Re: 0005-Search-all-dominated-blocks-for-expressions-to-hoist.patch

[Jeff Law <law at redhat dot com>]

On 06/16/10 10:27, Steven Bosscher wrote:
> On Wed, Jun 16, 2010 at 5:58 PM, Maxim Kuvyrkov<maxim@codesourcery.com>  wrote:
>    
> > 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.
> >      
> And makes the algorithm quadratic in the size of the CFG. You should
> limit the depth not only to avoid excessive live range lengths but
> also for corner cases of strangely-formed CFGs.
>    
Technically true, but we only care about the dominance tree here, not 
the entire CFG.  The change which made code hoisting only look at the 
immediate dominators was a mistake.  It's unfortunate that 
get_dominated_by only returns immediate dominators -- based on the name, 
one could reasonably expect to get the full set of dominators which I 
suspect happened back in 2002 when that change was made.

Maxim -- can you test f90-intrinsic-bit.f  with and without this change 
and report back on how the compilation time changes?

Thanks,
Jeff