Short circuit evaluation/expression rearrangement (2nd summary)

[Dale Worley]

   In article <16283 at smoke.brl.mil>, gwyn at smoke.brl.mil (Doug Gwyn) writes:
   > >... in "a + b + c",
   > >the first addition must be performed before the second, because it
   > >associates as "(a + b) + c", showing that the second addition has as
   > >an argument the result of the first.
   > 
   > The above is incorrect.  A conforming implementation is obliged to
   > perform the operations so that it doesn't create an exception where
   > one would not have occurred in evaluating the expression strictly
   > according to the precedence implied by the grammar, but within
   > those constraints it may perform the additions in any order.  To
   > force an order of evaluation (in Standard C), you must use parentheses.

However, if the computation is floating-point, problems can arise
above and beyond exceptions.  The usual problem is loss of
significance.  See section 3.3 of the Rationale of the C standard for
an explanation, which makes it clear that (in Standard C) a + b + c
*must* be evaluated as (a + b) + c if a, b, and c are floating point.

(Also, beware that the Rationale claims that Fortran has the same
no-regrouping rule as the C standard.  That's not true.)

Dale Worley		Compass, Inc.			worley at compass.com
--
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