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

[Kazu Hirata]

Hi Jim,

> 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.

Thanks.  Actually, I wanted to find a case where loading a constant in
a larger mode is as cheap as in a smaller mode (for a special constant
like 0) to tune move2add a little bit.

Kazu Hirata