llvm dev - Oct 2012 - [LLVMdev] "Multithreaded LLVM" in "Writing an LLVM Pass"

On Tue, Oct 09, 2012 at 08:35:57AM +0100, David Chisnall
wrote:
> On 8 Oct 2012, at 18:26, Lu Mitnick wrote:
> 
> > 1. Multiple threads handle different modules (Thread : Module = 1 : 1)
> > or
> 
> For ModulePasses, yes.
> 
> > 2. Multiple threads handle one module (Thread : Module = N : 1)
> 
> For FunctionPasses, yes.
> 
> Note that this part is in the future work section, meaning that no one has
done it yet.  FunctionPasses are independent of each other when running on
different functions, so it would be fairly simple to use something like
libdispatch to optimise every function within a module independently. 
FunctionPasses are not (in theory) allowed to modify any module state outside of
a function, so they should be able to be run concurrently.
  No one has done it yet? From [1], it seems parallel JIT is workable.
Or I just misread the question?

Regards,
chenwj

[1] http://lists.cs.uiuc.edu/pipermail/llvmdev/2012-January/046901.html
-- 
Wei-Ren Chen (陳韋任)
Computer Systems Lab, Institute of Information Science,
Academia Sinica, Taiwan (R.O.C.)
Tel:886-2-2788-3799 #1667
Homepage: http://people.cs.nctu.edu.tw/~chenwj

On 9 Oct 2012, at 08:55, 陳韋任 (Wei-Ren Chen) wrote:
>  No one has done it yet? From [1], it seems parallel JIT is workable.
> Or I just misread the question?
Your citation refers to a proprietary program that was presented at EuroLLVM -
their changes are not upstream.  They are running multiple MCJIT instances in
parallel, but they are completely independent - different modules and different
LLVMContexts, and so passes are completely independent (unless they do something
silly like have some mutable static variables).  This is a completely unrelated
problem.

David

2012/10/9 陳韋任 (Wei-Ren Chen) <chenwj at
iis.sinica.edu.tw>:
> On Tue, Oct 09, 2012 at 08:35:57AM +0100, David Chisnall wrote:
>> On 8 Oct 2012, at 18:26, Lu Mitnick wrote:
>>
>> > 1. Multiple threads handle different modules (Thread : Module = 1
: 1)
>> > or
>>
>> For ModulePasses, yes.
>>
>> > 2. Multiple threads handle one module (Thread : Module = N : 1)
>>
>> For FunctionPasses, yes.
>>
>> Note that this part is in the future work section, meaning that no one
has done it yet.  FunctionPasses are independent of each other when running on
different functions, so it would be fairly simple to use something like
libdispatch to optimise every function within a module independently. 
FunctionPasses are not (in theory) allowed to modify any module state outside of
a function, so they should be able to be run concurrently.
>
>   No one has done it yet? From [1], it seems parallel JIT is workable.
> Or I just misread the question?
MachinePasses don't change llvm::Module nor llvm::Function. I think
CodeGen could be prallelized.

In contrast, as Chandler mentions, Modifying Instruction(s) might
affect *hidden* state of globals, use-ref chain and refcount.

I think "transaction" might be useful here, too. See also
http://llvm.org/bugs/show_bug.cgi?id=2116

Hi David,

On 09/10/2012 09:02, David Chisnall wrote:
> On 9 Oct 2012, at 08:55, 陳韋任 (Wei-Ren Chen) wrote:
>
>>   No one has done it yet? From [1], it seems parallel JIT is workable.
>> Or I just misread the question?
>
> Your citation refers to a proprietary program that was presented at
EuroLLVM - their changes are not upstream.  They are running multiple MCJIT
instances in parallel, but they are completely independent - different modules
and different LLVMContexts, and so passes are completely independent (unless
they do something silly like have some mutable static variables).  This is a
completely unrelated problem.
As one of the authors of that paper, I'd just like to add a few points 
here. We didn't make any changes to the LLVM code base, so the idea can 
be implemented with the standard LLVM infrastructure. But you're right, 
we are exploiting parallelism on a higher level than Lu suggested: we 
are indeed using completely separate JIT instances (of the old JIT, in 
fact) so the problem of dependences between passes doesn't occur.

Tobias