On 08/01/2016 03:42 AM, Segher Boessenkool wrote:
> This is the second version.  Concern was renamed to component, and all
> other comments were addressed (I hope).

Not really, I'm afraid. There still seems to be no detailed explanation 
of the algorithm. Instead, there is a vague outline (which should be 
expanded to at least the level of detail found e.g. in tree-ssa-pre.c), 
and inside the code there are still meaningless comments of the form

/* Deselect those epilogue components that should not be inserted
    for this edge.  */

which don't tell me anything about what the intention is (why should 
they not be inserted?). The result is that as a reader, I still find 
myself unable to determine whether the algorithm is correct or not.

Worse, I'm still not entirely certain what it is intended to achieve: I 
asked for a motivating example or two, but haven't seen one in the 
comments anywhere. My most general question would be why the algorithm 
for placing individual prologue components would be different from the 
regular shrink-wrapping algorithm for full prologues. Examples and/or 
arguments relating to how this new code acts with regard to loops are 
also particularly needed.

So, I think improvement is necessary in these points, but I also think 
that there's room for experimental verification by way of self-tests. If 
done thoroughly (testing the transformation on several sufficiently 
random CFGs and maybe some manually constructed ones) it would go a long 
way towards showing that at least we don't have to worry too much about 
miscompilations. That's what I've been working on, and an initial patch 
is below. This is incomplete and posted more as an RFD since we're 
getting towards the end of the week: there are gaps in the test 
coverage, and it currently fails the selftest. I assume the latter is a 
problem with my code, but it wouldn't hurt if you could take a look; 
maybe I misunderstood something entirely about what the separate 
shrink-wrapping is supposed to achieve, or maybe I messed up the 
algorithm with my changes.


Bernd