[Bug tree-optimization/83142] Missed tail-call opportunity

[rguenther at suse dot de]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83142

--- Comment #2 from rguenther at suse dot de <rguenther at suse dot de> ---
On Tue, 28 Nov 2017, prathamesh3492 at gcc dot gnu.org wrote:

> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83142
> 
> prathamesh3492 at gcc dot gnu.org changed:
> 
>            What    |Removed                     |Added
> ----------------------------------------------------------------------------
>                  CC|                            |prathamesh3492 at gcc dot gnu.org
> 
> --- Comment #1 from prathamesh3492 at gcc dot gnu.org ---
> Hi,
> I had submitted a patch for a similar case involving __builtin_memcpy:
> https://gcc.gnu.org/ml/gcc-patches/2016-11/msg02485.html
> 
> IIUC, there was following issue:
> Marking it as tailcall at GIMPLE won't necessarily make it a tail-call during
> RTL tail-call emission. To address this issue, the patch created artificial lhs
> and returned that instead for the tail-call to be in more "natural" form:
> 
> _memcpy(dest, src, n)
> return dest
> to
> _1 = memcpy(dest, src, n)
> return _1
> 
> And I think later it was concluded that this was better handled in FRE/PRE, for
> which you had posted a patch:
> https://gcc.gnu.org/ml/gcc-patches/2016-12/msg00424.html

Yeah, that didn't work out so I implemented the reverse now, postponed
for GCC 9 because of this and other fallout.  So we should revisit
your original patch and adjust it to re-instantiate a LHS on the
call & propagate to dominating uses.