sergio masci wrote:

>>> 	byte_var = byte_var + word_var		// see (2)
>>> (2) ... add operation performed as two bytes
>> 
>> This is surprising, and may lead to a wrong result (for signed variables).
>> IMO this should at least trigger a compiler warning. 

I'd like to get your feedback on this. Is there something I was missing? 
Assuming all three being signed, at first I thought that something like 
(-127 + 254) would give a wrong result, but it doesn't: (0x81 + 0xfe) = 
0x7f = 127. Does this work for all cases where the result fits in a signed 
byte? Seems so... and is a bit counterintuitive for me.


> I got carried away in my enthusiasm and forgot about the promotion to 
> "int" in the conditional in C. But what I wrote also holds true for ints 
> and longs in XCSB whereas it does not in C

Yes, I thought about this, but since I didn't know whether you wanted to go 
there, I didn't include that in my reply.

> 	if unsigned_int_var > signed_int_var then
> 
> 		always true if unsigned_int_var > 32767
> 		always false if signed_int_var < 0
> 		otherwise true or false as expected
> 	endif

I think C treats the signed variable as unsigned, without changing its 
bits, so to speak. Which of course is not good, and may give an unexpected 
result. I tend to avoid that, unless I can guarantee the sign of the signed 
variable :) (And it definitely should trigger a warning. I'll have to try 
to see whether it does.) Your solution definitely makes more sense.


> and in anycase although the byte version has the same net effect under 
> both C and XCSB, the XCSB version does not need to generate zero and sign 
> extended ints prior to the comparison :-P

Since I have never worked with your compiler, I can't really say much. (I'd 
like to -- you have some interesting ideas --, but you know how that is... 
you have something that works, so you go with it :)  But at least the 
HiTech PICC compilers seem to do a good job of optimizing those operations, 
so that unnecessary promotions to int don't get executed. It's also 
sometimes effective (code-wise) to give the compiler hints in the form of 
type casts about the expected size of intermediate results.

Gerhard

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