Reordering two functions can slow down lld by 1.06 times

[Mehdi Amini via llvm-commits]

The attachment to this PR: https://llvm.org/bugs/show_bug.cgi?id=5615 <https://llvm.org/bugs/show_bug.cgi?id=5615>
Explains why this could happen.

(This could well be a different case here, but it may be related, or hint toward a similar type of problem).

OTH.

— 
Mehdi

> On Oct 21, 2016, at 7:30 AM, Rafael Espíndola via llvm-commits <llvm-commits at lists.llvm.org> wrote:
> 
> This is sufficiently crazy that I decided to create an easy reproducible.
> 
> I uploaded it to https://drive.google.com/open?id=0B7iRtublysV6WmZPZzh5LUpSZUU
> 
> I also tested it on a i7-3840QM where the problem reproduces exactly
> and on a AMD Opteron(tm) Processor 6380 where the two binaries have
> exactly the same performance.
> 
> Craig, all that I was able to find about branch prediction alias
> problems was a suggestion on the intel optimization manual to align
> branch targets, but looks like that is not the problem here. Any idea
> if there is anything that can be done to avoid this problem?
> 
> Thanks,
> Rafael
> 
> 
> On 20 October 2016 at 17:09, Rafael Espíndola
> <rafael.espindola at gmail.com> wrote:
>> I spend most of the day reducing an oddity I noticed while
>> benchmarking a small patch.
>> 
>> It turns out that just reordering two adjacent functions can have a
>> massive impact on performance. The two binaries are in
>> 
>> https://drive.google.com/open?id=0B7iRtublysV6VW5VVW1na2N1RGM
>> 
>> https://drive.google.com/open?id=0B7iRtublysV6MUJoeGVCRHpXVUU
>> 
>> And the total diff of the objdump is attached.
>> 
>> When linking xul with one of the binaries I get
>> 
>> 98,298,725      branch-misses             #    2.24% of all branches
>> 7.206486289 seconds time elapsed
>> 
>> With the other I get
>> 
>> 139,849,372      branch-misses             #    3.18% of all branches
>> 7.645573494 seconds time elapsed
>> 
>> Adding enough padding before the function gets the performance back,
>> which suggests an aliasing problem in the branch predictor.
>> 
>> The cpu is a E5-2697 (Ivy Bridge). Is anyone familiar with its branch
>> predictor and how to avoid hitting these problems?
>> 
>> Cheers,
>> Rafael
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