User's Manual

Use MMX™ PCMPEQD to Set All Bits in an MMX™ Register 119
22007E/0—November 1999 AMD Athlon™ Processor x86 Code Optimization
Use MMX™ PCMPEQD to Set All Bits in an MMX™ Register
To set all the bits in an MMX register to one, use:
PCMPEQD MMreg, MMreg
Note that PCMPEQD MMreg, MMreg is dependent on previous
writes to MMreg. Therefore, using PCMPEQD in the manner
described can lengthen dependency chains, which in return
may lead to reduced performance. An alternative in such cases
is to use:
ones DQ 0FFFFFFFFFFFFFFFFh
MOVQ MMreg, QWORD PTR [ones]
i.e., to load a quadword of 0xFFFFFFFFFFFFFFFF from a
statically initialized and properly aligned memory location.
However, loading the data from memory runs the risk of cache
misses. Cases where MOVQ is superior to PCMPEQD are
therefore rare and PCMPEQD should be used in general.
Use MMX™ PAND to Find Absolute Value in 3DNow!™ Code
Use the following to compute the absolute value of 3DNow!
floating-point operands:
mabs DQ 7FFFFFFF7FFFFFFFh
PAND MM0, [mabs] ;mask out sign bit
Optimized Matrix Multiplication
The multiplication of a 4x4 matrix with a 4x1 vector is
commonly used in 3D graphics for geometry transformation.
This routine serves to translate, scale, rotate, and apply
perspective to 3D coordinates represented in homogeneous
coordinates. The following code sample is a 3DNow! optimized,
general 3D vertex transformation routine that completes in 16
cycles on the AMD Athlon processor: