[fpc-devel] Division nodes

[J. Gareth Moreton]

So I need to ask... should the check for a divisor of -1 still be 
performed? The case of doing "min_int div -1", even with 
unsigned-to-signed typecasting, seems very contrived and the programmer 
should expect problems if "min_int" and "-1" appear as the operands.  Is 
there a specific example where this implicit check is absolutely 
necessary?  As others have pointed out, silently returning "min_int" as 
the answer seems more unexpected (granted this is just the behaviour of 
an optimisation that converts the nodes equating to "x div -1" to "-x", 
and Intel's NEG instruction doesn't return an error if min_int is its 
input operand, but I can't be sure if the same applies to non-Intel 
processors and their equivalent instructions).

Kit

On 17/05/2023 09:51, J. Gareth Moreton via fpc-devel wrote:
> Logically yes, but using 16-bit as an example, min_int is -32,768, and 
> signed 16-bit integers range from -32,768 to 32,767. So -32,768 ÷ -1 = 
> 32,768, which is out of range.  This is where the problem lies.
>
> Internally, negation involves inverting all of the bits and then 
> adding 1 (essentially how you subtract a number using two's 
> complement), so min_int, which is 1000000000000000, becomes 
> 0111111111111111 and then, after incrementing, 1000000000000000, which 
> is min_int again.
>
> Kit
>
> On 16/05/2023 13:13, Jean SUZINEAU via fpc-devel wrote:
>> Le 16/05/2023 à 01:47, Stefan Glienke via fpc-devel a écrit :
>>> min_int div -1
>>
>> "min_int div -1"  should give  "- min_int" ?
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