Postreload and STRICT_LOW_PART

[Paolo Bonzini]

On 09/20/2009 06:31 PM, Richard Guenther wrote:
> On Sun, Sep 20, 2009 at 6:10 PM, Andreas Schwab<schwab@linux-m68k.org>  wrote:
>> Why is postreload converting (set (REGX) (CONST_INT A)) ... (set (REGX)
>> (CONST_INT B)) into (set (STRICT_LOW_PART (REGX)) (CONST_INT B))?  That
>> looks like a pessimisation especially if the constants are small, since
>> STRICT_LOW_PART must not touch the high part.  Is there a way for the
>> backend to stop postreload from doing this if the constants are in some
>> range?  On the m68k, loading a constant in the range -128..127 in SI
>> mode is better than loading it in strict QI mode.
>
> It's probably an omission of a check if A&  ~GET_MODE_MASK (narrow_mode)
> is equal to zero.

Actually it is (A^B) & ~GET_MODE_MASK(narrow_mode) that has to be 0.

> And of course a cost check is completely missing.

I think that adding a cost check would be the right thing to do.  For 
size, a QImode move is probably better (it is on i386 for example).

Paolo