typedef long long s64;
From 
http://embed.cs.utah.edu/embarrassing/dec_09/harvest/gcc-head_llvm-gcc-head/

typedef unsigned long long u64;
union ktime
{
  s64 tv64;
};
typedef union ktime ktime_t;
ktime_t ns_to_ktime (u64 ns);
union ktime const ktime_zero = { 0LL };

ktime_t
ns_to_ktime (u64 ns)
{
  ktime_t __constr_expr_0;

  {
    __constr_expr_0.tv64 = (s64) ((u64) ktime_zero.tv64 + ns);
    return (__constr_expr_0);
  }
}

/* Checksum = 1A82CF6 */

this generates with 4.5.0 20091219 -O2 

 movq    %rdi, %rax
 addq    ktime_zero(%rip), %rax
 ret

llvm in this case uses the known const value of ktime_zero and just generates
an assignment (this makes more difference with -m32) gcc should too.