Re: [gentoo-user] mtune=k6-2 and a *small* upgrade

[Shawn Haggett]

Anthony Metcalf wrote:
> Alan McKinnon wrote:
>> Now the existing system should work with your new hardware and you can
>> update your CFLAGS and 'emerge -e world' at your leisure.
>>
>> That's the theory at least anyway :-)
>>
>>
> Well, exactly. That is the theory. I want to know the likelihood of
> success. I know that using mtune=k6-2 means it won't run on anything
> before a k6-2, and most likely not on anything Intel, due to the symbols
> and optimisations used. What I want is some idea of the chance it will
> run on a *later* AMD processor. Will an Athlon honour the k6-2
> optimisations?

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
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