Which of (set (reg:HI) (const_int 0)) and (set (reg:SI) (const_int 0)) is cheaper?

[Jim Wilson]

Kazu Hirata wrote:
> How would one know which of
>   (set (reg:HI) (const_int 0))
> and
>   (set (reg:SI) (const_int 0))
 > is cheaper?

One doesn't.  It is a historical error that constants don't have modes, 
and changing it now would be very hard.  However, it is a fairly safe 
assumption that SImode moves are more expensive than HImode moves if 
registers are HImode.  This is something you can check for.  At its 
simplest, you could check UNITS_PER_WORD and assume anything larger is 
more expensive than anything same sized or smaller.

It might even be reasonable to do something like this in rtx_cost in 
cse.c.  Currently it assumes that registers are 0 cost, regardless of 
the size of the register.

Jim