About Clear MMX & SSE Register...

CPU MMX	pxor	psub_	pcmp_
Pentium with MMX	1.0/0.5	1.0/0.5	1.0/0.5
Pentium II	1.0/0.5	1.0/0.5	1.0/0.5
Pentium 4	1.0/1.0	2.0/1.0	2.0/1.0
Prescott	1.0/1.0	1.0/1.0	2.0/1.0
Core 2 Duo	0.3/0.3	0.5/0.5	0.5/0.5
AMD K6-2	1.0/0.6	1.0/0.6	1.0/0.6
AMD Athlon	2.0/0.5	2.0/0.5	2.0/0.5
AMD Athlon 64	0.5/0.5	2.0/0.5	2.0/0.5
AMD Athlon 64 (90nm)	0.5/0.5	0.5/0.5	0.5/0.5

CPU SSE	pxor	psub_	pcmp_
Pentium !!!	2.0/2.0	N/A	N/A
Pentium 4	2.0/2.0	2.0/2.0	2.0/2.0
Pentium M	1.0/1.0	1.0/1.0	1.0/1.0
Core 2 Duo	0.3/0.3*	0.5/0.5	0.5/0.5
Core i7	0.3/0.3	0.5/0.5	0.5/0.5
AMD Athlon	2.0/2.0	N/A	N/A
AMD Athlon 64	1.0/1.0	2.0/1.0	2.0/1.0
AMD Athlon 64 (90nm)	1.0/1.0*	1.0/1.0	1.0/1.0
AMD Phenom	0.5/0.5*	0.5/0.5	0.5/0.5

PS:* included XORPS, XORPD.

Source : http://instlatx64.freeweb.hu/