Intel’s MMX

Intel’s MMX

Dr. Richard Enbody

CSE 820

Michigan State UniversityComputer Science and Engineering

Why MMX?

Make the Common Case Fast

• Multimedia and Communication consume significant computing resources.

• Providing specific hardware support makes sense.


Goals

• accelerate multimedia and communications applications.

• maintain full compatibility with existing operating systems and applications.

• exploit inherent parallelism in multimedia and communication algorithms

• includes new instructions and data types to improve performance.


First Step: examine code

• Examined a wide range of applications: graphics, MPEG video, music synthesis, speech compression, speech recognition, image processing, games, video conferencing.

• Identified and analyzed the most compute-intensive routines


Common Characteristics

• Small integer data types: e.g. 8-bit pixels, 16-bit audio samples

• Small, highly repetitive loops

• Frequent multiply-and-accumulate

• Compute-intensive algorithms

• Highly parallel operations


MMX Technology

A set of basic, general purpose integer instructions:

• Single Instruction, Multiple Data (SIMD)

• 57 new instructions

• Eight 64-bit wide MMX registers

• Four new data types


Data Types


Example

• Pixels are generally 8-bit integers. Pack eight pixels into a 64-bit MMX register.

• An MMX instruction takes all eight of the pixels at once from the MMX register, performs the arithmetic or logical operation on all eight elements in parallel, and writes the result into an MMX register.


Compatibility

• No new exceptions or states are added.

• Aliases to existing FP registers:The exponent field of the corresponding floating-point register (bits 64-78) and the sign bit (bit 79) are set to ones (1's), making the value in the register a NaN (Not a Number) or infinity when viewed as a floating-point value.


57 Instructions• Basic arithmetic: add, subtract, multiply,

arithmetic shift and multiply-add • Comparison• Conversion: pack & unpack• Logical• Shift• Move: register-to-register• Load/Store: 64-bit and 32-bit


Packed Add Word with wrap around

•Each Addition is independent•Rightmost overflows and wraps around


Saturation

• Saturation: if addition results in overflow or underflow, the result is clamped to the largest or smallest value representable.

• This is important for pixel calculations where this would prevent a wrap-around add from causing a black pixel to suddenly turn white


No Mode

There is no "saturation mode bit”:a new mode bit would require a change to the operating system. Separate instructions are used to generate wrap-around and saturating results.


Packed Add Word with unsigned saturation

•Each Addition is independent•Rightmost saturates


Multiply-Accumulate

multiply-accumulate operations are fundamental to many signal processing algorithms like vector-dot-products, matrix multiplies, FIR and IIR Filters, FFTs, DCTs etc


Packed Multiply-Add

Multiply bytes generating four 32-bit results.Add the 2 products on the left for one result and the 2 products on the right for the other result.


Packed Parallel Compare

• No new condition code flags

• No existing IA condition code flags are affected by this instruction.

• Result can be used as a mask to select elements from different inputs using a logical operation, eliminating branchs.


Packed Parallel Compare


Pack/Unpack

• Important when an algorithm needs higher precision in its intermediate calculations, as in image filtering.

• For example, image filtering involves a set of intermediate multiply operations between filter coefficients and a set of adjacent image pixels, accumulating all the values together.


Pack


Conditional Select

The Chroma Keying example demonstrates how conditional selection using the MMX instruction set removes branch mis-predictions, 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.


Chroma Keying


Chroma Keying (con’t)

• Take pixels from the picture with the woman on a green background.

• A compare instruction builds a mask for that data. That mask is a sequence of bytes that are all ones or all zeros.

• We now know what is the unwanted background and what we want to keep.


Create Mask

Assume pixels alternate green/not_green


Combine: !AND, AND, OR


Branch Removal

Without MMX technology, each pixel is processed separately and requires a conditional branch. Using MMX instructions, eight 8-bit pixels can be processed in parallel and no conditional branches are involved.


Vector Dot Product

• The vector dot product is one of the most basic algorithms used in signal-processing of natural data such as images, audio, video and sound.

• PMADD does 4 multiplies and 2 adds at a time. Coupled with PADD, eight multiply-accumulate operations can be performed: 2 PMADD and 2 PADD


Vector Dot Product


Vector Dot ProductAssuming precision is sufficient, a dot-

product on an 8-element vector can be completed using 8 MMX instructions: 2 PMADDs, 2 PADDs, two shifts (if needed to fix the precision after the multiply), and 2 loads for one of the vectors (the other vector is loaded by the PMADD instruction which can have one of its operands come from memory).


W/O MMX WITH MMX

Load 16 4

Multiply 8 2

Shift 8 2

Add 7 1

Misc -- 3

Store 1 1

Total 40 13

Compare


Compare

• With MMX technology, one third of the number of instructions is needed.

• Most MMX instructions can be executed in one clock cycle, so the performance improvement will be more dramatic than the simple ratio of instruction counts.


Matrix Multiply

3D games: computations that manipulate 3D objects use 4-by-4 matrices that are multiplied with 4-element vectors many times. Each vector has the X,Y, Z and perspective corrective information for each pixel. The 4-by-4 matrix is used to rotate, scale, translate and update the perspective corrective information for each pixel.


Compare

W/O MMX WITH MMX

Load 32 6

Multiply 16 4

Add 12 2

Misc 8 12

Store 4 4

Total 72 28


Matrix Multiply

• MMX required half the instructions.


Image Dissolve Using Alpha Blending

• Dissolve a Swan into a FlowerResult_pixel =

Flower_pixel * (alpha/255) + Swan_pixel * [1 - (alpha/255)]

• Assume 640x480 resolution


Dissolve: Millions of Inst.W/O MMX WITH MMX

Load 470 117

Unpack -- 117

Multiply 470 117

Add 235 58

Pack -- 58

Store 235 58

Total 1,400 525


Dissolve

1 billion fewer instructions for the 640x480 dissolve


Conclusion

• MMX appeared in 1997 in Pentium processors (with bigger cache).

• According to Intel, an MMX microprocessor runs a multimedia application up to 60% faster.In addition, it runs other applications about 10% faster

Intel’s MMX

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