[ponce]

I remember earlier AMD processors, supporting SSE1 (with the CPUID bit on), but this was so slow to be unusable in practice, slower than general instructions.

In this case, the optimal thing to do was not to use these instructions despite the processors claiming support.

[wmf]

I'd argue that the optimal thing for a compiler to do is to use SSE when the processor supports it... and then the optimal thing for customers to do is not buy processors that run reasonable code slowly.