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.