Visual Studio: use the results of Profile Guided Optimization from one exe to a different dll?

Question

I have a dll, call it core.dll which I want to optimize using Visual Studio's excellent Profile Guided Optimization. Most of the code is the dll actually compiles into a library called core.lib which is then wrapped by core.dll.
To unit-test this code I also have a tester executable called test_core.exe. this executable links to core.lib and activates various functions from it. The DLL core.dll has very few exports, only enough to start its main functionality. It cannot be unit tested fully using these exports.
What I want is to do the PGO data collection by activating some of the tests in test_core.exe and then to use this PGO data to link and optimize core.dll.

It seems that the Visual Studio framework was designed so that the collecting executable and optimized executable are the same.
One option is to add the relevant tests to be inside core.dll and run them using a special export but that would bloat core.dll with test code which is not used in any other circumstance.

Answer 1

It seems that the Visual Studio framework was designed so that the collecting executable and optimized executable are the same.

That was very, very intentional. Profile guided optimization can only work properly when it uses profile data that was collected from a realistic simulation of the way your users are going to run your program. That requires the actual executables as deployed to the user and using realistic data that's a reasonable match with the data the program is going to process at the user's site.

Trying to spike it with test unit profile results will achieve the opposite, your user isn't going to run the code the same way. Significant odds that you'd end up with a less optimized program it that was possible. The profile data you've gather is only good enough to optimize the unit test, that's not useful.

Don't try to cook the profile data, it can't work. This does mean that you can't necessarily easily measure the effectiveness of the optimization if you require a unit test to see a signal. In that case you need to just assume that PGO gets the job done.

Answer 2

It seems that the Visual Studio framework was designed so that the collecting executable and optimized executable are the same.

This is true, but in you're case, you want to optimize a DLL, not an executable. You can compile the static library and the DLL using the /GL switch and link the DLL using the /LTCG:PGINSTRUMENT switch. This creates a DLL that is instrumented. The test_core.exe image doesn't have to be instrumented, so you can just compile it normally (in Debug or Release mode). Then, by running test_core.exe, a PGC file will be generated containing a profile of the behavior of core.dll only. This profile can then be used to optimize core.dll by compiling it again and specifying the /LTCG:PGOPTIMIZE switch. As long as test_core.exe exercises core.dll for common usage scenarios, you'll certainly benefit from it. See this for more information.