code generation and backends Re: Design discussion for atomic primops to land in 7.8
carter.schonwald at gmail.com
Tue Aug 27 19:55:48 CEST 2013
Austin also raises an important point:
Its high time we explore changes to the ABI/per architecture calling
convention and how they might impact performance.
I think this is something worth exploring after the 7.8 release
(but not in the next month of prerelease engineering, though I need to suss
out some of that a bit more with the parties who care)
I had started working on a backwards compatible ABI change for llvm a few
months back that I could easily get LLVM folks to merge in, but I'd rather
only give the LLVM folks 1 set of ABI change patches within a 6month
period, where we've had the time to experiment with breaking changes as an
option too, rather than given them a partial patch now, and then a breaking
changes calling convention patch a few months from now.
(because if we don't bundle LLVM with GHC, we need to actually tie ABI
versions to LLVM major version releases, which isn't a problem, but just
requires thoughtful cooperation and such)
On Tue, Aug 27, 2013 at 12:51 PM, Austin Seipp <aseipp at pobox.com> wrote:
> 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
> > into backend optimisation in GHC, at least for those optimisations that
> > can already do.
> > But as for whether the NCG is needed at all - there are a few ways that
> > 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.
> > going via the textual intermediate syntax would probably help here.
> > 2. Shared library support (#4210, #5786). It works (or worked?) on a
> > of platforms. But even on those platforms it generated worse code than
> > NCG due to using dynamic references for *all* symbols, whereas the NCG
> > which symbols live in a separate package and need to use dynamic
> > 3. Some low-level optimisation problems (#4308, #5567). The LLVM backend
> > generates bad code for certain critical bits of the runtime, perhaps due
> > lack of good aliasing information. This hasn't been revisited in the
> > of the new codegen, so perhaps it's better now.
> > Someone should benchmark the LLVM backend against the NCG with new
> > 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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> Austin - PGP: 4096R/0x91384671
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