[LLVMdev] FW: Insight of 403050abcc091260be2e8f58484e7a39c0782b47? (original) (raw)

Lawrence lawrence at codeaurora.org
Tue Jul 14 17:42:56 PDT 2015

+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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