[LLVMdev] whole program optimization examples?

Filip Pizlo fpizlo at apple.com
Sat Oct 11 23:37:44 PDT 2014

> 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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