Re: Abusive (?) promotions to int

[Mike Stump <mrs at apple dot com>]

On Wednesday, September 24, 2003, at 03:15 AM, Andreas Schwab wrote:
> > I've been looking for macros to control integer promotions, 
> > especially for
> > temporary statements. Consider for instance the following code:
> >
> >     char c;
> >     ...
> >     if (c + 4 > 10) ...
> >
> > On my target, c gets zero_extended to int, then the addition and 
> > comparison
> > are performed in SImode, while it is not really necessary (could be 
> > done in
> > QImode as well and would save the zero_extend).
>
> No, it cannot.  You would get wrong behaviour if c >= 252 (assuming 
> char
> is unsigned 8 bit), because (char)(c + 4) < 10, but c + 4 > 10.

?  But, while we can envision bad code gen, can't we also envision good 
sequences as well:

cond = c + 4 > 10;
cond = ((int)c + 4) > 10;
cond = ((int)c) > 6);
cond = ((char)c) > (char)6);

or is there something I'm missing?  When the compiler is used on small 
machines, this type of optimization, if possible, is useful.