https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106594
Jakub Jelinek <jakub at gcc dot gnu.org> changed:
What |Removed |Added
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CC| |jakub at gcc dot gnu.org
--- Comment #20 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
(In reply to rsand...@gcc.gnu.org from comment #19)
> I completely agree with comment 18. See:
> https://gcc.gnu.org/pipermail/gcc-patches/2022-September/601472.html
> for some more on a similar theme.
>
> make_compound_operation_int already has code to convert
> (and (subreg X) (const_int N)) into (zero_extend X) when
> nonzero_bits indicates that that's safe. I think the AArch64
> regression can be fixed by extending that to:
>
> (and (mult (subreg x) (const_int N2)) (const_int N))
> -> (mult (sign_extend X) (const_int N2))
>
> for power-of-N2, with the nonzero_bits test adjusted for N2.
> (mult is the canonical form here, since it's part of an address.)
>
> Testing a patch for that. Hopefully that should be less
> risky than changing the expand/make_compound_operation
> dance in stage 4.
Complete agreement here, when I was seeing the gcc-patches thread during the
weekend that was exactly what I'd try to do if you haven't posted this comment.
In the nonzero_bits check for this one needs to verify that:
1) low log2(N2) bits don't really matter in N
2) bits above that if zero in N need to correspond to zero in nonzero_bits
after the shift
3) the most significant bit of the SUBREG_REG needs to be clear (so that it
doesn't
matter if it is zero or sign extension)
