13 May
2005
13 May
'05
12:11 p.m.
From: "Dave Dunfield" <dave04a at dunfield.com>
So, although unlikely that the compiler would defer
two operations and not
another, it is potentially possible that the "results" could also be 0 0
1,
0 0 2, 0 1 1 etc. (in any order).
Agreed.
Officially this type of code (that which modifies an
Lvalue multiple times
within the same sequence interval) invokes "undefined behaviour", which
means that the compiler can do "anything" - although highly unlikely, the
compiler can happily return 6 6 6 in this case, and leave the value of i
at
31415 (or anything else) ... and be performing in a
perfectly legal sense
- undefined operation is exactly that. The function performed is not
defined
by the standard.
I think the functions to be performed are defined, and only their order is
undefined.
On other words, it's not valid 'C'
(although it is syntactically correct,
and
most compilers will accept it without a diagnostic).
Which would make this an overstatement. I think it is perfectly valid C,
and the compiler has (after much deliberation) been allowed to generate the
most efficient code.
I think the OP's point has been completely missed
(that a valid program is
a
inherently very definitive and detailed
specification).
I thought the point was that, at the margins, the "valid program" is *not*
as detailed a specification as was claimed/expected. (Though I still don't
know of anything more detailed. In fact, I think most programs are
over-specifications of the desired behavior. That is, they require a lot of
specific behavior that is actually unimportant to the correct result.)
Vince