[LLVMdev] whole program optimization examples?

Mon Oct 13 18:52:33 PDT 2014

> On Oct 13, 2014, at 6:49 PM, Philip Reames <listmail at philipreames.com> wrote:
> 
> 
>> On 10/13/2014 06:17 PM, Filip Pizlo wrote:
>> 
>> 
>> On Oct 13, 2014, at 4:07 PM, Philip Reames <listmail at philipreames.com> wrote:
>> 
>>> 
>>>> On 10/13/2014 03:23 PM, Kevin Modzelewski wrote:
>>>> With the patchpoint infrastructure, shouldn't it now be relatively straightforward to do an accurate-but-non-relocatable scan of the stack, by attaching all the GC roots as stackmap arguments to patchpoints?  This is something we're currently working on for Pyston (ie we don't have it working yet), but I think we might get it "for free" once we finish the work on frame introspection.
>>> Take a look at the statepoint intrinsics up for review.  These are essentially exactly that, with two extensions:
>>> - A semantic distinction between gc roots and deopt state (since you may want both)
>>> - Support for explicit relocation of the gc root values (this could be made optional, but is currently not)
>>> 
>>> Though, you really don't want to emit these in your frontend.  You can, it'll work, but the performance will suffer.  Doing so will prevent many useful optimizations from running. 
>> 
>> You really should be specific here. The optimizations you're thinking of may be uninteresting to many clients.
> Assuming you have a VM which needs safepoints to occur at some fixed interval, you need to put a safepoint poll in *all* loops.  (Well, unless you can prove either a) the loop is bounded or b) there's another safepoint in the loop.)  Doing so, you introduce a call into the loop (using either approach).  This breaks loop recognition, complicates alias analysis and thus LICM, and is otherwise bad for the optimizer.

A multithreaded high-throughout VM will have deopt safe points in any loop that isn't proven to terminate in a timely fashion. Those safe points will take all live state and this will be superset of your GC pointers. 

> 
>> Also you won't lose any performance if your GC pointers are also needed for deopt (which happens to be the common case).
> 
>> I really do think that this whole discussion is tragicomic. Most clients of LLVM would be best served with mostly copying GC.
> I believe LLVM should not take a position in this debate and should try to support all collectors.  

It's good to encourage people to use state-of-the-art, easy-to-implement techniques rather than unnecessarily complicated ones. 

-Filip

>> 
>> -Filip
>> 
>> 
>>> Instead, you probably want to consider something like the late safepoint placement approach we've been pushing.  Hopefully, once the statepoint stuff lands, we can get that upstreamed fairly soon.  
>>> 
>>> Philip
>>> 
>>>> 
>>>>> On Sat, Oct 11, 2014 at 11:37 PM, Filip Pizlo <fpizlo at apple.com> wrote:
>>>>> 
>>>>> 
>>>>> > On Oct 10, 2014, at 6:24 PM, Hayden Livingston <halivingston at gmail.com> wrote:
>>>>> >
>>>>> > Hello,
>>>>> >
>>>>> > I was wondering if there is an example list somewhere of whole program optimizations done by LLVM based compilers?
>>>>> >
>>>>> > I'm only familiar with method-level optimizations, and I'm being told wpo can deliver many great speedups.
>>>>> >
>>>>> > My language is currently staticly typed JIT based and uses the JVM, and I want to move it over to LLVM so that I can have options where it can be ahead of time compiled as well.
>>>>> 
>>>>> As Philip kindly pointed out, WebKit uses llvm as part of a JavaScript JIT optimization pipeline. It works well for WebKit, but this was a large amount of work. It may not be the path of least resistance depending on what your requirements are.
>>>>> 
>>>>> >
>>>>> > I'm hearing bad things about LLVM's JIT capabilities -- specifically that writing your own GC is going to be a pain.
>>>>> 
>>>>> This is a fun topic and you'll probably get some good advice. :-)
>>>>> 
>>>>> Here's my take. GC in llvm is only a pain if you make the tragic mistake of writing an accurate-on-the-stack GC. Accurate collectors are only known to be beneficial in niche environments, usually if you have an aversion to probabilistic algorithms. You might also be stuck requiring accuracy if your system relies                   on being able to force *every* object to *immediately* move to a new location, but this is an uncommon requirement - usually it happens due to certain speculative optimization strategies in dynamic languages.
>>>>> 
>>>>> My approach is to use a Bartlett-style mostly-copying collector. If you use a Bartlett-style collector then you don't need any special support in llvm. It just works, it allows llvm to register-allocate pointers at will, and it lends itself naturally to high-throughput collector algorithms. Bartlett-style collectors come in many shapes and sizes - copying or not, mark-region or not, generational or not, and even a fancy concurrent copying example exists.
>>>>> 
>>>>> WebKit used a Bartlett-style parallel generational sticky-mark copying collector with opportunistic mark-region optimizations. We haven't written up anything about it yet but it is all open source.
>>>>> 
>>>>> Hosking's paper about the concurrent variant is here: http://dl.acm.org/citation.cfm?doid=1133956.1133963
>>>>> 
>>>>> I highly recommend reading Bartlett's original paper about conservative copying; it provides an excellent semi space algorithm that would be a respectable starting point for any VM. You won't regret implementing it - it'll simplify your interface to any JIT, not just llvm. It'll also make FFI easy because it allows the C stack to refer directly to GC objects without any shenanigans.
>>>>> 
>>>>> Bartlett is probabilistic in the sense that it may, with low probability, increase object drag. This happens rarely. On 64-bit systems it's especially rare. It's been pretty well demonstrated that Bartlett collectors are as fast as accurate ones, insofar as anything in GC land can be demonstrated (as in it's still a topic of lively debate, though I had some papers back in the day that showed some comparisons). WebKit often wins GC benchmarks for example, and we particularly like that our GC never imposes limitations on llvm optimizations. It's really great to be able to view the compiler and the collector as orthogonal components!
>>>>> 
>>>>> >
>>>>> > Anyways, sort of diverged there, but still looking for WPO examples!
>>>>> >
>>>>> > Hayden.
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>>>> 
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