[llvm-dev] Multi-Threading Compilers

Nicholas Krause via llvm-dev llvm-dev at lists.llvm.org
Thu Mar 19 17:37:48 PDT 2020

On 3/19/20 5:31 PM, Johannes Doerfert wrote:
> On 3/18/20 9:05 PM, Nicholas Krause wrote:
> >
> >
> > On 3/18/20 9:49 AM, Nicolai Hähnle wrote:
> >> On Wed, Mar 18, 2020 at 7:23 AM Nicholas Krause via llvm-dev
> >> <llvm-dev at lists.llvm.org> wrote:
> >>> On 3/3/20 8:37 PM, Chris Lattner wrote:
> >>>
> >>> On Feb 28, 2020, at 6:03 PM, Chris Lattner <clattner at nondot.org> 
> wrote:
> >>>
> >>>
> >>> On Feb 28, 2020, at 8:56 AM, Johannes Doerfert 
> <johannesdoerfert at gmail.com> wrote:
> >>>
> >>> On 02/28, Nicholas Krause via llvm-dev wrote:
> >>>
> >>> Anyhow what is the status and what parts are we planning to move to
> >>> MLIR in LLVM/Clang.  I've not seen any discussion on that other than
> >>> starting to plan for it.
> >>>
> >>>
> >>> As far as I know, there is no (detailed/discussed/agreed upon/...) 
> plan
> >>> to move any existing functionality in LLVM-Core or Clang to MLIR. 
> There
> >>> are some people that expressed interest in there is Chris's plan 
> on how
> >>> the transition could look like.
> >>>
> >>>
> >>> Yep, agreed, I gave a talk a couple days ago (with Tatiana) with a 
> proposed path forward, but explained it as one possible path.  We’ll 
> share the slides publicly in a few days after a couple things get 
> taken care of.
> >>>
> >>>
> >>> Hi all,
> >>>
> >>> Here is a link to the CGO presentation slides (outlining a 
> possible path to incremental adoption of MLIR in Clang) for anyone 
> curious.
> >>>
> >>> -Chris
> >>>
> >>> Greetings,
> >>> As to David Blaike's suggestion I'm merging the two threads for 
> this discussion. The original commenters is Johannes Doefert
> >>> starting with Hey,:
> >>>
> >>> 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.
> >>>
> >>> As was discussed on the previous thread - generally the assumption 
> is that one wouldn't try to run two function optimizations on the same 
> function at the same time, but, for instance - run function 
> optimizations on unrelated functions at the same time (or CGSCC passes 
> on distinct CGSCCs). But this is difficult in LLVM IR because use 
> lists are shared - so if two functions use the same global variable or 
> call the same 3rd function, optimizing out a function call from each 
> of those functions becomes a write to shared state when trying to 
> update the use list of that 3rd function. MLIR apparently has a 
> different design in this regard that is intended to be more amenable 
> to these situations.
> >> As mentioned on the other thread, the main challenge here is in the
> >> use lists of constant values (which includes also globals and
> >> functions). Right now, those are linked lists that are global for an
> >> entire LLVMContext. Every addition or removal of a use of a constant
> >> has to touch them, and doing such fine-grained locking doesn't seem
> >> like a great idea.
> > GCC has the same issues it terms of certain core structures so not
> > really surprised.
> >>
> >> So this is probably the biggest and seemingly most daunting thing
> >> you'd have to address first, but it's feasible and seems like a good
> >> idea to evolve LLVM IR in a direction where it ends up looking more
> >> like MLIR and can avoid these locks.
> > Sure that makes sense I will see what Johannes wants to start with.
> I think addressing this issue first makes sense. I would however start
> by determining the actual impact of different design choices here. I
> mean, do we know locks will be heavily contented? If I had to guess I'd
> say most passes will not create or modify functions nor add or remove
> calls. I further guess that passes which create/query llvm::Constant
> values will do so for ConstantInt between -1 and 2, I mean most of the
> time. This might be wrong but we should for sure check before we
> redesign the entire constant handling (as MLIR did). My suggestion is to
> profile first. What we want is to monitor the use-list of constants but
> I'm not sure if that is easy off the top of my head. What we can do
> easily is to print a message in the methods that are used to "create" a
> constant, thus the constructors (of llvm::Constant) and the
> ConstantXXX::get() methods. We print the pass names and these "constant
> generation" messages in a run of the test suite and analyze the result.
> What passes create constants, how often, which (kind of) constants, etc.
> We should also determine if any pass ever walks the use list of
> constants. I know we do it for global symbols but I don't know we do it
> for others. That said, I think it is sensible to distinguish global
> symbols and other constants at some point because (I think) we use them
> differently.
> From there we decide how to move forward. Localized constants, as MLIR
> has them, some locking (or similar solution), or maybe just restrictions
> on the parallel execution of passes.
> I hope this makes some sense.
If your talking about this class:

Then yes that makes sense in terms of getting the data. The only three 
questions I would
ask are:
1. There are methods in pass classes for getting the name of them. 
However the problem
would be adding a switch statement to check what were passing into the 
constructor. I'm
not sure of what the top level getName e.t.c. class is. Therefore if 
there is one I'm assuming
Value or ModulePass. This makes it easier as you don't have to wonder 
about data types
but walk down the virtual hierarchy for this. Names at least to my 
knowledge are a top
level feature and we should just use it there. So just do:
XXX->GetName() where XXX is the top level class(es).
2. Lock contention is rather vague. For example we could have multiple 
readers and few writers.
So we would also need to figure that out in order to choose the right 
locks. Thoughts?
3. Does LLVM have something like the GCC compile farm as at most point 
were going to need
to test on larger machines for scaling and race conditions.

The only other thing is LLVM seems to be late rc so it doesn't matter to 
me but do we want to
wait for LLVM 10 final to be released as this is new work. I don't think 
it matters frankly as this
is way off being in mainline anything soon.

Thanks and sorry if I mistaken,

> Cheers,
>   Johannes

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