Re: [PATCH v2 0/9] Separate shrink-wrapping

[Segher Boessenkool <segher@kernel.crashing.org>]

On Fri, Sep 09, 2016 at 09:59:03AM -0600, Jeff Law wrote:
> On 09/09/2016 09:17 AM, Segher Boessenkool wrote:
> >On Thu, Sep 08, 2016 at 10:41:37AM -0600, Jeff Law wrote:
> >>So can you expand on the malloc example a bit -- I'm pretty sure I
> >>understand what you're trying to do, but a concrete example may help
> >>Bernd and be useful for archival purposes.
> >
> >Sure, but it's big (which is the problem :-) )
> Yea :(  But it's likely a very compelling example of real world code. 
> It's almost certainly too big to turn into a testcase of any kind, but 
> just some before/after annotated code would be helpful.

I'll work on it, but it won't be before tonight, probably quite a bit
later.  Seeing the difference in generated machine code is probably
most useful?  Better than RTL ;-)

> Ideally we'd have some smaller testcases we could put in the testsuite 
> to ensure that the feature works over-time in the way intended would be 
> helpful as well.

I might be able to make some really tiny testcases that will not need
updates all of the time.  We'll see.

> Right.  That's always been my understanding of the key driver for 
> placement.  There's exactly one and will be executed one time or none 
> across all paths in the CFG.

But that is not because it is good to have only one!  GCC expects there
to be only one, instead.  Some ports might even use prologues that cannot
be duplicated at all.

> And in separate shrink-wrapping world, we're leaving that model behind 
> and I think that's one of the big things I'm struggling with -- we may 
> execute a prologue component more than once if I've read everything 
> correctly.

Yes, and that is perhaps radically new in the GCC world, but not anywhere
else.

[ ... ]

> This really feels comparable to block duplication for the purposes of 
> isolating a particular path through the CFG so that path can be modified 
> without affecting the behavior of other paths through the CFG.

Yes, very much.

> It's also directly comparable to block duplication to allow more 
> aggressive code motion in PRE-like algorithms.

Yeah.


Segher