Predefined Version expansion

[Don Clugston]

Dan wrote:
> Dave Wrote:
>> Just a thought for D -- add a global CPU type to the D standard runtime that is set during 
>> initialization (i.e.: in Dmain) that can be used by user code like BLADE and by the compiler itself 
>> for (future) processor determined optimization branches.
>>
>> Actually, I'm kind-of surprised one isn't there already (or is it?).
> 
> There is already a cpu variable/object/module or something within the phobos library which provides cpu related information.  The problem being discussed is that this information is stored and branches are made based on the cpu type during *runtime*.
> 
> If instead, it was pre-determined during compile time, these branches could be optimized out.  Since version() is designed to perform precisely this function, it makes sense to continue to use version() to identify and target cpu's during compile time.

Yes. All that's required is for the spec to include standard names for 
the CPU types. I don't think any DMD compiler changes are required.

How about these:

X86_MMX   // necessary? (MMX is dead technology).
X86_SSE
X86_SSE2
X86_SSE3
X86_SSSE3 // is this really necessary?
X86_SSE4

Only change would be that the GDC compiler for X86_64 should set all of 
the above.
Right now, the predefined version identifiers cannot be set from the 
command line, so eventually the compiler would need a CPU switch to 
control them - it would specify that the compiler has freedom to use 
recent opcodes, but of course it could continue to generate exactly the 
same code. I think such a switch would be necessary for supporting the 
3- and 4-element array types with swizzle functions discussed recently.

However, if we all agreed to use the same set of version identifiers, we 
can get going immediately.