Shawn Haggett wrote:
>
> There's two points that come to mind.
>
> 1) mtune is a request for the compiler to make the code more suited to 
> the given processor, but without breaking compatibility. march is 
> telling the compiler, do everything you can to make this code fastest 
> on this processor.
>
> From the GCC docs for 4.2.3:
> "-mtune=cpu-type: Tune to cpu-type everything applicable about the 
> generated code, except for the ABI and the set of available 
> instructions."
> "-march=cpu-type: Generate instructions for the machine type cpu-type. 
> The choices for cpu-type are the same as for -mtune. Moreover, 
> specifying -march=cpu-type implies -mtune=cpu-type."
>
> So mtune shouldn't be using any instructions that are in K-6 that 
> weren't in a 386.
>
> 2) I believe x86 hardware never goes backwards. That is, if a new 
> feature is added, all future versions of the chip have that feature, 
> just with more added. Of course Intel and AMD both have their separate 
> additions, but since your staying with AMD, moving to a new processor 
> shouldn't break anything (even if you had used march).
>
> Disclaimer: I'm not an expert on hardware architectures or compilers, 
> so I might be wrong.
>
> Shawn

Thanks Shawn, that's probably the best answer I'm going to get, I doubt 
many of the AMD chip designers hang around here... :)