net
>> > <mailto:mainland at apeiron.net>
>> > > <mailto:mainland at apeiron.net
>> > <mailto:mainland at apeiron.net>>>
>> > > <mailto:mainland at apeiron.net
>> > <mailto:mainland at apeiron.net>
>> > > <mailto:mainland at apeiron.net
>> > <mailto:mainland at apeiron.net>>
>> > > <mailto:mainland at apeiron.net
>> > <mailto:mainland at apeiron.net>
>> > > <mailto:mainland at apeiron.net
>> > <mailto:mainland at apeiron.net>>>>>
>> > > wrote:
>> > > >
>> > > > On 09/11/2013 07:44 PM,
>> > Johan Tibell
>> > > wrote:
>> > > > > On Wed, Sep 11, 2013 at
>> > 4:40 PM,
>> > > Geoffrey Mainland
>> > > > <mainland at apeiron.net
>> > <mailto:mainland at apeiron.net>
>> > > <mailto:mainland at apeiron.net
>> > <mailto:mainland at apeiron.net>>
>> > > <mailto:mainland at apeiron.net
>> > <mailto:mainland at apeiron.net>
>> > > <mailto:mainland at apeiron.net
>> > <mailto:mainland at apeiron.net>>>
>> > > <mailto:mainland at apeiron.net
>> > <mailto:mainland at apeiron.net>
>> > > <mailto:mainland at apeiron.net
>> > <mailto:mainland at apeiron.net>>
>> > > <mailto:mainland at apeiron.net
>> > <mailto:mainland at apeiron.net>
>> > > <mailto:mainland at apeiron.net
>> > <mailto:mainland at 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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>
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