Chundles said:
Adding Altivec support to Rosetta would be absolute nonsense and counter-productive.
Yes, there are cases where Altivec code on a G4 can run ten times faster than non-Altivec code, because it does ten times more work in the same time. However, Altivec code is ten times harder to emulate. If your G3 code takes ten seconds, then Rosetta might take ten seconds on a more powerfull Pentium processor to emulate that code. Altivec code might do the same task in one second, but then Rosetta will take ten seconds to emulate that code. What is gained? Nothing.
But that is just the best possible situation. Sometimes you have code that isn't very suitable for Altivec. Then some genius of a programmer comes up with a method using Altivec, that actually does five times more work, but does it ten times faster on a G4 for a net gain of fifty percent. When this kind of code were to run under Rosetta with Altivec emulator, you would really lose out: It still does five times more work, but because emulating Altivec is so much harder, it actually takes five times longer than emulating equivalent G3 code would do!
As an example: The simple loop
for (i = 0; i < 16; ++i) dst = table [src ];
looking up sixteen one-byte values from a 256 byte lookup table would usually be done with sixteen load and sixteen store instructions, and that is what both G3 code and Rosetta-emulated G3 code would do.
Someone at Apple found an ingenious way to do the same thing with eight vector permute instructions, plus about twelve other vector instructions. The result runs twice as fast on a G4 with Altivec. However, emulating a _single_ vector permute instruction needs sixteen loads and stores. Emulating eight of them takes 128 loads and stores, plus the emulation of all the other instructions. As a result, emulated Altivec code would run at least ten times slower in this case than emulated G3 code!
As a reminder from history: Most people think the Classic environment on MacOS X emulates an ancient 68040 processor. It doesn't. It emulates the even more ancient 68020 processor. Emulating a more powerful processor doesn't gain anything, because it takes longer to emulate.
