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.
