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.
