Re: Variable "foo" is not available

[Daniel Jacobowitz <drow at false dot org>]

On Mon, Apr 04, 2005 at 10:30:23PM +0300, Eli Zaretskii wrote:
> But this kind of code is only possible if the compiler examines all
> the callers of `bar' and finds that none of them uses the value of
> `bar's argument after `bar' returns.  So such code is probably only
> possible in practice with static functions, right?  Or am I missing
> something?

That would be true if the ABI labeled the stack slots as preserved
across the function call.  The consensus seems to be that it does not;
at the point of the function call they become part of the callee's
stack frame and it is free to modify them.

> Also, is such optimizations really worth it?  I mean, the more
> traditional code will mov the argument into a register and do the math
> there; is adding to a memory location really faster than a mov and a
> register-based add?

Yes, especially on decoding-limited processors.

Also, sibling calling does the same thing.  It reuses the stack slots
to pass arguments to a new function, and then jumps to the function.

> > GCC won't reuse the slot for an unrelated variable at present. 
> > However, in the future, it would be a valid optimization.
> 
> Again, only if the compiler has enough information about the callers,
> right?

No, as above.

-- 
Daniel Jacobowitz
CodeSourcery, LLC