On Wed, Aug 11, 2021 at 11:28 AM Richard Sandiford <richard.sandif...@arm.com> wrote: > > Richard Biener <richard.guent...@gmail.com> writes: > > On Tue, Aug 10, 2021 at 10:33 AM Jojo R via Gcc-patches > > <gcc-patches@gcc.gnu.org> wrote: > >> > >> Some target like RISC-V allow to group vector register as a whole, > >> and only operate part of it in fact, but the 'init-regs' pass will add > >> initialization > >> for uninitialized registers. Add this hook to reject this action for > >> reducing instruction. > > > > Are these groups "visible"? That is, are the pseudos multi-reg > > pseudos? I wonder > > if there's a more generic way to tame down initregs w/o introducing a new > > target > > hook. > > > > Btw, initregs is a red herring - it ideally should go away. See PR61810. > > > > So instead of adding to it can you see whether disabling the pass for RISC-V > > works w/o fallout (and add a comment to the PR)? Maybe some more RTL > > literate (in particular DF literate) can look at the remaining issue. > > Richard, did you > > ever have a look into the "issue" that initregs covers up (whatever > > that exactly is)? > > No, sorry. I don't really understand what it would be from the comment > in the code: > > [...] papers over some problems on the arm and other > processors where certain isa constraints cannot be handled by gcc. > These are of the form where two operands to an insn my not be the > same. The ra will only make them the same if they do not > interfere, and this can only happen if one is not initialized. > > That would definitely be an RA bug if true, since the constraints need > to be applied independently of dataflow information. But the comment > and code predate LRA and maybe no-one fancied poking around in reload > (hard to believe). > > I'd be very surprised if LRA gets this wrong.
OK, we're wondering since quite some time - how about changing the gate of initregs to optimize > 0 && !targetm.lra_p ()? We'll hopefully figure out the "real" issue the pass is papering over. At the same time we're leaving old reload (and likely unmaintianed) targets unaffected. Richard. > Thanks, > Richard
