Note that GCC actually has a bunch of bugs related to this where it will give you different results depending on whether a value gets computed at compile time or runtime.
Been a while since I was actively involved in GCC development, but back then most issues like this AFAIR were due to math functions and not basic arithmetic. GCC doesn't control the target libm, and correctly rounded libm's are rare and much slower than ones which permit a bit more slop in the results.
