[LLVMdev] -O0 compile time speed

[Chris Lattner]

The place to start would be to make 'llc' parallel on functions.  This  
would directly benefit lto and normal single file compiles.

-Chris

On Nov 23, 2009, at 12:21 AM, Nick Lewycky <nicholas at mxc.ca> wrote:

> Renato Golin wrote:
>> 2009/11/22 Jon Harrop<jon at ffconsultancy.com>:
>>> What about parallelization?
>>
>> I thought about that for a while, but if you keep your classes/files
>> small, intra-unit parallelization gains are probably not worth the
>> time invested. Compiling multiple files is embarrassingly parallel.
>> [1]
>
> Compiling multiple files is embarrassingly parallel ... which doesn't
> matter if your developers are doing a build+edit cycle on a single  
> file.
>
> I don't know about the backend, but I've thought about how to make the
> middle parallelized. We already have the notion of FunctionPass, but
> this running FPs in parallel will encounter a lot of contention around
> the reference counts for things like ConstantInt 'i32 0'. That led  
> me to
> my next idea, which is to turn the use lists for llvm::Constant  
> (except
> llvm::GlobalValue) off. Just remove that and you lose the lock
> contention. That means that you can't look at a Constant and iterate
> over its list of users, but then again, you already weren't doing  
> that.
>
> I might experiment with trying out a parallel optimizing LLVM --  
> mostly
> to find out what the problems really are -- but I suspect it won't
> really matter for performance until the backend is parallel.
>
> Nick
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