[llvm-dev] Reach Out about Module Infrastructure Multi-Threading

Nicolai Hähnle via llvm-dev llvm-dev at lists.llvm.org
Wed Mar 18 06:47:15 PDT 2020

Hi Nick,

On Wed, Mar 18, 2020 at 5:07 AM Nicholas Krause via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
> I was researching this on and off for the last few months in terms of figuring out how to make the pass manager itself async. Its not easy and I'm not even
> sure if that's possible. Not sure about GIMPLE as I would have to ask the middle end maintainer on the GCC side but LLVM IR does not seem to have shared
> state detection or the core classes and same for the back ends. So yes this would interest me.
> The first place to start with is which data structures are shared for sure. The biggest ones seem to be basic blocks and function definitions in terms of shared state, as
> those would be shared by passes running on each function.  We should start looking at implementing here locks or ref counting here first if your OK with that.
> It also allows me  to understand a little more concrete the linkage between the core classes as would be required for multi threading LLVM. In addition,
> it allows us to look into partitioning issues with threads at the same thing in terms of how to do it.

I think it's great that you're looking into this!

As David already mentioned, the gnarliest immediate issue is that
constant values (which includes global variables and functions) track
uses via a linked list. So in the current design, being able to do
concurrent transformations within the same LLVMContext would require
taking a lock whenever a use of a constant is added or removed. That's
obviously too much overhead.

So one of the first steps you'd have to do would be to change that.
That may seem a bit daunting initially, but it's feasible and seems
like a good idea to evolve LLVM IR in the direction of what MLIR does


> If others want to chip it as I've CCed the list, that's fine as well and this should be up for discussion with the whole community.
> I've given up on the idea of a async pass manager as it seems to require IR level detection of changed state between passes but maybe I'm wrong,
> Nick
> What sounds interesting to you?
> Cheers,
>   Johannes
> On Tue, Mar 17, 2020, 08:09 Nicholas Krause <xerofoify at gmail.com> wrote:
>> Johanne,
>> I'm assuming you've been busy as so have I am so I'm pinging this email.
>> Nick
>> On 3/6/20 7:57 PM, Nicholas Krause wrote:
>> > Greetings Johannes,
>> >
>> > I'm pinging you as you wanted to discuss the ModulePass and related
>> > infrastructure in terms of
>> > making it either multi-threading or preparing for that. You stated to
>> > wait to this week so now
>> > seems a good time.
>> >
>> >
>> > Regards,
>> > Nick
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