How to profile a shared object without profiling the host app?

Question

I have a host application and I have written a plug-in. I compile my plug-in down to a shared object (say foo.so), and the host application will load it via dlopen. In this case, my host application is the opt tool from llvm (though I don't think that's important to the question).

I'd like to compile my plug-in with profiling enabled (i.e. g++ -pg, gprof). However, when I do this, the profile output file gmon.out is never created. Maybe gprof expects someone to call a setup routine, or something like that.

For various reasons, I would like to avoid recompiling the host app with -pg. I am curious if it is possible to profile a shared object foo.so without profiling the host application opt.

I've also looked into other profiling tools; qprof from HP should be able to handle this situation, but it is unable to resolve the names of functions in the shared object (it falls back to addr2line in a very naive way).

Thanks, Nick

Answer 1

I assume the reason you want to do this is to find ways to optimize the plugin (as opposed to just getting timing information).

Can you run the host app under a debugger or IDE? Does the IDE have a pause button, or can you interrupt it with Ctrl-C or some such key?

Then you can quickly find the costly code by using this technique. Only take samples when your plugin is running, or if you can't do that, just ignore samples that don't end in your plugin.

Even if you get gprof to work, or a similar profiler, you're likely to be disappointed.