Re: [Qemu-devel] [PATCH for-1.4 qom-cpu 8/9 v6] target-i386: Topology & APIC ID utility functions

[Eduardo Habkost]

On Fri, Jan 25, 2013 at 06:27:47PM +0000, Blue Swirl wrote:
[...]
> >> > [1] If I set gcov-files-test-x86-cpuid-y = target-i386/topology.h, I get:
> >> >
> >> >   GTESTER tests/test-x86-cpuid
> >> >   Gcov report for target-i386/topology.h:
> >> >   target-i386/topology.gcno:cannot open graph file
> >> >
> >> > It looks like the .gcno file generation is per-object-file, not 
> >> > per-source-file
> >> > (gcov-files-*-y being a list of .c files confused me). If that's the 
> >> > case, then
> >> > the only valid value for gcov-files-test-x86-cpuid-y is really
> >> > tests/test-x86-cpuid.c, because all the tested code is being compiled 
> >> > inside
> >> > tests/test-x86-cpuid.o.
> >>
> >> But we are not interested in the coverage of the test suite code.
> >>
> >> Perhaps the design of gcov is not so well thought out and it is not
> >> very suitable for this kind of files.
> >
> > I agree it is not perfect, but including test-x86-cpuid.c in the list is
> > the only way to get a report of the topology.h test coverage. Do you see
> > any reason to not do it?
> 
> The statistics for topology.h would be slightly biased by
> test-x86-cpuid.c, meaning that even if test-x86-cpuid.c didn't test
> anything but still gained some coverage, the some total coverage would
> be reported. Also the opposite case could decrease 100% test coverage.

Good point. I believe it is more than "slightly biased", as all the
functions in topology.h are static inline: any code that wouldn't be
covered by the test generate any machine code at all.

I quickly tested this by adding extra functions to topology.h without
adding any test code, and coverage report was still 100%, as the
functions are inline and didn't generated any machine code in
test-x86-cpuid.o. That makes the topology.h coverage statistics useless
and misleading.

> 
> But I think this would only matter if we made a huge effort to reach
> 100% coverage for whole of QEMU and the opposite case somehow became
> the only block keeping us from reaching that 100%. Until that day
> comes (in the late 2050s perhaps, or it could be never if new code is
> always added faster than coverage is gained) slightly biased
> statistics are much better than no statistics at all.

In the topology.h case I would call it "completely useless" statistics,
as any percentage reported for that file would mean nothing. I prefer to
not have any coverage statistics from that file at all, than having
misleading statistics.

So I am OK with simply removing gcov-files-test-x86-cpuid-y from my
patch.

The only thing that gcov-files-test-x86-cpuid-y does is to make "make
check" print the (useless) percentages. Coverage reports can still be
generated manually by simply running "gcov tests/test-x86-cpuid.gcno" if
somebody wants to see the line-by-line coverage report.

-- 
Eduardo