Compile FreeBSD kernel with gcc48

Hi,

Anyone have experience compiling FreeBSD kernel with gcc48

I have tried it but no success with this in src.conf

C= /usr/local/bin/gcc48
CXX= /usr/local/bin/g++48
CWARNFLAGS= -Wall -Wredundant-decls -Wnested-externs -Wstrict-prototypes
\-Wmissing-prototypes -Wpointer-arith -Winline -Wcast-qual
\-Wno-pointer-sign -D__FreeBSD_cc_version=0 \-Wno-unused-function
-Wno-inline -Wno-format \-Wno-uninitialized -Wno-array-bounds
-Wno-missing-prototypes
NO_WERROR= 1

Another question, is there any benchmark for LLVM/Clang vs gcc45, gcc46,
gcc47 or gcc48?

Regards,
Alie T