prefetch is a noop on GPUs. It is useful in CPU kernels only to prefetch data in CPU cache (same as what _mm_prefetch() does).
if defined VECTORS4
(v.s0==g.s0) ? uint nonce = (W[3].s0); #endif : ();
(v.s1==g.s1) ? uint nonce = (W[3].s1); #endif : ();
(v.s2==g.s2) ? uint nonce = (W[3].s2); #endif : ();
(v.s3==g.s3) ? uint nonce = (W[3].s3); #endif : ();
...
#endif
This is also not possible, it's an illegal construction that would fail the compilation. (v.s0==g.s0) is evaluated at run-time and the results are unknown to the preprocessor. If you need to terminate execution before write, you can just do that:
if (!nonce) return;
I am not sure it would make much of a difference though.