discouraging register use

[DJ Delorie]

> If the read-modify-write operand has double cost for the memory
> access, it should be more like:

Oh, right.  Good idea.  Thanks!

> To allow realod inheritance to work where the same value happens to lie around in a 'c'
> class register, you also have to extend all the 'm' alternatives with a 'c':
>   [(set (match_operand:SI 0 "" "c,c,??mc,??mc")

How does this related to Richard's comment that this denigrates the "c" class also?

>         (plus:SI (match_operand:SI 1 "" "0,0,0,0")
> 		 (match_operand:SI 1 "" "c,?mc,c,?mc")))]
> and make sure the union of the two classes is a register class.

What about when there are three classes, say c, d, and m - do I need
cm, dm, and cdm unions, or just cm and dm?

> Another thing to consider is if you want register class preferencing to ignore or further
> penalize the 'm' class.
> E.g. this would triple the penalty for register preferencing:
>   [(set (match_operand:SI 0 "" "c,c,??mc#????,??mc#????")

Ok, someone needs to write a book about all this ;-)

> You might also consider making your addsi commutative, i.e. adding a '%' constraint modifier.

Yeah, the real code has that.