[tree-ssa] Out of SSA status and issues

[Andrew MacLeod]

On Mon, 2003-05-12 at 12:04, Michael Matz wrote:
> Hi,
> 
> On 12 May 2003, Andrew MacLeod wrote:
> 
> > I beleive the bvrute force approach is to flag any variables which are
> > elements of PHI's acroiss an abnormal ciroitcal edge, and never copy
> > propagate them  ie, so in the example, when you look at the PHI, you see
> > p_9 and p_14 are used across abnormal critical edges, so you stick them
> > in a list of variables never to copy propagate.
> 
> A better heuristic is to start with ignoring abnormal critical edges.
> Then _if_ there are copies inserted on such edges, mark the involved
> variables as never-propagate.  In that case rerun the pass.  Yes, this
> sometimes is slower, but produces better code (i.e. something or -O3
> depending on how slow it actually is).
> 

Unfortunately, we don't know which edges we are going to need to insert
copies on until we are exiting SSA form and have built the interference
graph et all. So by that point, its too late to make a decision not to
do something in an earlier pass.

Andrew