Thanks for the valuable posts everybody.
Yes, I guess you are right I didn't really realise the things about new compiler versions and about C time critical stuff etc.

The reason why I am doing this compatibility stuff with SDCC and Keil is because we want to give a possibility to use our API which we are developing also for students and universities without buying the expensive Keil compiler (unfortunately the limited 4k version of Keil does not offer enough space).

So I guess the best solution is to leave the code intact for Keil and implement it in assembler for SDCC. I just wanted to avoid to use different files for the same project.

greetings
Attila