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bug#8545: issues with recent doprnt-related changes

From: Paul Eggert
Subject: bug#8545: issues with recent doprnt-related changes
Date: Sat, 30 Apr 2011 22:41:38 -0700
User-agent: Mozilla/5.0 (X11; U; Linux i686; en-US; rv: Gecko/20110223 Thunderbird/3.1.8

On 04/30/11 14:03, Richard Stallman wrote:
>        long
>        foo (char *p, int i)
>        {
>          return &p[i + 1] - &p[i];
>        }
> ...
> i+1 is computed as an integer, but then it gets converted to a long.

Unfortunately that doesn't explain FOO's behavior.

If i+1 is computed as an int and if wraparound is required, then
when i is INT_MAX, i+1 must be INT_MIN.  (FOO does not convert
i+1 to long; but even if it did, INT_MIN's value would be unchanged by
that conversion.)  If p is pointing into a large array, &p[INT_MIN]
and &p[INT_MAX] can both be valid addresses, and in that case
foo (p, INT_MAX) would have to yield -2**32 + 1 on a typical 64-bit host
where signed integer arithmetic wrapped around.

But in my experience no compiler does it that way.  FOO always returns 1.

> What happens here seems to be an issue about type conversion combined
> with addition -- not addition itself.

I'm afraid not.  First, FOO doesn't have any type conversions.
If I is an int, A[I] doesn't convert I to any other type; it
simply uses I's value without conversion.

Second, it's easy to construct an example that involves only "int":

   bar (int i)
     return i < i + 1;

With many compilers, BAR always returns 1, even when i == INT_MAX.

> These compilers are taking a strange liberty.
> Why isn't that a bug?

Well, for starters, most programmers *expect* FOO to return 1,
and similarly for BAR.  Why would a programmer file a bug report
when the program is behaving as expected?

>     printf ("%d", INT_MAX+1);
> will output INT_MIN.

That's true for all systems I have ready access to, yes.  And I
expect there are other cases where int arithmetic wraps around
reliably.  But there are many practical cases where it doesn't.
We cannot simply advise programmers to assume that adding 1
to INT_MAX always results in INT_MIN, as that assumption is
often incorrect nowadays.

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