User's Manual

118 Use MMX™ PXOR to Clear All Bits in an MMX™ Register
AMD Athlon™ Processor x86 Code Optimization
22007E/0—November 1999
/* block fill (destination QWORD aligned) */
__asm {
mov edx, [dst_ptr]
mov ecx, [blk_size]
shr ecx, 6
movq mm0, [fill_data]
align 16
$fill_nc:
movntq [edx], mm0
movntq [edx+8], mm0
movntq [edx+16], mm0
movntq [edx+24], mm0
movntq [edx+32], mm0
movntq [edx+40], mm0
movntq [edx+48], mm0
movntq [edx+56], mm0
add edx, 64
dec ecx
jnz $fill_nc
femms
sfence
}
Use MMX™ PXOR to Clear All Bits in an MMX™ Register
To clear all the bits in an MMX register to zero, use:
PXOR MMreg, MMreg
Note that PXOR MMreg, MMreg is dependent on previous
writes to MMreg. Therefore, using PXOR in the manner
described can lengthen dependency chains, which in return
may lead to reduced performance. An alternative in such cases
is to use:
zero DD 0
MOVD MMreg, DWORD PTR [zero]
i.e., to load a zero from a statically initialized and properly
aligned memory location. However, loading the data from
memory runs the risk of cache misses. Cases where MOVD is
superior to PXOR are therefore rare and PXOR should be used
in general.