Help with ivopts

[Michael Matz]

Hi,

On Wed, 6 Jul 2011, Richard Sandiford wrote:

> The individual difference_cost and add_cost seem reasonable (4 in each 
> case). I don't understand the reasoning behind the division though.  Is 
> the idea that this should be hoisted?

Yes, it should be hoisted outside the loop.  The difference is between two 
loop-invariant values (the bases), and hence is also loop-invariant.  Some 
tree optimizer should do this already, possibly the casts confuse us.

> If so, then:
> 
> (a) That doesn't happen at the tree level.  The subtraction is still inside
>     the loop at RTL generation time.
> 
> (b) What's the advantage of introducing a new hoisted subtraction that
>     is going to be live throughout the loop, and then adding another IV
>     to it inside the loop, over using the original IV and incrementing it
>     in the normal way?

It can reduce address complexity for one of the addresses.  E.g. given:

 i=0; i < end; i+=4 
   p[i];
   q[i];

-->

 n=p; n < p+end; n+=4
   [n];
   (q-p)[n];

Here (q-p) is loop-invariant, and the complexity of the first address is 
lower (no offset).  In fact the register pressure is lower by one too 
(three instead of four, including the end/p+end bound).


Ciao,
Michael.