[LLVMdev] Testing the new CFL alias analysis

Mon Sep 15 14:56:18 PDT 2014

On CINT2006 ARM64/ref input/lto+pgo I practically measure no performance difference for the 7 benchmarks that compile. This includes bzip2 (although different source base than in CINT2000), mcf,  hmmer, sjeng, h364ref, astar, xalancbmk

On Sep 15, 2014, at 11:59 AM, Hal Finkel <hfinkel at anl.gov> wrote:

> ----- 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?
Ok, I’ll see that I can get a small test case.
> 
>> 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.
Maybe. But I prefer the OoO HW to handle hoisting though. It is hard to tune in the compiler. 
I’m also curious about the impact on loop transformations.
> 
>> 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

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