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

[Jeff Law <law at redhat dot com>]

On 12/02/2016 01:33 AM, Richard Biener wrote:
> > The LHS on the assignment makes it easier to identify when a tail call is
> > possible.  It's not needed for correctness.  Not having the LHS on the
> > assignment just means we won't get an optimized tail call.
> >
> > Under what circumstances would the LHS possibly be removed?  We know the
> > return statement references the LHS, so it's not going to be something that
> > DCE will do.
>
> Well, I thought Prathamesh added the optimization to copy-propagate
> the lhs from the returned argument.  So we'd have both transforms here.
That seems like a mistake -- the fact that we can copy propagate the LHS 
from the returned argument is interesting, but in practice I've found it 
to not be useful to do so.

The problem is it makes the value look live across a the call and we're 
then dependent upon the register allocator to know the trick about the 
returned argument value and apply it consistently -- which it does not 
last I checked.

I think we're better off leaving the call in the form of LHS = call () 
if the return value is used.  That's going to be more palatable to tail 
calling.

> Of course as always the user could have written the code in this way.
>
> If the LHS is not required for correctness then I don't think we need
> to put it there - Pratamesh verified the call is tail-called already
> if marked by the tailcall pass, even if the LHS is not present.
But if the function returns the value from the tail call, then going 
through an LHS is the right thing to do.  Using the magic "argX will be 
the return value" seems clever, but actually hurts in practice.
jeff