C \"optimization\"

[VLD/VMB]

The so-called "optimizer" in the older UNIX C compilers
is really an "object code improver" that runs over the
ALREADY-GENERATED assembly code and substitutes better
instruction sequences, rearranges blocks of code to
eliminate unnecessary branches, etc.  Most compilers
also have a very modest amount of expression tree
rearrangement, but this is intended more to assist in
code generation than to really optimize execution.

Several independent compiler vendors have added true
"optimization" that works directly with the compiler's
internal representation of the program being compiled.
One such optimizer for PCC was described a few USENIXes
ago (Data General, I think), but that product was not
going to be made available to the UNIX community at large.

Over-optimization can actually be a bad thing.  Suppose
one is assigning to a shared memory location or device
register or something like that.  You want to know that
the compiler will generate code that does what you tell
it, even though it "looks" pointless.  Since C was
developed for just this kind of programming, that is an
important consideration.  However, a hook ("volatile")
is being added to the language in the forthcoming ANSI
X3J11 standard that will permit highly optimizing
compilers and a guaranteed way of telling the compiler
to do what you are trying to in such cases.  X3J11 also
defines the concept of "sequence points" which are those
places in code that execution side-effects have to be
caught up.

("Optimizers" that turn correct programs into non-
working code have been quite common in the past, but
that is a different type of problem.)

I don't worry much about optimizing code like that of
your example, since a programmer would presumably not
be performing arithmetic on variables if constants
would do.  If he does, it "must" be for some good reason.