Comment by sparkie

GCC and Clang both provide `_readfsbase_u64()`, which presumably is just a wrapper around `__asm__("rdfsbase %0" : "=r"(result))`. Also gcc lets you define variables like `int__seg_fs *foo;`if the `__SEG_FS` preprocessor symbol is defined. With Clang, you can use `int __attribute__((address_space(257))) *foo`