[LLVMdev] [cfe-dev] [3.6 Release] RC3 has been tagged

[Jack Howarth]

On Wed, Feb 18, 2015 at 9:37 AM, Hal Finkel <hfinkel at anl.gov> wrote:
> ----- Original Message -----
>> From: "Joerg Sonnenberger" <joerg at britannica.bec.de>
>> To: cfe-dev at cs.uiuc.edu, "llvmdev" <llvmdev at cs.uiuc.edu>
>> Sent: Wednesday, February 18, 2015 7:42:11 AM
>> Subject: Re: [LLVMdev] [cfe-dev]  [3.6 Release] RC3 has been tagged
>>
>> On Tue, Feb 17, 2015 at 11:12:20AM -0800, Hans Wennborg wrote:
>> > I asked a few of the other developers, and the consensus was that
>> > while unfortunate, we won't block the release on a perf regression
>> > like this, at least not at this stage in the release process.
>>
>> Wasn't the result of the discussion much stronger -- the performance
>> regression seems to be more a result of hypersensitivity to specific
>> code generation choices and less a fundamental issue?
>
> There are several commits involved, and I don't believe we understand whether the issues are all hypersensitivity, or some other more systematic interactions. There is one issue, which involved a correctness fix for runtime unrolling that was done in a sub-optimal way (causing it to introduce an unnecessary comparison) that I think is well understood, and for which we have a fix (http://reviews.llvm.org/D7715). We should pick up this in the release. I think that, unfortunately, for the others, there's not sufficient time to investigate before the release.
>
>  -Hal

Hal,
     In my testing, we only had two commits that triggered the 22%
performance regression (r217953 and r222451) with each contributing to
half of the performance lost. The proposed fix in
http://reviews.llvm.org/D7715 entirely eliminates the regression due
to  r222451. Committing this fix will at least mitigate the regression
down to 11%.
             Jack

>
>>
>> Joerg
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