This section describes example uses of the MMX instruction set to implement basic coding structures.
Conditional Select
Operating on multiple data operands using a single instruction presents an interesting issue: what happens when a computation is only done if the operand value passes some conditional check? For example, in an absolute value calculation, only if the number is already negative a 2?s complement is performed on it:
for i = 1 to 100
if a[i] < 0
then b[i] = – a[i]
else b[i] = a[i]
There are different approaches possible, and some are simpler than others. Using a branch approach does not work well for two reasons: a branch-based solution is slower because of the inherent branch misprediction penalty, and because of the need to convert packed data types to scalars. Direct conditional execution support does not work well for the x86 IA since it requires three independent operands (source, source/destination, and predicate vector).
The MMX technology adopts a simpler design: a conditional execution is converted into a conditional assignment. MMX compare operations result in a bit mask corresponding to the length of the operands: for example, a compare operation operating on packed byte operands produce byte-wide masks. These masks then can be used in conjunction with logical operations to achieve conditional assignment. Consider the following example:
If True
then Ra := Rb
else Ra := Rc
Assuming that register Rx contains all 1?s if the condition is true and all 0?s if the condition is false, Ra can be computed with the following logical expression:
Ra = (Rb AND Rx) OR (Rc ANDNOT Rx)
This approach works for operations with a register as the destination. Conditional assignment to memory can be implemented as a sequence of load, conditional assignment, and store.
The Chroma Keying example demonstrates how conditional selection using the MMX instruction set removes branches, in addition to performing multiple selection operations in parallel. Text overlay on a pix/video background, and sprite overlays in games are some of the other operations that would benefit from this technique.
Most have seen the television weather man overlaid on the image of a weather map. In this example a blue screen is used to overlay an image of a woman on a background picture.
PCMPEQ (packed compare for equality) is performed on the weathercaster and blue-screen images, yielding a bitmask that traces the outline of the weathercaster.
This bitmask image is PANDNed (packed and not) with the weathercaster image, yielding the first intermediate image: now the weathercaster has no background behind her.
The same bitmask image is PANDed (packed and) with the weather map image, yielding the second intermediate image.
The two intermediate images are PORed (packed or) together, resulting in final composite of the weathercaster over weather map
