[Oberon] Runtime Overflow Checks

[Jörg]

I just tried

j := 1;
FOR i := 1 TO 128 DO
	INC(j, j);
	IF SYSTEM.COND(2) (* C flag *)                   THEN Out.printf("the CPU register has %d bits.\n", i, 0, "")
	ELSIF j < 0 (* N flag = SYSTEM.COND(0) *) THEN Out.printf("positive INTEGERs have %d bits.\n", i, 0, "")
	END
END;

It outputs:
positive INTEGERs have 31 bits.
the CPU register has 32 bits.

In both cases the V flag is set.

br
Jörg

Am 03.05.20, 13:55 schrieb "Oberon im Auftrag von Joerg" <oberon-bounces at lists.inf.ethz.ch im Auftrag von joerg.straube at iaeth.ch>:

    BTW: according to the RISC5 architecture guide overflow bit is only set with ADD/SUB not with multiplication, as the high part of the multiplication is stored in H register.
    
    Your loop works if you use  INC(j, j)
    
    Jörg
    
    > Am 03.05.2020 um 13:43 schrieb Chris Burrows <chris at cfbsoftware.com>:
    > 
    > Thanks Jörg that's useful to know. I did wonder what SYSTEM.COND was intended to be used for.
    > 
    >> -----Original Message-----
    >> From: Jörg [mailto:joerg.straube at iaeth.ch]
    >> Sent: Sunday, 3 May 2020 7:58 PM
    >> To: chris at cfbsoftware.com; ETH Oberon and related systems
    >> Subject: Re: [Oberon] Runtime Overflow Checks (was: Negative integer
    >> literals in Oberon)
    >> 
    >> Chris
    >> 
    >> Such a hidden feature does exist in the Oberon-07 compiler as well:
    >>    i := 47000;
    >>    i := i * 47000;
    >>    IF SYSTEM.COND(0) THEN Out.String("overflow"); Out.Ln END;
    >> 
    >> The parameter is the condition you like to check.
    >> 
    >> The case above could be catched with this as well
    >>    IF i < 0 THEN Out.String("overflow"); Out.Ln END;
    >> 
    >> br
    >> J�rg
    >> 
    >> ?Am 03.05.20, 02:22 schrieb "Oberon im Auftrag von Chris Burrows" <oberon-
    >> bounces at lists.inf.ethz.ch im Auftrag von chris at cfbsoftware.com>:
    >> 
    >>> -----Original Message-----
    >>> From: Oberon [mailto:oberon-bounces at lists.inf.ethz.ch] On Behalf Of
    >>> dave at brownsmeet.com
    >>> Sent: Saturday, 2 May 2020 11:49 PM
    >>> To: ETH Oberon and related systems
    >>> Subject: Re: [Oberon] Negative integer literals in Oberon
    >>> 
    >>> Personally I am surprised there is not more support in compilers (of
    >> all
    >>> languages) for generating run time integer overflow checking code.
    >>> 
    >> 
    >>    Inevitably run time integer overflow checking code leads to an impact on
    >> performance. Assuming that such an impact is considered to be acceptable,
    >> the next question is: What should a runtime system do when it detects an
    >> integer overflow? Some might say it should cause an application to come to
    >> an abrupt halt in mid-flight. That is not always desirable behaviour.
    >> 
    >>    To be palatable, systems that do generate runtime integer overflow
    >> checks also need to allow the programmer to disable them. If the checks can
    >> be turned on and off at will it becomes more difficult to verify that the
    >> application is correct. Hence, the scope of such switches must be as local
    >> as possible e.g. a single statement.
    >> 
    >>    An alternative solution is to give the programmer a means to detect when
    >> overflow has occurred rather than generating runtime overflow checks by
    >> default. The programmer would have a much better idea than the compiler
    >> implementer of when they were required and the best course of action to take
    >> when they were detcted. One example I have seen of this was in Wirth's
    >> Oberon-07 ARM compiler. This had an additional function SYSTEM.OVFL. My
    >> understanding is that this returns TRUE if the previous assignment generated
    >> an overflow condition that was detectable by the hardware. E.g. you could
    >> write something like:
    >> 
    >>    (* untested! *)
    >>    j := 1;
    >>    FOR i := 1 TO 128 DO
    >>      j := j * 2;
    >>      IF SYSTEM.OVFL(j) THEN Out.String("Size of INTEGER in bits = ");
    >> Out.Int(i, 0) END
    >>    END;
    >> 
    >>    Regards,
    >>    Chris Burrows
    >>    CFB Software
    >>    https://www.astrobe.com
    >> 
    >> 
    >> 
    >> 
    >>    --
    >>    Oberon at lists.inf.ethz.ch mailing list for ETH Oberon and related systems
    >>    https://lists.inf.ethz.ch/mailman/listinfo/oberon
    >> 
    > 
    > 
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