[PATCH][libbacktrace] Declare external backtrace fns noinline

[Ian Lance Taylor]

On Fri, Feb 8, 2019 at 9:26 AM Thomas Schwinge <thomas@codesourcery.com> wrote:
>
> On Fri, 8 Feb 2019 10:41:47 +0100, Tom de Vries <tdevries@suse.de> wrote:
> > The backtrace functions backtrace_full, backtrace_print and backtrace_simple
> > walk the call stack, but make sure to skip the first entry, in order to skip
> > over the functions themselves, and start the backtrace at the caller of the
> > functions.
> >
> > When compiling with -flto, the functions may be inlined, causing them to skip
> > over the caller instead.
>
> So, when recently working on the OpenACC Profiling Interface
> implementation in libgomp, where I'm using libbacktrace to figure out the
> caller of certain libgomp functions, I recently wondered about the very
> same issue, that we reliably have to skip a few initial frames.
>
> So, "noinline" is how to do that reliably...  ;-/ That might be
> non-obvious for the casual reader, so they might not understand...
>
> > Fix this by declaring the functions with __attribute__((noinline)).
>
> ... this alone.
>
> I'd suggest to have a common "#define LIBBACKTRACE_NOINLINE [...]" (or
> similar), together with the explanatory comment given above, and use that
> at the respective definition (or declaration?) sites.  Can that go into
> the public libbacktrace "*.h" file, so that it can also be used
> elsewhere, as described above?
>
> If you agree, want me to prepare a patch?

I think that at least for backtrace_full and backtrace_print we are
arguably looking at the SKIP parameter in the wrong place.  We
shouldn't look at it in unwind before calling backtrace_pcinfo.  We
should count the inlined functions found by backtrace_pcinfo against
the SKIP parameter.

Ian