Binary compatibility between avr-gcc 3.4.0 and avr-gcc 4.3.x

Question

I have inherited an application that links to a library which MAY HAVE been built with gcc3. Or maybe with the imagecraft compiler. That information has now vanished to the heavenly bitfield and I am left with a libXXX.a library against which to link my app. I cannot recompile the libXXX.a because it requires certain unknown headers from imagecraft and somewhere else which at a certain point may have been ubiquitous in my environment but now are nowhere to be found.

My question is this, provided that my compiling my app with avr-gcc version 3.4.0 (and linking to that "special" libXXX) resulted in a working binary image, is it reasonable to expect that I could compile all the other parts of my app with avr-gcc 4 (this action having some very nice and proven benefits), link with libXXX and still get a working program?

Essentially, it all boils down to: is avr-gcc binary compatible with "mysterious compiler X which just may have been avr-gcc 3.something"?

To be honest, I have successfully compiled the rest of my app with avr-gcc4 and linked it with the library, and verified that the result works, but what kind of side effects or quirks should I be on the lookout for?

Answer 1

Linking libraries from different compilers (or -versions) will work reliably if both compilers use the same ABI (Application Binary Interface)

The ABI of a specific platform is typically specified by the dominant compiler for that platform, but that could be done by referencing an external specification.
ABI changes are rare, especially if the platform supports third-party libraries/applications, because an ABI change means that literally everything has to be rebuilt.