register variable???

[Norman Diamond]

In article <30585 at srcsip.UUCP> sklee at srcsip.UUCP () writes:

>  i = 0;
>  while (i < 4)  list[i] = list[++i];
>
>... if I define "int i" instead of "register int i",
>the result is [different]

This question seems to come up twice a week now.

list    [    i    ]    =      list    [    ++    i    ]
        [b]  [a]       [last] [4]     [3]  [2]   [1]

There are several possible sequences of evaluation.
Operation [1] (fetching value of i) must precede [2] (increment).
Operation [2] must precede [3] (array indexing) because it is a
pre-increment operation.
Operation [3] must precede [4] (fetching value of array element).
Operation [a] (fetching value of i) must precede [b] (array indexing).
All of the above must precede the assignment [last].

Notice what isn't specified?  Does operation [a] come before or after
operation [2]?  [a] could even come before [1] or after [4].

On the right-hand side, the rules of the language specify that
list[++i] uses the new value of i.  On the left-hand side, the rules
do not specify whether list[i] uses the old or new value of i.

If you were lucky, the compiler would call a random number generator
to decide whether to use the old value of i or the new value.
Unfortunately it chose one way for "register" and the other way
for non-"register".  Therefore you were confused, and you thought
that "register" makes a difference.  It does not.

--
-- 
Norman Diamond, Sony Corporation (diamond at ws.sony.junet)
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