Metadata and compile time performance

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.

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

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. 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() 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

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

> 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.

-Hal

Could you share more detailed profiling information? It’s possible you’re tripping across a compile time bug or that seeing what sections are hot would given insights into how to tweak your code to avoid the problem. In general, if you can give “easier” answers, that’s always a good thing. If you’re using LLVM for inlining, you may find that “noalias” attributes are quite useful for controlling compile time (i.e. enable optimizations of callee before inlining). Without knowing more about your source language and where you’re currently spending time, I can’t really help more than that.