Re: PowerPC shrink-wrap support 3 of 3

[Alan Modra <amodra at gmail dot com>]

On Wed, Oct 26, 2011 at 03:01:01PM +0200, Bernd Schmidt wrote:
> On 10/26/11 14:27, Alan Modra wrote:
> > Committed revision 180522.  It turns out that shrink-wrapping isn't as
> > effective as it used to be with the 20110915 based sources I was using
> > originally.  povray Ray_In_Bound no longer gets the benefit of shrink
> > wrap, likely due to some cfg optimization.  We end up with a simple
> > block that just does r3=1 then jumps to last_bb being reached from
> > blocks that need a prologue as well as blocks that don't.  That's
> > enough to kill our current shrink wrap implementation.  What we need
> > is something to duplicate these tail blocks..
> 
> Would it work to insert the epilogue on some edges to this R3=1 block,
> and not on the others?

Wouldn't you need to modify all the target epilogue code?  Our
epilogues return.

> (How many edges of each kind are there?)
In the povray case there was one edge of each kind, but I have seen
other cases where there were 4 edges from blocks needing no prologue
and 2 edges from blocks needing a prologue.  I can't tell you what the
testcase was now;  It was something I looked at when ironing out bugs
in my code.  You wouldn't believe how many ways it is possible to
write buggy cfg manipulation code..

I guess the tradeoff between the classic shrink-wrap epilogue scheme
and my duplicate tail idea is whether duplicating tail blocks adds
more code than duplicating epilogues.  From what I've seen, the
duplicate tails are generally very small.  I guess I should dump out
some info so we can get a better idea.

-- 
Alan Modra
Australia Development Lab, IBM