[PATCH] D44847: [profile] Fix value profile runtime merging issues

[Xinliang David Li via llvm-commits]

On Mon, Mar 26, 2018 at 11:24 AM Rong Xu via Phabricator <
reviews at reviews.llvm.org> wrote:

> xur added inline comments.
>
>
> ================
> Comment at: lib/profile/InstrProfilingFile.c:230
> +  // (which can leave garbage at the end of profile file).
> +  if (!VPMergeHook)
> +    truncate(ProfileFileName, 0);
> ----------------
> davidxl wrote:
> > xur wrote:
> > > davidxl wrote:
> > > > xur wrote:
> > > > > davidxl wrote:
> > > > > > non value profiling data has fixed size, so why is this needed?
> > > > > Because there are still value counts in both in-memory and in-disk
> profiles.
> > > > >
> > > > > Using the test in the patch as the example (running on my machine).
> > > > > assuming the binary is named as 'gen' and VPMergeHook null.
> > > > >
> > > > > if we don't do online merge,
> > > > > "gen" will generate a profile of size 320 bytes.
> > > > > "gen 1" will generate a profile with size of 352 bytes
> > > > >
> > > > > If we run "gen 1 && gen", the result binary is still 352 bytes.
> > > > > But the effective size is 320 bytes. The tail 32 bytes are trash.
> > > > >
> > > > >
> > > > Without VpMergeHook, we should assert that there is no value profile
> data -- it should not be dumped in the first place.
> > > current behavior is to write out the in-memory value profile data and
> discard the value profile data in the file. if there is not value profile
> from the file, the result is still correct.
> > >
> > > I'm not familiar with the use cast the not having libc (where
> VPMergeHook will be nullptr). Is it not suppose to have any value profile?
> > >
> > > I can assert VPMergeHook==nullptr and in-memory valuesites are not
> empty. That will be easy. Is this what you suggested?
> > In lprofGetVPDataReader method, returns null if VPMergeHook is null.
> This will generate incorrect profile: the valuesite in the data section
> says there are valuesites but VP count is empty.
>
>
We should assert that when VPMergeHook is null, the data section has no
value sites.  Do you see a possible case this does not hold?

David




> if we do this, the behavior would be: llvm-profdata show or merge command
> will stop silently at the first function that has the value-site
> instrumentation. The rest profile will be discarded.
>
> Note, of course, this only applies to  VPMergeHook==nullptr case.
>
>
> https://reviews.llvm.org/D44847
>
>
>
>
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