Re: c++/1687: Extreme compile time regression from 2.95.2

["Zack Weinberg" <zackw at Stanford dot EDU>]

On Wed, Feb 14, 2001 at 06:07:08AM -0500, Scott A Crosby wrote:
> On Tue, 6 Feb 2001, Zack Weinberg wrote:
> 
> > Using the 'visit each node only once' mechanism of walk_tree
> > thoroughly squelches the performance problem.  (One can't use
> > walk_tree_without_duplicates blindly - slightly more cleverness is
> > required.  It's still simple.)  We get sub-second compile time all the
> > way up to -O2 and 32 input mux().
> 
> > HOWEVER: I am not certain that the change is correct.  Suppose that a
> > function A is a candidate for inlining, and it's called twice from the
> > same function B.  If the two call_expr nodes are shared, we won't
> > inline both calls.  There may be other problems as well.
> > 
> > Patch follows.  Commentary from C++ team appreciated.  Will bootstrap
> > and report.
> 
> 
> Could you workaround this by walking the tree normally, and try to inline
> at all of the nodes, but if you find out that you are scanning too many
> nodes, you abort and use a workaround strategy, say, walk-each-node once?
> 
> Catch the pathalogical cases and shunt them elsewhere, and behave normally
> otherwise?

Let's find out if always walking each node once is safe, first.  It
doesn't cause any C++ regressions, but I still don't know if it
inhibits optimizations.  Paging C++ team...

zw