[llvm] r372232 - [SampleFDO] Minimize performance impact when profile-sample-accurate

[Hans Wennborg via llvm-commits]

This caused a ~70 KB size regression for Chrome on Android. Is this expected?

Also +gbiv for Chrome Android pgo.

We're tracking this in
https://bugs.chromium.org/p/chromium/issues/detail?id=1006159

On Wed, Sep 18, 2019 at 6:05 PM Wei Mi via llvm-commits
<llvm-commits at lists.llvm.org> wrote:
>
> Author: wmi
> Date: Wed Sep 18 09:06:28 2019
> New Revision: 372232
>
> URL: http://llvm.org/viewvc/llvm-project?rev=372232&view=rev
> Log:
> [SampleFDO] Minimize performance impact when profile-sample-accurate
> is enabled.
>
> We can save memory and reduce binary size significantly by enabling
> ProfileSampleAccurate. However when ProfileSampleAccurate is true,
> function without sample will be regarded as cold and this could
> potentially cause performance regression.
>
> To minimize the potential negative performance impact, we want to be
> a little conservative here saying if a function shows up in the profile,
> no matter as outline instance, inline instance or call targets, treat
> the function as not being cold. This will handle the cases such as most
> callsites of a function are inlined in sampled binary (thus outline copy
> don't get any sample) but not inlined in current build (because of source
> code drift, imprecise debug information, or the callsites are all cold
> individually but not cold accumulatively...), so that the outline function
> showing up as cold in sampled binary will actually not be cold after current
> build. After the change, such function will be treated as not cold even
> profile-sample-accurate is enabled.
>
> At the same time we lower the hot criteria of callsiteIsHot check when
> profile-sample-accurate is enabled. callsiteIsHot is used to determined
> whether a callsite is hot and qualified for early inlining. When
> profile-sample-accurate is enabled, functions without profile will be
> regarded as cold and much less inlining will happen in CGSCC inlining pass,
> so we can worry less about size increase and be aggressive to allow more
> early inlining to happen for warm callsites and it is helpful for performance
> overall.
>
> Differential Revision: https://reviews.llvm.org/D67561