Help with casts

[Richard Caley]

In article <414 at ceco.ceco.com>, Garry Garrett (gg) writes:

gg> Bear in mind that C is one of the hardest languages in the world
gg> to optimize.  For instance, in pascal the index of a for loop
gg> cannot be referenced outside of that loop, thus the code can be
gg> optimized by making that index a register variable.

a) Whether you reference outside will not affect whether it can be
made a regiser variable (though it might affect the advisability).

b) it is amazingly trivial for the compiler to look and see if it is
referenced outside.

gg> You must explicitly ask the compiler to make a varible into
gg> a register variable by declairing it as such. 

There is no way to do this. `register' is, at best, a hint. The
compiler will stick what it likes into registers.

gg> There are many reasons why C is harder to optimize, 

To coin the old phrase -- name three.

The only ones which comes to mind are pointers and casts, both of
which will only be a problem if you use them, so there is no loss. 

gg> so when you are programming in C, you must not count on an
gg> optimizer to clean things up for you.

If any of the trivial micro-optimisations which were listed in that
article make any difference, I think someone needs to kick the
compiler writer:-)

Counting down rather than up is only faster if compare against 0 is
faster on your machine. The compiler is more likely to knowthat than
you. 

gg> Sorry, but my code is still faster.

It's only faster if it runs faster, arguing about hypothetical
(broken) compilers on hypothetical machines proves nothing. 

--
rjc at cstr.ed.ac.uk