[LLVMdev] Testing the new CFL alias analysis

[Hal Finkel]

----- Original Message -----
> From: "Gerolf Hoflehner" <ghoflehner at apple.com>
> To: "Jiangning Liu" <liujiangning1 at gmail.com>, "George Burgess IV" <george.burgess.iv at gmail.com>, "Hal Finkel"
> <hfinkel at anl.gov>
> Cc: "LLVM Dev" <llvmdev at cs.uiuc.edu>
> Sent: Sunday, September 14, 2014 12:15:02 AM
> Subject: Re: [LLVMdev] Testing the new CFL alias analysis
> 
> In lto+pgo some (5 out of 12 with usual suspect like perlbench and
> gcc among them using -flto -Wl,-mllvm,-use-cfl-aa
> -Wl,-mllvm,-use-cfl-aa-in-codegen) the CINT2006 benchmarks don’t
> compile.

On what platform? Could you bugpoint it and file a report?

> Has the implementation been tested with lto?

I've not.

> If not, please
> stress the implementation more.
> Do we know reasons for gains? Where did you expect the biggest gains?

I don't want to make a global statement here. My expectation is that we'll see wins from increasing register pressure ;) -- hoisting more loads out of loops (there are certainly cases involving multiple-levels of dereferencing and insert/extract instructions where CFL can provide a NoAlias answer where BasicAA gives up). Obviously, we'll also have problems if we increase pressure too much.

> Some of the losses will likely boil down to increased register
> pressure.

Agreed.

> 
> 
> Looks like the current performance numbers pose a good challenge for
> gaining new and refreshing insights into our heuristics (and for
> smoothing out the implementation along the way).

It certainly seems that way.

Thanks again,
Hal

> 
> 
> Cheers
> Gerolf
> 
> 
> 
> 
> 
> 
> 
> 
> 
> On Sep 12, 2014, at 1:27 AM, Jiangning Liu < liujiangning1 at gmail.com
> > wrote:
> 
> 
> 
> Hi Hal,
> 
> I run on SPEC2000 on cortex-a57(AArch64), and got the following
> results,
> 
> (It is to measure run-time reduction, and negative is better
> performance)
> 
> spec.cpu2000.ref.183_equake 33.77%
> spec.cpu2000.ref.179_art 13.44%
> spec.cpu2000.ref.256_bzip2 7.80%
> spec.cpu2000.ref.186_crafty 3.69%
> spec.cpu2000.ref.175_vpr 2.96%
> spec.cpu2000.ref.176_gcc 1.77%
> spec.cpu2000.ref.252_eon 1.77%
> spec.cpu2000.ref.254_gap 1.19%
> spec.cpu2000.ref.197_parser 1.15%
> spec.cpu2000.ref.253_perlbmk 1.11%
> spec.cpu2000.ref.300_twolf -1.04%
> 
> So we can see almost all got worse performance.
> 
> The command line option I'm using is "-O3 -std=gnu89 -ffast-math
> -fslp-vectorize -fvectorize -mcpu=cortex-a57 -mllvm -use-cfl-aa
> -mllvm -use-cfl-aa-in-codegen"
> 
> I didn't try compile-time, and I think your test on POWER7 native
> build should already meant something for other hosts. Also I don't
> have a good benchmark suit for compile time testing. My past
> experiences showed both llvm-test-suite (single/multiple) and spec
> benchmark are not good benchmarks for compile time testing.
> 
> Thanks,
> -Jiangning
> 
> 
> 2014-09-04 1:11 GMT+08:00 Hal Finkel < hfinkel at anl.gov > :
> 
> 
> Hello everyone,
> 
> One of Google's summer interns, George Burgess IV, created an
> implementation of the CFL pointer-aliasing analysis algorithm, and
> this has now been added to LLVM trunk. Now we should determine
> whether it is worthwhile adding this to the default optimization
> pipeline. For ease of testing, I've added the command line option
> -use-cfl-aa which will cause the CFL analysis to be added to the
> optimization pipeline. This can be used with the opt program, and
> also via Clang by passing: -mllvm -use-cfl-aa.
> 
> For the purpose of testing with those targets that make use of
> aliasing analysis during code generation, there is also a
> corresponding -use-cfl-aa-in-codegen option.
> 
> Running the test suite on one of our IBM POWER7 systems (comparing
> -O3 -mcpu=native to -O3 -mcpu=native -mllvm -use-cfl-aa -mllvm
> -use-cfl-aa-in-codegen [testing without use in code generation were
> essentially the same]), I see no significant compile-time changes,
> and the following performance results:
> speedup:
> MultiSource/Benchmarks/mafft/pairlocalalign: -11.5862% +/- 5.9257%
> 
> slowdown:
> MultiSource/Benchmarks/FreeBench/neural/neural: 158.679% +/- 22.3212%
> MultiSource/Benchmarks/MiBench/consumer-typeset/consumer-typeset:
> 0.627176% +/- 0.290698%
> MultiSource/Benchmarks/Ptrdist/ks/ks: 57.5457% +/- 21.8869%
> 
> I ran the test suite 20 times in each configuration, using make -j48
> each time, so I'll only pick up large changes. I've not yet
> investigated the cause of the slowdowns (or the speedup), and I
> really need people to try this on x86, ARM, etc. I appears, however,
> the better aliasing analysis results might have some negative
> unintended consequences, and we'll need to look at those closely.
> 
> Please let me know how this fares on your systems!
> 
> Thanks again,
> Hal
> 
> --
> Hal Finkel
> Assistant Computational Scientist
> Leadership Computing Facility
> Argonne National Laboratory
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-- 
Hal Finkel
Assistant Computational Scientist
Leadership Computing Facility
Argonne National Laboratory