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ac6 >> ac6-training >> Processors >> ARM Cores >> NEON-v8 programming Download Catalog Download as PDF Write us Printable version

RC2 NEON-v8 programming

This course explains how to use ARMv8 NEON SIMD instructions to boost multimedia algorithms

  • This course has been designed for programmers wanting to run multimedia algorithms on NEON Single Instruction Multiple Data execute units on ARMv8 processors.
  • Evolution of the NEON architecture between ARMv7 and ARMv8 is detailed.
  • Each instruction family is detailed, first at assembly level, and then at C level using macros developed present in arm_neon.h file.
  • Several tricky usage of processing instructions are provided.
  • Vector and vector element load / store instructions are studied and guidelines for organizing data in memory are provided to minimize the number of memory accesses.
  • The underlying cache operation as well as preload mechanisms (instruction and hardware prefetch) are detailed to explain how a processing can be pipelined .
  • The course shows how DSP typical algorithms such as FIR and FFT can be vectorized and then optimized to be executed on NEON unit.

  • Cryptographic operations are also detailed, with explanation of the supported algorithms.
Labs are compiled with GCC and run on a Linux Cortex-A53 board or a simulator
A more detailed course description is available on request at training@ac6-training.com
  • Knowledge of ARMv7 instruction sets.

Day 1
Introduction to NEON
  • Clarifying the resources shared by NEON and the scalar floating point engine
  • Explaining the AArch32 and AArch64 differences
  • NEON Register banks
    • S, D and Q registers (AArch32)
    • B, H, S, D and V registers (AArch64)
  • Data types
  • Vector vs scalar
  • Related system registers
  • Alignment issues
  • Enabling NEON
  • Differences between NEONv7 and NEONv8
NEON instruction syntax
  • Instructions producing wider / narrower results
  • Instructions modifiers
  • Selecting the shape
  • Selecting the operand / result type
  • Syntax flexibility
  • Declaring initialized vectors in C language
  • Using unions with vectors and arrays of vectors to simplify the debug
  • Casting vectors
Data transfer instructions
  • Move
  • Swap
  • Table lookup
  • Vector transpose
  • Vector zip / unzip
  • Data transfer between NEON and integer unit
    • Practical lab: clarifying narrow and long instructions, building a vector from bytes selected from a pair of vectors
Exercise:  Example: managing audio samples
Exercise:  Using load with de-interleaving instructions to store all right lane samples into a vector and left lane samples into another vector
Exercise:  Clarifying narrow and long instructions, building a vector from bytes selected from a pair of vectors
Arithmetic Instructions
  • Arithmetic instructions
    • Add, modulo vs saturated arithmetic
    • Halving / Doubling the result
    • Rounding
    • Subtract
    • Multiply
    • Multiply accumulate / Multiply subtract
    • Absolute value
    • Min / Max
Exercise:  Implementing a complex multiply accumulate with NEON
  • Conversion instructions
    • Converting Floating Point numbers into Fixed point numbers
    • Converting Fixed point numbers into Floating point numbers
Exercise:  Converting fixed-point elements into single precision floating point values and adding the resulting elements
  • Advanced arithmetic instructions
    • Reciprocal estimate, reciprocal square root estimate, Newton-raphson algorithm
    • Pairwise instructions
Day 2
Logic and Bitfield Instructions
  • Element comparison
  • Logic instructions
    • Logical AND, Bit Clear, OR, XOR
    • Operations with immediate values
  • Bitfield instructions
    • Count Leading zeros, ones, signs
    • Bitwise insert instructions
    • Conditional bitwise insert instructions, avoiding branches
    • Shifts with possible rounding and saturation
    • Bitfield reverse
Exercise:  Transposing a matrix, shifting a large bitmap using vector instructions
NEON Cryptography Extension
  • The Cryptography extension
  • Algorithms supported
    • AES
    • SHA1
    • SHA256
Optimizing techniques
  • Automatic vectorization
  • Tuning loops for optimal results
    • Avoid loop feedbacks
    • Avoid loop-dependent conditionals
    • Avoid early termination
    • Padding loops
Exercise:  Experimenting with loop auto-vecorization
  • Pointers and arrays
    • indirect addressing
    • pointer aliasing and restrict
Exercise:  Using restrict to eliminate dependencies
  • Function calls and inlining
    • promises
Exercise:  Making promises to help the compiler optimize
  • Avoiding data dependencies
NEON coding examples
  • FIR filter
    • Converting the scalar algorithm into a vector algorithm
    • Finding the NEON instructions to encode the vector algorithm
    • Optimizing the code
    • Using the performance monitor to tune the algorithm
  • FFT (DFT)
    • Converting the scalar algorithm into a vector algorithm, understanding how circle properties can be used to process 4 angles concurrently
    • Finding the NEON instructions to encode the vector algorithm
    • Optimizing the code
    • Using the performance monitor to tune the algorithm