Introducing the PowerPC SIMD unit

An anonymous reader writes "AltiVec? Velocity Engine? VMX? If you've only been casually following PowerPC development, you might be confused by the various guises of this vector processing SIMD technology. This article covers the basics on what AltiVec is, what it does -- and how it stacks up against its competition."

Comments

AltiVec is nice...

[Grand V'izer]

I've done some altivec programming in the past, and discovered it was a very effective use of my time. Since there's no mode-switching penalty for using the vector instructions you can use it for some very trivial-but-common tasks, like replacing strlen(), vector operations on small tables, etc.. I knocked a lot of computation time (25%) from one of my projects just by vectorizing three functions. Of course there's a hitch: vector processing only works for certain kinds of algorithms and requires a change in mindset. In spite of that it's a great tool to have in your box.

Re:AltiVec is nice...

[evn]

The other nice thing about Altivec on OS X is that Apple has done a fairly good job of making it accessible without forcing the programmer to learn and use assembly language. These libraries will automatically fall back to a scaler code path if they're running on a G3 so it saves you from a fair bit of work there too. They have included a number of optimized libraries that use Altivec that are ready to go "out of the box" with xCode including:

• Vimage: for image processing
• vDSP: for signal processing
• BLAS: the name says it all: "Basic Linear Algebra"
• LAPAC: for solving systems of equations and matrix factorization
• Vector Math Library: unloads common operations like square root, transcendental functions, division, etc to VMX
• vBasicOps: for simple algebra operations like integer addition, subtraction, etc.
• VBN: for dealing with 256-1024bit numbers easily

Apple has documentation and source code for the libraries on their Developer Connection Website. What good are vector units if nobody can make use of them? I can't wait for Apple to put the GPUs image processing abilities into my hads with CoreImage/Video.

Re:Altivec and OS X

[ip_fired]

I googled around and found this article on Macworld:

According to several developers Macworld talked to who are currently working on OS X applications, anytime the OS can take advantage of the AltiVec engine, it does. This ensures that the parts of the OS that can utilize AltiVec, such as working in the new user interface, experience a significant increase in performance.

I don't know how much of OS X has AltiVec code, but there are many other apple apps that use it. iTunes uses it for encoding music. I'm sure the video codecs in Quicktime use it as well.

The Mac has a really nice optimization tool called shark which will help you find things that can be put into the AltiVec processor (it also helps with general optimization).