[fpc-devel] Division nodes

J. Gareth Moreton gareth at moreton-family.com
Mon May 15 19:47:46 CEST 2023

Thank you!  I knew it must have been something simple like that!

I just did a quick test with x86 assembly language - this is the code 
that used to run for dividing min_int by -1:

MOV EAX, $80000000

EAX contains $80000000 as a result.  When using IDIV:

MOV EAX, $80000000
MOV EDX, $FFFFFFFF // Sign extension

Exception SIGFPE raised.

I understand now.  Thanks for pointing out min_int div -1.  I'll 
experiment a bit more though to see if generating the setup is necessary 
in all circumstances (e.g. if an unsigned operand is being expanded into 
a larger signed value, it isn't necessary). At the very least, I'll add 
a new test for this and also put a comment in the code so it's clear why 
-1 needs special treatment (the fact I got no regressions means this 
very specific set-up is not tested properly).


On 15/05/2023 17:43, Kirinn via fpc-devel wrote:
> I didn't see a mention of it in this discussion thread, but dividing the
> largest negative integer by -1 does cause an overflow error of some sort,
> because of two's complement.
> Just to make sure, do (min_int div -1) and (min_int mod -1) behave the same
> way before and after this proposed change? And, what min_int is of course
> depends on whether targeting a 32-bit or 64-bit system, so best check both
> cases.
> ~Kirinn
> On Mon, 15 May 2023 17:21:30 +0100
> "J. Gareth Moreton via fpc-devel" <fpc-devel at lists.freepascal.org> wrote:
>> I made a merge request that removes the comparison against -1.
>> x86_64-win64 and i386-win32 pass without any regressions, so the reason
>> behind the change still eludes me.  Maybe my fix for #39646 removes the
>> need for it?
>> https://gitlab.com/freepascal.org/fpc/source/-/merge_requests/418
>> Kit
>> On 11/05/2023 23:04, J. Gareth Moreton via fpc-devel wrote:
>>> Fair enough, but I would like an explanation as to why it's necessary,
>>> because the reason for testing -1 in particular is very unclear, and I
>>> wonder if there's a known misbehaviour with a particular division
>>> function with -1.
>>> Kit
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