Help with -finstrument-functions

Help with -finstrument-functions

[thulasidhar]

Hi,
I am facing a strange problem when I tried to profile the call trace of my application using -finstrument-functions switch in gcc.

We have our application built as a shared library and we use python scripting language to test all the interfaces provided by this shared library.
It is the call trace of this shared library that I want to print.

When I create an executable by linking this shared library to a main.o (as follows), I get the call trace.
$ gcc main.o libtest_app.so -o app
$ app

This main.o contains only a main function that calls one of the interfaces present in libtest_app.so.

But when I import this shared library (libtest_app.so) into python, I do not get the call trace.

I analyzed the scenario in detail and these are my observations.
I have defined the functions __cyg_profile_func_enter and __cyg_profile_func_exit in my application, and I can see them in nm output.

$ nm libtest_app.so | grep __cyg
00177518 T __cyg_profile_func_enter
001775c4 T __cyg_profile_func_exit

Case 1: When I debug my application, built as an exe (app) I see that whenever it enters any function, it jumps to these above defined functions in libtest_app.so

Case 2: But when I debug my application, imported as an module in python, I see that whenever it enters any function it jumps to some symbol of the same name (__cyg_profile_func_enter) in glibc (source filename is displayed as noophooks.c).

The library is the same in both cases. What I don't understand is that if the shared library is built and linked, why should a reference to one function (__cyg_profile_func_enter) result in different jumps in different situations. Ideally all jumps to __cyg_profile_func_enter should have been resolved to refer to the symbol defined in my library, when I build the shared library (as follows),
$ gcc -shared -L/usr/lib/python2.2/config -lpython2.2 -o test/lib/libtest_app.so -lapp.a -lpthread -lrt -lm -ldl -lutil

Why doesn't this happen in the second case? Am I missing something here?

GCC version 3.3.1


Warm regards,
Thulasidhar J.K 






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Re: help with -finstrument-functions

[Jonathan Wakely]

On Sun, 4 Apr 2021, 09:31 Akshit Sharma via Gcc-help, <gcc-help@gcc.gnu.org>
wrote:

> I am trying to profile the program with gcc. I am inserting the data into
> an array which is later used to generate a file.
>
> I am having a problem using template class (std::atomic<int>). Program is
> multithreaded. So, I need to use atomics.
> Right now, the __cyg_profile_func_enter is called for
> std::__atomic_base<int>::operator++ as well.
>
> I even tried setting the compile parameters to ignore it with:
> -finstrument-functions
>
> "-finstrument-functions-exclude-function-list=std::__atomic_base<int>::operator++>"
>

Looks like you have a stray '>' here, but I would expect that you have to
use the mangled name for C++ functions.



> But I am unable to make the compiler ignore the function.
> Is there a way for the compiler to make ignore a predefined function?
>


N.B. this function is not "predefined", it's defined in the C++ Standard
Library.

Re: help with -finstrument-functions

[Thomas Bleher]

* Akshit Sharma via Gcc-help <gcc-help@gcc.gnu.org> [2021-04-04 10:30]:
> I am trying to profile the program with gcc. I am inserting the data into
> an array which is later used to generate a file.
>
> I am having a problem using template class (std::atomic<int>). Program is
> multithreaded. So, I need to use atomics.

Depending on how you manage your threads, you might be able to avoid
atomics in __cyg_profile_func_* by using thread-local variables.

E.g. collect all data per-thread in thread-local variables. Then disable
profiling (you can't disable the calls to __cyg_profile_func_*, but you
can exit immediately at the top of the function if a flag is set), and
copy the data to global storage. This can work if you have control over
all your threads.

Hope this helps you.
Thomas

help with -finstrument-functions

[Akshit Sharma]

I am trying to profile the program with gcc. I am inserting the data into
an array which is later used to generate a file.

I am having a problem using template class (std::atomic<int>). Program is
multithreaded. So, I need to use atomics.
Right now, the __cyg_profile_func_enter is called for
std::__atomic_base<int>::operator++ as well.

I even tried setting the compile parameters to ignore it with:
-finstrument-functions
"-finstrument-functions-exclude-function-list=std::__atomic_base<int>::operator++>"

But I am unable to make the compiler ignore the function.
Is there a way for the compiler to make ignore a predefined function?

Any pointers about using the __cyg_profile_func_* functionality will be
helpful.

Thanks,
- Akshit