Paul Schlie wrote:
Gabriel Dos Reis wrote:
I predict we will end up changing nothing for GCC; I also predict that
he'll be unsatisfied with unsupported claims; then, there is a high
chance the discussion will be restarted again in a different form
(hey, this is not the first time you, Paul and me are involved in this
kind of undefined discussion). If we have good answers with backed up
claims, we could just reference it later.
...
I think supported answers will help diminish undefined disucssion frequency.
If you don't have a definition for "good reason", that is fine. Let's move
on something else; this is a new year.
Gentlemen, I honestly don't want to promote a frivolous debate either; as
such, if it is generally agreed there are no valid conceivable reasons to
enable GCC to allow a target to specify the semantics of its particular
implementation to enable optimizers to utilize that information such that
its otherwise non-optimized behavior may be preserved during optimization,
then I simply accept that I'm alone in the believe of its significance and
utility; if not, then it seems that the only questions remaining are related
to how may this be done relatively easily within the current optimization
and target definition framework?
If specific examples are required, although likely considered obvious,
here's a few:
- x[y] = 0; // may also be undefined if y is one past the
if (x[y]) y = y+1; // extent of x[], and/or result in an overflow
// of x[y] one past the it's upper bound if
// allocated at the upper range of the address
// space, and/or if allocated at the base
// address of 0 and y is 0 or negative; but
// non-the less the machine evaluating the code
// will do something which is likely well
// defined, and must be defined if that logical
// behavior is to be predictable and thereby
// preferable during optimization.
We have been through this before, there is no requirement that
optimization preserve the behavior of
undefined programs (indeed that is often one of the primary motivations
in making things undefined).
It is fine to argue that defining the semantics is useful in a
particular case, but arguing solely from the
point of view of trying to preserve observed behaviort is a poor
argument. Indeed the point is that
optimization is not changing the behavior, the behavior is
non-deterministic, and can change from
one compilation to the next even if optimization does not change.
- x = 5 << z; // where although implementation specific, must
if (x > 0) z = 2; // to be definable if to be utilized as the
// basis of a target specific behavior
// preserving optimization.
ditto
