[Info-vax] Poulson info from Dave Cantor

[Jan-Erik Soderholm]

On 2010-11-19 12:47, Michael Kraemer wrote:
> glen herrmannsfeldt schrieb:
>
>> Well, for RISC, and especially for VLIW, the problem isn't allowing it to
>> run, but to be able to extend the architecture
>> and allow older programs to run reasonably fast.
>>
>> Expecting the compiler to optimize for a specific processor tends
>> to cause it to be much less optimal for other similar processors.
>
> This seems to be much more of a problem for that EPIC contraption
> than for traditional RISC. Newer versions of the same RISC
> chip will execute old code considerably faster,
> without any modification, that's what the vendors
> claim and that's my experience. Of course one can "tune"
> for a specific CPU version, but then one may loose compatibility
> with future chip versions. It's up to the developer to decide
> whether the gain in speed is worth it.
>
>> For VLIW, a new implementation might have completely different
>> timing requirements, but is still forced to allow for the old ones.
>
> one reason one might consider VLIW to be a dead end.
>
>> There is one solution, though rarely used, and that is to have
>> the compilers output intermediate code, before the final bundling.
>> Then, either at program load time or install time, run the bundling
>> program for the specific processor available. It will then generate
>> the optimal code for that specific version.
>
> Isn't that what RISC chips do automagically via instruction
> scheduling? If the newer CPU has more functional units
> it could throw more instructions at them.
>

Not that it realy matters, but...
What if the Alpha chips had got the same development
efforts and money now spent on IA64 ? I think that
Alpha was a quite nice architecture as such.

No, it realy doesn't matter...