Effect of -mtune=* on x86 or x86-64 systems?

[Wirawan Purwanto <wirawan0@xxxxxxxxx>]

Hi,

What is, and how much is, the effect of -mtune (or its implication in 
-march) switch of gcc (version 4.3, that is)? Suppose I use 
-march=pentium4 and then run the binary on pentium M machine (both support 
MMX, SSE, and SSE2), what kind of penalty will I get? I don't have a 
particular code in mind, but think of a numerical-type code with lots of 
double precision math.

Thanks,
Wirawan