VSX? VMX? Altivec? VR? VSR?! How these PowerPC SIMD acronyms relate to each other?

I've a heard a lot about Altivec registers but I don't even find it on a POWER9 ISA. On that document I only find VR and VSR registers under Chapter 6 "Vector Facility" and Chapter 7 "Vector-Scalar Floating-Point Operations".

On ABI I noticed how the registers are physically allocated:

and I still didn't get why there are 2 groups. Is Altivec legacy?

As far as I understood, the following is what happened:

Altivec / VMX (Vector Multimedia eXtensions) is an old SIMD technology for PowerPc. I see it as the old Intel MMX.
The new VSX (Vector Scalar eXtensions) are an upgrade of it but to avoid breaking compatibility with Altivec, they remained 32 registers (VSR32-VSR63) only and Altivec operations can't handle the other VSR0-VSR31. The VSX instructions, however, can operate on all 64 registers (VSR0-VSR63). I see it as the new Intel SSE.

Is that correct?

Solution

To clarify the terminology a little:

VMX is the earlier implementation of vector support for POWER/PowerPC processors.
Altivec is a trademark for VMX; functionally, it should be equivalent
VSX is a newer implementation of vector support for POWER processors

VSX adds more vector registers (32 more, but still 128-bits per register), and VSX-specific instructions. As you've found, the higher-numbered VSX registers are shared with those used for VMX operations. Therefore, it'd be inadvisable to mix VMX and VSX instructions in a single process, unless you're careful with register allocations.

I'm not too familiar with Intel's vector implementations, but your analogies sound correct to me.