Design discussion for atomic primops to land in 7.8

[Austin Seipp]

To do this, IMO we'd also really have to start shipping our own copy
of LLVM. The current situation (use what we have configured or in
$PATH) won't really become feasible later on.

On platforms like ARM where there is no NCG, the mismatches can become
super painful, and it makes depending on certain features of the IR or
compiler toolchain (like an advanced, ISA-aware vectorizer in LLVM
3.3+) way more difficult, aside from being a management nightmare.

Fixing it does require taking a hit on things like build times,
though. Or we could use binary releases, but we occasionally may want
to tweak and/or fix things. If we ship our own LLVM for example, it's
reasonable to assume sometime in the future we'll want to change the
ABI during a release.

This does bring other benefits. Max Bolingbroke had an old alias
analysis plugin for LLVM that made a noticeable improvement on certain
kinds of programs, but shipping it against an arbitrary LLVM is
infeasible. Stuff like this could now be possible too.

In a way, I think there's some merit to having a simple, integrated
code generator that does the correct thing, with a high performance
option as we have now. LLVM is a huge project, and there's definitely
some part of me that thinks this may not lower our complexity budget
as much as we think, only shift parts of it around ('second rate'
platforms like PPC/ARM expose way more bugs in my experience, and
tracking them across such a massive surface area can be quite
difficult.) It's very stable and well tested, but an unequivocal
dependency on hundreds of thousands of lines of deeply complex code is
a big question no matter what.

But, the current NCG isn't that 'simple correct thing' either, though.
I think it's easily one of the least understood parts of the compiler
with a long history, it's rarely refactored or modified (very unlike
other parts,) and it's maintained only as necessary. Which doesn't
bode well for its future in any case.


On Mon, Aug 26, 2013 at 3:19 PM, Simon Marlow <marlowsd at gmail.com> wrote:
> On 26/08/13 08:17, Ben Lippmeier wrote:
>>
>>
>>>     > Well, what's the long term plan?  Is the LLVM backend going to
>>>     become the only backend at some point?
>>>
>>>     I wouldn't argue against ditching the NCG entirely. It's hard to
>>>     justify fixing NCG performance problems when fixing them won't
>>>     make the NCG faster than LLVM, and everyone uses LLVM anyway.
>>>
>>>     We're going to need more and more SIMD support when processors
>>>     supporting the Larrabee New Instructions (LRBni) appear on
>>>     people's desks. At that time there still won't be a good enough
>>>     reason to implement those instructions in the NCG.
>>>
>>> I hope to implement SIMD support for the native code gen soon. It's
>>> not a huge task and having feature parity between LLVM and NCG would
>>> be good.
>>
>>
>>  Will you also update the SIMD support, register allocators, and calling
>> conventions in 2015 when AVX-512 lands on the desktop? On all supported
>> platforms? What about support for the x86 vcompress and vexpand
>> instructions with mask registers? What about when someone finally asks
>> for packed conversions between 16xWord8s and 16xFloat32s where you need
>> to split the result into four separate registers? LLVM does that
>> automatically.
>>
>> I've been down this path before. In 2007 I implemented a separate graph
>> colouring register allocator in the NCG to supposably improve GHC's
>> numeric performance, but the LLVM backend subsumed that work and now
>> having two separate register allocators is more of a maintenance burden
>> than a help to anyone. At the time, LLVM was just becoming well known,
>> so it wasn't obvious that implementing a new register allocator was a
>> largely a redundant piece of work -- but I think it's clear now. I was
>> happy to work on the project at the time, and I learned a lot from it,
>> but when starting new projects now I also try to imagine the system that
>> will replace the one I'm dreaming of.
>>
>> Of course, you should do what interests you -- I'm just pointing out a
>> strategic consideration.
>
>
> The existence of LLVM is definitely an argument not to put any more effort
> into backend optimisation in GHC, at least for those optimisations that LLVM
> can already do.
>
> But as for whether the NCG is needed at all - there are a few ways that the
> LLVM backend needs to be improved before it can be considered to be a
> complete replacement for the NCG:
>
> 1. Compilation speed.  LLVM approximately doubles compilation time. Avoiding
> going via the textual intermediate syntax would probably help here.
>
> 2. Shared library support (#4210, #5786).  It works (or worked?) on a couple
> of platforms.  But even on those platforms it generated worse code than the
> NCG due to using dynamic references for *all* symbols, whereas the NCG knows
> which symbols live in a separate package and need to use dynamic references.
>
> 3. Some low-level optimisation problems (#4308, #5567).  The LLVM backend
> generates bad code for certain critical bits of the runtime, perhaps due to
> lack of good aliasing information.  This hasn't been revisited in the light
> of the new codegen, so perhaps it's better now.
>
> Someone should benchmark the LLVM backend against the NCG with new codegen
> in GHC 7.8.  It's possible that the new codegen is getting a slight boost
> because it doesn't have to split up proc points, so it can do better code
> generation for let-no-escapes. (It's also possible that LLVM is being
> penalised a bit for the same reason - I spent more time peering at
> NCG-generated code than LLVM-generated code).
>
> These are some good places to start if you want to see GHC drop the NCG.
>
> Cheers,
>         Simon
>
>
>
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-- 
Regards,
Austin - PGP: 4096R/0x91384671