[LLVMdev] FW: Insight of 403050abcc091260be2e8f58484e7a39c0782b47?

[Lawrence]

+llvmdev

 

From: Lawrence [mailto:lawrence at codeaurora.org] 
Sent: Tuesday, July 14, 2015 5:42 PM
To: 'dberlin at dberlin.org'
Cc: 'Ana Pazos'; 'Sanjin Sijaric'; 'zinob at codeaurora.org'
Subject: RE: Insight of 403050abcc091260be2e8f58484e7a39c0782b47?

 

Thanks Daniel for your prompt response.

 

I understand the problem is a combination of multiple things, I have opened
at least one RA bug for it.

 

One thing I am not sure is should we add some register pressure mechanism to
your patch (or even PRE) or fix later optimization such as
rematerialization/split/spiller/coalescing(?) to handle that.

 

+llvmdev for more discussion

 

Will attached two log more detail to individual if anyone ask.

 

Regards.

 

Lawrence Hu

 

 

From: Lawrence [mailto:lawrence at codeaurora.org] 
Sent: Tuesday, July 14, 2015 3:16 PM
To: 'dberlin at dberlin.org'
Cc: 'Ana Pazos'; 'Sanjin Sijaric'; 'zinob at codeaurora.org'
Subject: Insight of 403050abcc091260be2e8f58484e7a39c0782b47?

 

Hi, Daniel:

 

I am investigating the performance of a version of open source
bzip2/decompress, I found out that your patch 

 

commit 403050abcc091260be2e8f58484e7a39c0782b47

Author: Daniel Berlin <dberlin at dberlin.org <mailto:dberlin at dberlin.org> >

Date:   Tue Feb 3 20:37:08 2015 +0000

 

    Allow PRE to insert no-cost phi nodes

 

changed some getelementptr to PHIs, which introduced a lot more long live
ranges, that leaded to almost 3 times more Spill/Reloads from register
allocation, as a result, it degraded the performance of bzip2/decompress by
16%.

 

Attached are two log files, log.good without your change, log.bad with your
change.

 

 

I couldn't understand your patch well just base on the description and
comments, could you please give me some insight about the intention of the
patch and possible way of limiting it to avoid that performance degradation?

 

Your help is highly appreciated.

 

Regards

 

Lawrence Hu.

 

 

 

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