Re: Proposal to extend -Wcoverage-mismatch diagnostic cases

[Martin Liška <mliska at suse dot cz>]

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    changed, the profile data may be out of date [-Werror=coverage-mismatch]"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?

> 
> 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.
>