[LLVMdev] Should we enable Partial unrolling and Runtime unrolling on AArch64?

[Hal Finkel]

----- Original Message -----
> From: "Kevin Qin" <kevinqindev at gmail.com>
> To: "LLVM Developers Mailing List" <llvmdev at cs.uiuc.edu>
> Sent: Thursday, July 31, 2014 3:03:19 AM
> Subject: [LLVMdev] Should we enable Partial unrolling and Runtime unrolling	on AArch64?
> 
> 
> 
> 
> 
> Hi all,
> 
> 
> Partial unrolling and runtime unrolling are enabled by default in
> aarch64 gcc which is help to get performance better. But these two
> methods are enabled for only several backends in LLVM which are X86,
> PowerPC and R600. I don't know the history of these two kinds of
> unrolling, and why they are not widely used. I also want to know is,
> for aarch64 backend, is it intentionally to get them disabled?
> 
> I've did some experiment around this and see the performance is
> indeed impacted. Overall, partial unrolling can bring small benefit
> on most cases of Benchmark and regression is major and small.
> Runtime unrolling can bring huge improvement on some certain cases
> but also huge regression on others. The proportion of improvement
> and regression varies in different Benchmark . Also, code size is
> increased for two both.
> 
> 
> I will show more information before this be changed. Here I just want
> to know more backgrounds of two unrolling methods.

These unrolling methods have been available in LLVM for several years, but the pass-manager setup and TTI hooks that enable backends to enable these in a target-specific way is relatively new. As you've noticed, per-target tuning is required. Patches are certainly welcome; if you have a modification for AArch64 that provides significant benefits and little downside, please send it to llvm-commits for review.

Thanks for looking at this.

 -Hal

> 
> 
> --
> 
> Best Regards,
> 
> 
> Kevin Qin
> 
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-- 
Hal Finkel
Assistant Computational Scientist
Leadership Computing Facility
Argonne National Laboratory