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
