-ffreestanding 
 
   Assert that compilation takes place in a freestanding environment. 
   This implies -fno-builtin.  A freestanding environment is one in 
   which the standard library may not exist, and program startup may 
   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 
   not necessarily be at "main".  The most obvious example is an OS 
   kernel.  This is equivalent to -fno-hosted. 
 
$ cat switch64.c 
 
#include "/usr/src/linux/include/linux/types.h" 
__u64 test(__u64 val) 
{ 
    switch (val) 
    { 
        case 0: 
            return 0; 
            break; 
        case 1: 
            return 1; 
            break; 
        default: 
            return -1; 
    } 
} 
 
$ gcc switch64.c -c -ffreestanding 
$ objdump -t switch64.o 
 
switch64.o:     file format elf32-sparc 
 
SYMBOL TABLE: 
00000000 l    df *ABS*  00000000 switch64.c 
00000000 l    d  .text  00000000 
00000000 l    d  .data  00000000 
00000000 l    d  .bss   00000000 
00000000 l    d  .note.GNU-stack        00000000 
00000000 l    d  .comment       00000000 
00000000         *UND*  00000000 __ucmpdi2 
                                 ^^^^^^^^^ 
00000000 g     F .text  0000009c test 
 
$ objdump -T /lib/libgcc_s.so|grep __uc 
0000181c g    DF .text  00000038  GCC_3.0     __ucmpdi2 
 
$ ldd /lib/libgcc_s.so 
        libc.so.6 => /lib/libc.so.6 (0x70028000) 
        /lib/ld-linux.so.2 => /lib/ld-linux.so.2 (0x08000000) 
 
In the kernel sources such situations happen very often. 
In the case of __ucmpdi2, it appears to be a combination of kernel 
and compiler stupidity - there's no reason why __ucmpdi2 should 
not be done inline by the compiler, but at the same time there 
is probably also little reason to use a slow "long long" comparison 
in the kernel. Of course I can force a static linking with libgcc.a 
but in the above case gcc should report an error or do inline and warn user.