[kelnos]

GCC far far far predates LLVM. They do not share code.

[fweimer]

The libsanitizer library is imported from the LLVM sources into the GCC source tree. There are probably more examples of code sharing. Both projects are quite large.

[steve1977]

I would assume the code sharing is one-way only due to the licenses. I.e. a GPL project can use MIT licensed code, but not vice versa (AFAIK).