Intel

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Intels MMX

• Dr. Richard Enbody
• CSE 820

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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 registersThe 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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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 bita 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 (cont)

• 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 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

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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 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).

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Compare
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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
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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 FlowerResult_pixel
  Flower_pixel (alpha/255)
  Swan_pixel 1 - (alpha/255)
• Assume 640x480 resolution

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Dissolve Millions of Inst.
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Dissolve

• 1 billion fewer instructions for the 640x480
  dissolve

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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