Re: "Interesting" C behaviours
From: Rennie deGraaf (ca.ucalgary.cpsc_at_degraaf)
Date: 11/27/04
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Date: Sat, 27 Nov 2004 07:08:01 GMT
Malcolm wrote:
> "Rennie deGraaf" <ca.ucalgary.cpsc@degraaf> wrote
>>2. In C99, the expression 'a%b' where a<0 and b>0 will return a negative
>>result (K&R lets this be machine dependent).
>>
>
> That would probably be driven by architecture. The idea is that a % b would
> compile to a single machine instruction (on K and R's original platform).
> Just a guess, but I suspect this is the motivation.
It was architecture dependent in K&R, but C99 standardized it (see
http://home.tiscalinet.ch/t_wolf/tw/c/c9x_changes.html#Semantics,
section 25). I know that the x86 idiv instruction spits out negative
residues, but is the fact that a common architecture does something
unusual a reason to standardize a programming language on an unusual
behaviour? It would make more sense to me to work around the
architecture when it does something weird.
GCC, for instance, frequently doesn't even compile a%b to a single idiv
instruction - it compiles it to a big mess of imul, shift, and leal
instructions.
Rennie
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