Currently, code hoisting only checks immediately-dominated blocks for 
expressions to hoist.  I wonder if limiting the search for expressions 
is intentional.

This patch makes code hoisting search through all dominated blocks for 
expressions to hoist.

On one hand, hoisting expressions from all dominated blocks, not just 
the immediate dominees, provides significantly greater unification 
opportunities.  On the other hand, it can also substantially extend live 
ranges of pseudos, and increase register pressure.

Even considering the negatives, hoisting expressions from non-immediate 
dominees seems like the right choice to me.  Most expressions that can 
be moved several basic blocks up are constants, and IRA/reload should be 
able to rematerialize those under high register pressure.  On the 
flip-side, if an expression is complex, than it would be less costly to 
spill/restore it instead of calculating it in dominated blocks.

A compromise may be to limit the depth of search with a parameter.

OK to apply?

Thank you,

-- 
Maxim Kuvyrkov
CodeSourcery
maxim@codesourcery.com
(650) 331-3385 x724