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

[Jeff Law <law at redhat dot com>]

On 12/01/2016 06:22 AM, Richard Biener wrote:
> > Well after removing DECL_BY_REFERENCE, verify_gimple still fails but
> > differently:
> >
> > tail-padding1.C:13:1: error: RESULT_DECL should be read only when
> > DECL_BY_REFERENCE is set
> >  }
> >  ^
> > while verifying SSA_NAME nrvo_4 in statement
> > # .MEM_3 = VDEF <.MEM_1(D)>
> >     nrvo_4 = __builtin_memset (nrvo_2(D), 0, 8);
> > tail-padding1.C:13:1: internal compiler error: verify_ssa failed
>
> Hmm, ok.  Not sure why we enforce this.
I don't know either.  But I would start by looking at tree-nrv.c since 
it looks (based on the variable names) that the named-value-return 
optimization kicked in.

> Note that in the end this patch looks fishy -- iff we really need
> the LHS on the assignment for correctness if we have the tailcall
> flag set then what guarantees that later passes do not remove it
> again?  So anybody removing a LHS would need to unset the tailcall flag?
>
> Saying again that I don't know enough about the RTL part of tailcall
> expansion.
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.

jeff