Hi Tom!
On Fri, 8 Feb 2019 10:41:47 +0100, Tom de Vries <tdevr...@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?
Grüße
Thomas
> [libbacktrace] Declare external backtrace fns noinline
>
> 2019-02-08 Tom de Vries <tdevr...@suse.de>
>
> * backtrace.c (backtrace_full): Declare with __attribute__((noinline)).
> * print.c (backtrace_print): Same.
> * simple.c (backtrace_simple): Same.
>
> ---
> libbacktrace/backtrace.c | 2 +-
> libbacktrace/print.c | 2 +-
> libbacktrace/simple.c | 2 +-
> 3 files changed, 3 insertions(+), 3 deletions(-)
>
> diff --git a/libbacktrace/backtrace.c b/libbacktrace/backtrace.c
> index 29204c63313..c579e803825 100644
> --- a/libbacktrace/backtrace.c
> +++ b/libbacktrace/backtrace.c
> @@ -98,7 +98,7 @@ unwind (struct _Unwind_Context *context, void *vdata)
>
> /* Get a stack backtrace. */
>
> -int
> +int __attribute__((noinline))
> backtrace_full (struct backtrace_state *state, int skip,
> backtrace_full_callback callback,
> backtrace_error_callback error_callback, void *data)
> diff --git a/libbacktrace/print.c b/libbacktrace/print.c
> index b2f45446443..0767facecae 100644
> --- a/libbacktrace/print.c
> +++ b/libbacktrace/print.c
> @@ -80,7 +80,7 @@ error_callback (void *data, const char *msg, int errnum)
>
> /* Print a backtrace. */
>
> -void
> +void __attribute__((noinline))
> backtrace_print (struct backtrace_state *state, int skip, FILE *f)
> {
> struct print_data data;
> diff --git a/libbacktrace/simple.c b/libbacktrace/simple.c
> index d439fcee8e2..118936397da 100644
> --- a/libbacktrace/simple.c
> +++ b/libbacktrace/simple.c
> @@ -90,7 +90,7 @@ simple_unwind (struct _Unwind_Context *context, void *vdata)
>
> /* Get a simple stack backtrace. */
>
> -int
> +int __attribute__((noinline))
> backtrace_simple (struct backtrace_state *state, int skip,
> backtrace_simple_callback callback,
> backtrace_error_callback error_callback, void *data)
