This is the mail archive of the mailing list for the GCC project.

Index Nav: [Date Index] [Subject Index] [Author Index] [Thread Index]
Message Nav: [Date Prev] [Date Next] [Thread Prev] [Thread Next]
Other format: [Raw text]

Re: [PATCH] Introduce run-time library (PR gcov-profile/84107).

On Wed, 29 Aug 2018, Martin Liška wrote:
> >> That said, it looks the shared library is overkill.
> > 
> > It feels like that somehow.
> > 
> >> Alexander may explain how that can be beneficial?
> >>
> >> Note that I can fix the segfault being caused by inter-DSO calls.
> > 
> > I think that would be good and also amenable for backports.  Just
> > check for NULL before
> > dereferencing.

Note if you have a call chain foo->bar->baz such that foo and baz are
in the same DSO but bar is in another, with the NULL fix instead of
segfaulting you'll probably record baz as direct callee of foo, which
is not correct (but is a pre-existing problem, in the sense that it
would also happen if the DSO with 'bar' was not instrumented at all).

> Sure. Let's wait for Alexander. Maybe he's got something we missed.

Well, here's the backstory. When people were discussing PR 83879
(multiple instances of __gcov_master due to dlopen) on IRC, with
the fix being "advise the user to pass --dynamic-list-data", I butted in
to say,

>> imho it's a design mistake: gcov symbols that need to work across dso
>> boundaries should have been a part of (currently non-existent)

>> indirect call profiling across exe/library boundary is broken for a similar reason

and later filed PR 84107 for the latter claim.

I think in general if a design requires an object to exist in one instance
like __gcov_master, for dynamic linking it should reside in a dynamic library.
Otherwise, it puts the onus on users to ensure that it's exported from the
main executable, not concealed in libraries by their version scripts. And
even then it still might come up in two copies with dlopen (RTLD_LOCAL).

In theory, another approach would be to have all gcov symbols have hidden
visibility, no exceptions, and redesign the logic around __gcov_master to
work properly when multiple instances are active in one program.


Index Nav: [Date Index] [Subject Index] [Author Index] [Thread Index]
Message Nav: [Date Prev] [Date Next] [Thread Prev] [Thread Next]