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From: Andrew Dunstan on 19 Nov 2009 15:38


Boszormenyi Zoltan wrote:
>
> Is it *really* a bug? I recalled a comment from my C teacher
> in '92 or '93 about this exact issue, that the prefix/postfix
> increment/decrement operators are executed in the
> statement in an implementation-defined order,
>

Not if they come after a short-circuit operator such as && - after all,
that's what short-circuit evaluation implies. If the left hand operand
of && is false the right hand should not be evaluated at all.

cheers

andrew

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From: Tom Lane on 19 Nov 2009 15:51
Andrew Dunstan <andrew(a)dunslane.net> writes:
> Boszormenyi Zoltan wrote:
>> Is it *really* a bug? I recalled a comment from my C teacher
>> in '92 or '93 about this exact issue, that the prefix/postfix
>> increment/decrement operators are executed in the
>> statement in an implementation-defined order,

> Not if they come after a short-circuit operator such as && - after all,
> that's what short-circuit evaluation implies. If the left hand operand
> of && is false the right hand should not be evaluated at all.

Yes. && is a sequence point and the compiler is not allowed to move
side-effects across a sequence point. What your C teacher was warning
you against was things like
a[i] = i++;
'=' is not a sequence point so it's undefined which array index
will be stored into.

regards, tom lane

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From: Boszormenyi Zoltan on 19 Nov 2009 16:00
Tom Lane �rta:
> Andrew Dunstan <andrew(a)dunslane.net> writes:
>
>> Boszormenyi Zoltan wrote:
>>
>>> Is it *really* a bug? I recalled a comment from my C teacher
>>> in '92 or '93 about this exact issue, that the prefix/postfix
>>> increment/decrement operators are executed in the
>>> statement in an implementation-defined order,
>>>
>
>
>> Not if they come after a short-circuit operator such as && - after all,
>> that's what short-circuit evaluation implies. If the left hand operand
>> of && is false the right hand should not be evaluated at all.
>>
>
> Yes. && is a sequence point and the compiler is not allowed to move
> side-effects across a sequence point. What your C teacher was warning
> you against was things like
> a[i] = i++;
> '=' is not a sequence point so it's undefined which array index
> will be stored into.
>
> regards, tom lane
>

Thanks to both of you, this was really informative.
Actually my C teacher didn't mention such optimization barriers.
It seems I need to look up the raw C language specs...

Best regards,
Zolt�n B�sz�rm�nyi

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than these cometh of evil." (Matthew 5:37) - basics of digital technology.
"May your kingdom come" - superficial description of plate tectonics

----------------------------------
Zolt�n B�sz�rm�nyi
Cybertec Sch�nig & Sch�nig GmbH
http://www.postgresql.at/


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