a question about MMX instruction

[Huimin Cui]

Hi,
   I've seen that in the file i386.md there exists some MMX insn templates. But it seems that it doesn't create mmx instructions at all. 
   And I heard that many multimedia programs need to be rewritten by hand to use the MMX instructions for optimization. If the compiler can generate MMX instructions, why do they write them by hand? 
My gcc version is 3.2(and I have tried 2.96 and failed). 
Can anyone give me some advice?Thanks for you all.