[fpc-devel] Division nodes

[Florian Klämpfl]

Am 19.05.23 um 21:14 schrieb J. Gareth Moreton via fpc-devel:
> So I need to ask... should the check for a divisor of -1 still be 
> performed? 

Yes. This is the result of "down sizing" a division. In case of

longint(int64 div int64) can be converted only into longint(int64) div 
longint(int64) if this check is carried out. longint($80000000 div 
$ffffffff) must silently result in $8000000 in this case.

> 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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