[llvm-dev] Recent -Os code size regressions

Smith, Kevin B via llvm-dev
Sat, 21 Nov 2015 00:50:47 -0000

Maybe adjust this to be different for –Os, -Oz than for –O2?

Kevin Smith

From: llvm-dev [mailto:llvm-dev-bounces at lists.llvm.org] On Behalf Of James
Molloy via llvm-dev
Sent: Friday, November 20, 2015 4:05 PM
To: Steve King <steve at metrokings.com>; Renato Golin <renato.golin at
linaro.org>
Cc: llvm-dev <llvm-dev at lists.llvm.org>
Subject: Re: [llvm-dev] Recent -Os code size regressions

Hi Steve,

That commit gives significant performance improvements, so I'm not happy
reverting it because of the code size increase. A lot of the code size increase
is backend dependent, and I have a set of patches that improves the codegen
drastically on ARM at least. The midend is generating better code, and more
optimisable code (and it's about 30% more performance too).

James
On Sat, 21 Nov 2015 at 00:01, Steve King <steve at
metrokings.com<mailto:steve at metrokings.com>> wrote:
On Thu, Nov 19, 2015 at 1:10 PM, Renato Golin <renato.golin at
linaro.org<mailto:renato.golin at linaro.org>>
wrote:
> On 19 November 2015 at 19:08, Steve King via llvm-dev
> <llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>>
wrote:
>> Does the community have bots or humans tracking code size for -Os
>> builds?
>
> Hi Steve,
>
> I still haven't got around doing a CI for EEMBC or SPEC on ARM. I do
> track performance every release, but not code size at -Os.
>
>>  I've noticed troubling regressions lately.  Sometime near Nov
>> 5, the EEMBC bitmnp01 benchmark grew by 25% for ARMv7m and 35% for
>> i586.  That's ghastly.  This week, the EEMBC matrix01 workload grew
by
>> 5% for ARMv7m and 3% for i586.
>
> Hum, v7M is even lower priority for me at the moment. :)
>
> Though, I have to say, 25% is really bad. Can you bisect to see which
> commit was that?

Hi Renato, Thanks for advising.  The commit is:

[llvm] r252152 - [SimplifyCFG] Tweak heuristic for merging conditional stores

Can this be reverted until the surprising code size impact is
understood?  I'm about to leave for the week, so I can't delve further
anytime soon.

Regards,
-steve
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