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

[Jeff Law <law at redhat dot com>]

On 09/09/2016 09:28 AM, Segher Boessenkool wrote:
> > Segher's code essentially allows individual components of the prologue
> > to sink to different points within the function rather than forcing the
> > prologue to be sunk as an atomic unit.
>
> It also allows prologue an epilogue components to be placed in multiple
> places,
Right.  I need to keep that in the forefront of my mind.



and even allows them to be executed more than once, if that is
> cheaper.
This is the part that I'm still struggling with.

> > > > The full-prologue algorithm makes as many blocks run without prologue as
> > > > possible, by duplicating blocks where that helps.  If you do this for
> > > > every component you can and up with 2**40 blocks for just 40 components,
> > >
> > > Ok, so why wouldn't we use the existing code with the duplication part
> > > disabled? That's a later addition anyway and isn't necessary to do
> > > shrink-wrapping in the first place.
> > I think the concern here is the balance between code explosion and the
> > runtime gains.
>
> The code explosion can be terrible if you have only a few components,
> already.  The runtime gains are not so great either.  Here's a simple
> example, showing part of a cfg, the exits to the right are calls to
> abort and need some prologue component:
>
> |
> 1
> |\
> | \
> 2  X1
> |\
> | \
> |  X2
> |
  3
> With the "old" algorithm, you need to place the prologue at 1 (because
> you can only have one), and then the fall-through path also necessarily
> gets that component's prologue.  With the "separate" algorithm, the
> component prologues can be placed at X1 and X2 instead (and that will
> most likely be cheapest according to the cost model, too).
Thanks.  That really helps highlight some of the key differences between 
the old and new model.


> You can also put this in a loop.  I'll try to make a simple example
> with that.
That would be great.  And a case where they execute more than once too.

Jeff