Re: [PATCH] tree-optimization/110434 - avoid <retval> ={v} {CLOBBER} from NRV

[Jakub Jelinek <jakub@redhat.com>]

On Wed, Jun 28, 2023 at 10:21:45AM +0000, Richard Biener via Gcc-patches wrote:
> When NRV replaces a local variable with <retval> it also replaces
> occurences in clobbers.  This leads to <retval> being clobbered
> before the return of it which is strictly invalid but harmless in
> practice since there's no pass after NRV which would remove
> earlier stores.
> 
> The following fixes this nevertheless.
> 
> Bootstrapped and tested on x86_64-unknown-linux-gnu, OK?
> 
> Thanks,
> Richard.
> 
> 	PR tree-optimization/110434
> 	* tree-nrv.cc (pass_nrv::execute): Remove CLOBBERs of
> 	VAR we replace with <retval>.

This is in a loop over all basic blocks in a function.
Do we want to kill all clobbers, or just the ones at the end of functions
(i.e. after the <result> = VAR; assignment that we also remove)?
Complication is that doesn't necessarily have to be just the rest of
a single basic block, but all basic blocks from that point until end of
function.
I mean, if we have
  var = whatever;
  use (var);
  var = {CLOBBER};
  ...
  var = whatever_else;
  <result> = var;
  var = {CLOBBER};
killing the first clobber might result in missed optimizations later on.

On the other side, could there be partial clobbers for the var -> <result>,
  var.fld = {CLOBBER};
?  Or even worse, indirect clobbers (MEM_REF with SSA_NAME pointing to
var or parts of it)?

	Jakub