Re: Optimization question

[James E Wilson <wilson at specifixinc dot com>]

Piotr Wyderski wrote:
> Why does it generate the instructions pointed to by (<=== HERE)?
> The most mysterious one is at 46fa2: the value is never used.
> If it were eliminated, the stack frame management code would be
> useless too. Where's the source of such artifacts and how can
> I avoid them?

You didn't provide a compilable testcase.  Without a testcase, we can't 
say with any certainty what happened.

It appears that the iterator i got allocated to a stack slot.  Once we 
have writes to the stack, it is very difficult to eliminate them, as 
that requires a fully general alias analysis pass, which is a hard 
problem to solve.

There is a good chance that this is already fixed in gcc mainline.  gcc 
has always had a tendency to allocate variables to regs or stack before 
performing optimizations, which caused optimization problems for 
everything that got allocated to a stack slot.  Current mainline will do 
some optimizations before putting variables into stack slots, thus 
sometimes avoiding problems like this.
--
Jim Wilson, GNU Tools Support, http://www.SpecifixInc.com