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					Intel’s MMX
Dr. Richard Enbody CSE 820

Why MMX?
Make the Common Case Fast
• Multimedia and Communication consume significant computing resources. • Providing specific hardware support makes sense.
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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.
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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
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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
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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
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Data Types

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Data Types

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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.
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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.
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Michigan State University Computer Science and Engineering

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
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Packed Add Word
with wrap around

•Each Addition is independent •Rightmost overflows and wraps around

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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
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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.

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Packed Add Word
with unsigned saturation

•Each Addition is independent •Rightmost saturates

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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

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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.

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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.

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Packed Parallel Compare

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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.
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Pack

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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.
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Chroma Keying

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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.
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Create Mask
Assume pixels alternate green/not_green

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Combine: !AND, AND, OR

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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.

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Vector Dot Product
• The vector dot product is one of the most basic algorithms used in signalprocessing 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
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Vector Dot Product

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Vector Dot Product

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Vector Dot Product
Assuming precision is sufficient, a dotproduct 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).
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Compare
Load Multiply Shift Add Misc Store Total W/O MMX 16 8 8 7 -1 40
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WITH MMX 4 2 2 1 3 1 13

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.
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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.
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Compare
Load Multiply Add Misc Store Total W/O MMX 32 16 12 8 4 72 WITH MMX 6 4 2 12 4 28

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Matrix Multiply
• MMX required half the instructions.

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Image Dissolve Using Alpha Blending
• Dissolve a Swan into a Flower Result_pixel = Flower_pixel * (alpha/255) + Swan_pixel * [1 - (alpha/255)]

• Assume 640x480 resolution
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Dissolve: Millions of Inst.
Load Unpack Multiply Add Pack Store Total W/O MMX 470 -470 235 -235 1,400 WITH MMX 117 117 117 58 58 58 525

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Dissolve
1 billion fewer instructions for the 640x480 dissolve

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Michigan State University Computer Science and Engineering

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
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posted:11/25/2009
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