Proposal to add FDO profile quality related diagnostics

Indu Bhagat
Tue Nov 27 23:34:00 GMT 2018

Usecase : The usecase is to get a high-level assessment of the quality of
function profiles available (-Wcoverage-mismatch and -Wmissing-profile are
useful in the same spirit). For large codebases, it is useful to make the
profile quality information more handy for ease of development. Doing a full
gcov run is more cumbersome than keeping the warning flag ENABLED in the
codebase at compile/development time to catch these.

For these reasons, I do think that atleast adding the summary-level diagnostic
is useful in general -
  "warning: XX out of YY functions not exercised in training run [-Wprofile-quality]"
This lets the user make the decision of whether and how to drill down.

Now for drill down, I agree gcov/lcov is a good choice. But a full gcov run is
sometimes a deterrent for large codebases.
For this reason, my proposal has a second-level option in the form of
opt-info msgs/-Wprofile-quality=all (depending on whether we choose OPTION 1 or

Regarding the function level detail being too noisy : I sort of agree with that
comment. But I am of the opinion that I would rather leave it to the user to
infer the profile quality as per the application characteristics.


On 11/22/2018 03:58 AM, Martin Liška wrote:
> On 11/21/18 4:47 AM, Andi Kleen wrote:
>> Indu Bhagat<>  writes:
>>> Proposal to add diagnostics to know which functions were not run in the
>>> training run in FDO.
>> Don't you think the warning will be very noisy? I assume most programs
>> have a lot of cold error handling functions etc. that are never executed
>> in a normal execution.
>> Like how does it look like for a gcc build for example?
> I guess like this:
> Indu, have you thought about using gcov/lcov rather than inventing a new warning
> (or opt info)?
> Can you please define exactly your use case?
> Martin
>> I suspect it would need some heuristics to cut it down at least, like if
>> the function ends with exit (perhaps propagated through the call tree)
>> it's not flagged.  Or if there is warning for a function don't warn
>> about callees that are not called from somewhere else.
>> -Andi

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