[LLVMdev] -O0 compile time speed

[Nick Lewycky]

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