Any docs about gcov impl change from 3.3 to 3.4

[Fei, Fei]

Hi, 

I am working on investigating some low level coverage tool, including
gcov. As I find out, the difference of implementation of gcc in gcov
part btw 3.3 and 3.4 is relevant. Can anyone direct me to some document
regarding this change?

Thanks,
Fei