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==foo+bar)
    Something();

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==((unsigned char)(foo+bar)))
    Something();

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