[Oberon] Assumed issue in the Lola-2 compiler

Hellwig Geisse hellwig.geisse at mni.thm.de
Tue Apr 16 09:42:39 CEST 2019

Hi all,

just two remarks:

On Mo, 2019-04-15 at 16:53 +0200, joerg.straube at iaeth.ch wrote: 
> In my point of view the Oberon spec is rather clear:
> q = x DIV y, r = x MOD y, x = q*y + r, 0 <= r < y

This implies 0 < y and leaves the case y <= 0 unspecified
(y = 0 can of course be excluded).

On Mo, 2019-04-15 at 14:35 +0200, rochus.keller at bluewin.ch wrote:
> Ok, I think I found a decent solution. 
> [..]
> But I can avoid this indetermination by using the following
> two functions and re-using features of the C language.

The C language spec is rather reluctant to exactly
define the result of the % operator (at least the
C89 version; I didn't look into the newer standards).
It says (K&R, 2nd edition, A7.6 Multiplicative Operators):
".. it is always true that (a/b)*b + a%b is equal to a.
If both operands are non-negative, then the remainder is
non-negative and smaller than the divisor; if not, it is
guaranteed only that the absolute value of the remainder
is smaller than the absolute value of the divisor."

The implication of this is that you cannot use %
consistently across implementations (and not even
within an implementation) if any of its arguments
could be negative.


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