llvm calling convention matters

Wed Sep 18 20:49:52 UTC 2013

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
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).

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.

-Carter

On Wed, Sep 18, 2013 at 4:22 PM, Simon Marlow <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>> 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<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>>**>
>>>>> 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>>**>
>>>>> 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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