"In the dark old days, in the time that most of you hadn't even heard of the word "Linux", the kernel was once modified to be compiled under g++. That lasted for a few revisions. People complained about the performance drop. It turned out that compiling a piece of C code with g++ would give you worse code. It shouldn't have made a difference, but it did. Been there, done that."
I wonder if it is still true. C++ compilers have come a long way. (as have C compilers). C++ is 99% a superset of C, I'm not sure how much of that last is used in the kernel, so it might be too much effort, but C++ is in a few cases stricter than C in ways that compilers can use to optimize. Many C programs run faster when compiled in C++ these days.
If there is a difference (either way) I'd expect it to be something you can measure, but not something you would notice in the real world on one computer. (though at google scale it probably shows up)
