Re: Proposal to extend -Wcoverage-mismatch diagnostic cases

[Indu Bhagat <indu dot bhagat at oracle dot com>]

On 08/13/2018 06:32 AM, Martin Liška wrote:
> On 08/09/2018 08:05 PM, Indu Bhagat wrote:
> > Wcoverage-mismatch is meant to "Warn in case profiles in -fprofile-use do not
> > match"
> >
> > Currently, gcc compiler warns about the following two cases of source code
> > changes which alert the programmer to re-generate the profiles :
> >
> > 1. Control flow mismatch
> >     If functions' control flow has been altered after profile generation, the
> >     profile count data will not be able to match the cfg_checksum.
> >     Hence the warning, "warning: the control flow of function 'XXX' does not
> >     match its profile data (counter 'YYY') [-Werror=coverage-mismatch]"
> >
> > 2. Function is physically moved to a different location
> >     If existing functions are physically swapped in the source file, gcc alerts
> >     the user about potentially stale profiles for all the affected functions.
> >
> >     Hence the warning, "warning : source locations for function 'XXX' have
> >     changed, the profile data may be out of date [-Werror=coverage-mismatch]"
> >
> >     So, if new function foo() is added in the source file before existing
> >     functions, gcc alerts the user about potentially stale profiles of the
> >     already seen functions.
> >           Note however, there is no warning about the new function foo(). Hence, my
> >     proposal #3 below which will cover foo() in this scenario.
> Hello.
>
> I would appreciate a warning for that. Will you work on a patch or should I do
> it myself?

Thanks for your response. Yes, I will work on this. I will create a 
bugzilla for both these new warnings.
Indu

> > Proposing to add the following cases which are not currently covered :
> >
> > 3. If a new function foo() is added to the file. Proposal to add a warning along
> >     the lines of -
> >     "warning : profile for function ‘XXX’ not found in profile data [-Wcoverage-mismatch]"
> >        The corner case of foo() being added to the end of the file is exclusively
> >     covered by #3.
> >
> > 4. A new source file is added to the project after profile generation phase.
> >     Proposal to add a warning per function along the lines of -
> >     "warning : profile count data file not found, regenerating profiles may help [-Wcoverage-mismatch]"
> I was considering some time ago a warning when a profile is missing for a compilation unit. It's not
> only situation when a file is added. But one can also wipe somehow .gcda files in make clean and
> it worth informing a user. I tend to come up with a new warning maybe for it. Again are you planning
> to work on that?
>
> May I please ask you for a bugzilla entry both these issues?
>
> Thanks,
> Martin
>
> > For both #3 and #4, gcc currently does not warn. Although, one can infer that
> > the count profile data for a specific function / file are missing from the dump
> > files, adding warnings #3 and #4 makes the coverage-mismatch set of warnings
> > more complete for a better end user experience.