Re: seperate keyword and seperate compilation with Gnat?

[eachus@spectre.mitre.org (Robert I. Eachus)]

   Someone asked:

   > For example, suppose someone starts worrying whether it is valid to
   > compute a + b as (4 * a + b - 3 * a), which could raise intermediate
   > overflow.

    Lets look at this from a practical point of view.  Assume that 4 *
a + b was computed earlier and is available in a register.  The
compiler can save one load of b this way, so it could be a
"worthwhile" optimization.  Multiplying a by 3 can't cause an overflow
when 4 * a won't, so if you are in the same exception handler scope,
it looks like a valid optimization, even if for other reasons, the
computation of 4 * a + b didn't overflow.  (Perhaps 2 * a + b was in a
register, and b is a large negative number.)

    So I think it is legal unless it could cause an exception in a
different handler scope, again assuming that there was a partial lying
around.
--

					Robert I. Eachus

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