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

[Richard Biener <rguenther@suse.de>]

On Wed, 28 Jun 2023, Jakub Jelinek wrote:

> 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.

As said there's nothing run after NRV.

> 
> 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)?

We know that 'var' is not address taken, not sure about the partial
clobbers.  We could deal with this in the walk_gimple_op case and
simply remove a clobber when data.modified.

I went at it under the presumption that <retval> never goes out of
scope so we shouldn't have any CLOBBER for it.  You could also say
that NRV should operate flow-sensitive, going from returns backwards
and simply stop replacing when the var it substitutes for is clobbered.

I'll do the adjustment handling var.fld = {CLOBBER};

If we don't remove earlier clobbers shouldn't that prevent the NRV
then?

Richard.