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

[Nicholas Krause via llvm-dev]

On 3/17/20 6:16 PM, Johannes Doerfert wrote:
> Hey,
>
> Apologies for the wait, everything right now is going crazy..
Compiler Folks are very busy people as there aren't as much of us 
unfortunately so no need to
apologize. I've yet to heard from someone on the GCC side and will wait 
until after GCC 11
is released due to this. Also not to mention the health issues of 
Coronavirus-19.
>
> I think we should early in move this conversation on the llvm Dev list 
> but generally speaking we can see three options here:
> 1) parallelize single passes or a subset of passes that are known to 
> not interfer, e.g. the attributor,
> 2) parallelize analysis pass execution before a transformation that 
> needs them,
> 3) investigate what needs to be done for a parallel execution of many 
> passes, e.g. How can we avoid races on shared structure such as the 
> constant pool.
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.

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 
> <mailto: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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