/usr/include/clc/integer/mul_hi.h is in libclc-dev 0~git20140101-1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 | #define __CLC_BODY <clc/integer/mul_hi.inc>
#include <clc/integer/gentype.inc>
|