[llvm-dev] LLVM as a back end for HHVM

[Brett Simmers via llvm-dev]

On 9/10/15 9:43 AM, Philip Reames wrote:
> On 09/08/2015 05:29 PM, Brett Simmers wrote:
>> On 9/8/15 9:35 AM, Philip Reames wrote:
>>> On the VM state synching side of things, I'm curious about the tradeoffs
>>> involved in the approach you've used.  I'm guessing from what you said
>>> that you're essentially pre-reserving a set of allocas for the spill
>>> locations, somehow registering those with your runtime once, then
>>> emitting stores down the unlikely path into those allocas.  Is that
>>> roughly right?
>>
>> It's even simpler than that: the stores to spill VM state go directly
>> to the VM locations where those values belong. We only ever side-exit
>> at bytecode boundaries, so every live value has a place to live on the
>> eval stack or in a local variable (every PHP function has a fixed
>> number of local variables, and space for those is allocated on the
>> eval stack). This means that the side-exit path doesn't have to know
>> if the next bytecode is going to executed by the interpreter or by
>> more jitted code, since they'll both read values from the same locations.
> Ah, gotcha.  I'd assumed your were intermixing your language and
> execution stacks.  Yeah, if you're maintaining them separately,
> transitioning becomes much easier.
>
> Just as a sanity check, the compiled code does end up forwarding all the
> loads and keeping everything in registers within a single compilation
> right?  I'd assume you'd have to for decent performance, but your
> description almost makes it sound like it doesn't.

Ah yeah, forgot to mention that we had a separate eval stack :). We've 
tossed around a few ideas about how to merge the C++ and PHP stacks but 
it would be a lot of work and I don't expect it to happen any time soon.

And yeah, within a single compilation unit we keep whatever we can in 
registers. There are a few cases where I've seen our optimizations miss 
something that LLVM cleaned up later but those are rare and tend to be 
in patterns that we haven't seen show up in hot code.

-Brett