The AltiVec Fractal Carbon demo contains code for both the z -> z^2 + c and z -> z^4 + c Mandelbrot-style iteration methods. Although the z^4 case is selected by default, the z^2 case is shown below for simplicity.

The innermost loop of the iteration for the regular FPU case is:

//12 flops per iteration (counted in the assembly language) { float tr; count++; tr=ca+za*za-zb*zb; zb=cb+zb*za+zb*za; za=tr; tr=tr*tr+zb*zb; if (tr>sqrmaxrad) {     } }

To rewrite the above code for AltiVec, the multiplications, additions, and subtractions must be reexpressed in AltiVec macros, such as vec_madd and vec_nmsub. The comparison operations must be performed in vector form as well, here using vec_cmpgt, vec_and, and vec_any_ne. Note that there is no such thing as a simple floating-point multiply in AltiVec; you must add to something after the multiply. Thus an add to zero is required to complete the iteration loop.

The corresponding loop, converted to AltiVec is:

//13 flops - 1 add to zero = 12 flops per iteration { vector float tr; count++; tr=vec_madd(za,za,vec_nmsub(zb,zb,ca)); zb=vec_madd(zb,za,vec_madd(zb,za,cb)); za=tr; tr=vec_madd(zb,zb,vec_madd(tr,tr,zero)); { vector bool int exitMask=vec_and(vec_cmpgt(tr, sqrmaxrad),loopMask); if (vec_any_ne(exitMask,zeroInt)) {     } } }

This is the primary difference between the AltiVec and non-AltiVec version of the z^2 code. The remaining differences involve preparing the vector variables with the appropriate data prior to the iteration loop and saving the final data out into a regular array of floating-point numbers for later conversion to pixel data.

For a slightly more complex example of vectorized code, compare sections of the DoSqueezeToLetterboxEffect() routine in the source code of the iMovie Squeeze to Letterbox plug-in on the Other Software page.