[mythtv] C++ style

[John P. Poet]

On Wed, 11 Feb 2004, Paul Woodward wrote:

> > On Wednesday 11 February 2004 13:25, steve at nexusuk.org wrote:
> > > On Wed, 11 Feb 2004, John P. Poet wrote:
> > > > For example, he says to always use pre-decrement/increment instead
> > > > of post-decrement/increment -- when possible.  The "pre" versions
> > > > can be converted into more efficient assembly language code, than
> > > > the "post" versions.  The only time you should use the "post"
> > > > version, is when you really need the increment/decrement to happend
> > > > after the *current* value of the variable has been used in the
> > > > expression.
> > >
> > > I would hope the optimizer on the compiler would optimize correctly,
> > > afterall, it's not hard for the optimizer to spot that you're not
> > > assigning the value to anything (in which case it doesn't matter if
> > > you do post or pre, so the optimizer can pick the fastest).
> >
> > That's only half true.  pre/post inc/decrement is also significant in
> > non-assignment expressions, like "if" conditions.  In fact, the only
> > time I can think of where it doesn't matter is if it's in it's own
> > statement by itself, with no assignment, like:
> >
> > 	foo++;
> > is equivalent to
> > 	++foo;
> > or
> > 	for (int i = someval; i < someotherval; i++)
> > is equivalent to
> > 	for (int i = someval; i < someotherval; ++i)
> >
> > but
> > 	if (somevar == ++i)
> > is definitely different than
> > 	if (somevar == i++)
> >
>
> I don't think even Microsoft could come up with up with a compiler that
>  generates such poor code that it takes a significant time to INC a
>  register on an nGhz chip!

True.  A post operation does incure the extra over-head of saving the
current value of the variable, before doing the operation.  The question
raised though, is the compiler smart enough to NOT save the current value
when it is not actually needed?

John