I've read what the others said, and if the following was the case
you'd expect it to have been said. This is somewhat implicit in at
least one reply.

As
=A0=A0=A0=A0=A0=A0 foo+bar

is capable of overflowing an 8 bit word, then at least bat needs to be
int for safety.

Once you start going 'int char char'=A0 you can imagine the compiler
wanting to have consistent type, even though obviously unnecessary.

Using logical operators and two 8 bit words, which cannot ever produce
a non char result doesn't trigger the action.



=A0 Russell




On 30 March 2010 11:06, Bob Blick <bobblick@ftml.net> wrote:
>
> I'm using HiTech C on a 16F688 and notice a promotion I didn't think
> should happen. Here's a little example. "Something" is unimportant but
> included for context.
>
> unsigned char foo;
> unsigned char bar;
> unsigned char bat;
>
> if(bat=3D=3Dfoo+bar)
> =A0 =A0Something();
>
> The compiler promotes the unsigned chars to integers, does the add and
> does the comparison. In the past I always thought HiTech used the native
> data size unless there was a reason not to (although I can't seem to
> find reference to that right now). And since everything, including the
> left operator "bat" are unsigned chars, I assumed (incorrectly) that all
> the math and comparisons would be 8 bit. Optimization is set as high as
> it goes, and this is the paid version of the compiler.
>
> Manually casting does fix it:
>
> if(bat=3D=3D((unsigned char)(foo+bar)))
> =A0 =A0Something();
>
> And something like this also does efficient 8 bit math:
>
> if(!(bat^(foo&bar)))
>
> but this doesn't:
>
> if(!(bat^(foo+bar)))
>
> so it isn't that the comparison with a constant (the implied "0") is the
> reason for the promotion, it's the use of addition.
>
> I haven't been able to find any reference that covers these cases in
> 8-bit implementations of "C" and my HiTech manual hasn't given me
> anything either.
>
> Any idea where I can look for definitive rules on this?
>
> Thanks,
>
> Bob
>
>
>
> --
> http://www.fastmail.fm - Send your email first class
>
> --
> http://www.piclist.com PIC/SX FAQ & list archive
> View/change your membership options at
> http://mailman.mit.edu/mailman/listinfo/piclist

-- =

http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist