[llvm-dev] Metadata and compile time performance

Hal Finkel via llvm-dev llvm-dev at lists.llvm.org
Fri Feb 19 11:40:59 PST 2016

----- Original Message -----

> From: "John Criswell via llvm-dev" <llvm-dev at lists.llvm.org>
> To: "Norilo Vesa-Petri" <vesa-petri.norilo at uniarts.fi>,
> llvm-dev at lists.llvm.org
> Sent: Friday, February 19, 2016 7:54:30 AM
> Subject: Re: [llvm-dev] Metadata and compile time performance

> On 2/19/16 7:47 AM, Norilo Vesa-Petri via llvm-dev wrote:

> > Dear LLVMers,

> > I’m investigating the response time of my JIT, and according to
> > profiling, optimization takes 85% of the compile time, while the
> > rest is being split evenly between the front-end and machine code
> > generation. Much of the optimizer time is spent in various alias
> > analysis passes.
> One thing you might want to do is look at the pass pipeline and see
> if the alias analysis passes are being re-run multiple times. If
> they are, then maybe the issue is getting alias analysis to run
> fewer times instead of trying to get it to run faster.

Unfortunately, our AA is mostly stateless, so this is not really going to be an issue. "Running" BasicAA, which is the most-expensive one, does not really do anything. Every time to make an AA query it walks use/def chains to determine an answer. 

Counterintuitively, using TBAA metadata also won't really help because the BasicAA query is always done first regardless. This is because for its primary use case (modeling C/C++ TBAA rules), we need to catch cases of disallowed aliasing from common type-punning idioms and still do what the user expects (i.e. recognize the aliasing). Adding restrict (noalias) on function arguments where you can should help (that will make the associated BasicAA logic cheaper). 

Depending on what you're doing, generating fewer GEPs that BasicAA needs to look through will also speed things up. 


> There could be two culprits for running analysis passes over and over
> again. First, it's possible that an analysis pass (or passes) are
> invalidated over and over again by optimizations. Changing the
> ordering of the optimizations or having key optimizations update the
> analysis information might fix that problem.

> Second, unless it's been changed, using a FunctionPass from within a
> ModulePass forces the FunctionPass to be re-run from scratch
> whenever the getAnalysisUsage<FunctionPass>() method is called. A
> poorly written ModulePass may cause a FunctionPass to be run over
> and over again needlessly. I doubt that the in-tree optimizations
> make this sort of mistake, but it is conceivably possible.

> To be clear, I don't know that this is the cause of your overhead,
> but if it were me, I'd check this first before trying to make alias
> analysis run faster.

> Regards,

> John Criswell

> > I’m happy with the generated code quality and wouldn’t like to
> > lower
> > the optimization level (O2).

> > Would generating alias metadata from the front-end help and speed
> > up
> > optimization?

> > If so, what kind of AA would have the best return on investment in
> > terms of compile time? TBAA is not applicable to my language, but I
> > could provide C99 restrict and related semantics fairly easily.

> > Thanks!!

> > Vesa Norilo
> > Lecturer
> > Centre for Music and Technology
> > University of Arts Helsinki

> > _______________________________________________
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> > llvm-dev at lists.llvm.org
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> --
> John Criswell
> Assistant Professor
> Department of Computer Science, University of Rochester
> http://www.cs.rochester.edu/u/criswell
> _______________________________________________
> LLVM Developers mailing list
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Hal Finkel 
Assistant Computational Scientist 
Leadership Computing Facility 
Argonne National Laboratory 
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