Re: [tree-tailcall] Check if function returns it's argument

[Jeff Law <law at redhat dot com>]

On 12/09/2016 01:10 AM, Richard Biener wrote:

> > Yea.  Not sure how often something like that would happen in practice, but
> > using the equivalence to simplify rather than for propagation seems like the
> > way to go.
> >
> > I keep thinking about doing some similar in DOM, but haven't gotten around to
> > seeing what the fallout would be.
>
> Shouldn't be too bad (it would require to keep an additional
> what-to-substitute-for-value-X lattice during the DOM walk).  But it
> will still require some "magic" to decide about those conditional
> equivalences... (I think).
>
> Separating "values" from what we substitute during elimination is a good
> thing in general, so we can be more aggressive with the value parts.
I think we'd just need to change SSA_NAME_VALUE from a vector of trees 
to a pair representation or to have an on-the-side bitmap to indicate 
values that can be used for simplification but not for propagation.

It shouldn't be terrible.
jeff