Compilation of fresh (a few minutes old sources) amd64 kernel
with COMPAT_LINUX fails:

#   compile  DUUNI/linux_machdep.o
cc -mcmodel=kernel -mno-red-zone -ffreestanding -fno-zero-initialized-in-bss -O2 
-Werror -Wall -Wno-main -Wno-format-zero-length -Wpointer-arith 
-Wmissing-prototypes -Wstrict-prototypes -Wswitch -Wshadow -Wcast-qual 
-Wwrite-strings -Wno-sign-compare -Wno-pointer-sign -Wno-attributes 
-fno-strict-aliasing -Damd64 -Dx86_64 -I. -I../../../../../common/include 
-I../../../../arch -I../../../.. -nostdinc -DLKM -DMAXUSERS=32 -D_KERNEL 
-D_KERNEL_OPT -I../../../../lib/libkern/../../../common/lib/libc/quad 
-I../../../../lib/libkern/../../../common/lib/libc/string 
-I../../../../lib/libkern/../../../common/lib/libc/arch/x86_64/string -c 
../../../../compat/linux/arch/amd64/linux_machdep.c
--- /usr/src/sys/arch/amd64/compile/DUUNI/lib/compat/libcompat.a ---
making sure the compat library is up to date...
--- linux_machdep.o ---
cc1: warnings being treated as errors
../../../../compat/linux/arch/amd64/linux_machdep.c: In function 
'linux_sys_rt_sigreturn':
../../../../compat/linux/arch/amd64/linux_machdep.c:406: warning: declaration of 
'fxsave' shadows a global declaration
./x86/cpufunc.h:95: warning: shadowed declaration is here