[PATCH] Adding target hook allows to reject initialization of register

[Richard Sandiford]

Richard Biener <richard.guenther@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.

Thanks,
Richard