https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98190
--- Comment #14 from rsandifo at gcc dot gnu.org <rsandifo at gcc dot gnu.org>
---
(In reply to Jakub Jelinek from comment #12)
> (In reply to rsand...@gcc.gnu.org from comment #10)
> > If we can't assert, I guess the rule is that we need to extend
> > whenever we're storing to the MSB of the inner register. We can
> > do that either by extending the source value and the range to
> > the outer register, or by assigning to the inner register and
> > then extending it separately.
>
> So perhaps:
> --- gcc/expr.c.jj 2020-12-09 00:00:08.622548080 +0100
> +++ gcc/expr.c 2020-12-09 10:36:12.198801940 +0100
> @@ -5451,6 +5451,33 @@ expand_assignment (tree to, tree from, b
> mode1, to_rtx, to, from,
> reversep))
> result = NULL;
> + else if (SUBREG_P (to_rtx)
> + && SUBREG_PROMOTED_VAR_P (to_rtx))
> + {
> + /* If to_rtx is a promoted subreg, this must be a store to the
> + whole variable, otherwise to_rtx would need to be MEM.
> + We need to zero or sign extend the value afterwards. */
> + gcc_assert (known_eq (bitpos, 0)
> + && known_eq (bitsize,
> + GET_MODE_BITSIZE (GET_MODE (to_rtx))));
> + if (TREE_CODE (to) == MEM_REF && !REF_REVERSE_STORAGE_ORDER (to))
> + result = store_expr (from, to_rtx, 0, nontemporal, false);
> + else
> + {
> + result = store_field (to_rtx, bitsize, bitpos,
> + bitregion_start, bitregion_end,
> + mode1, from, get_alias_set (to),
> + nontemporal, reversep);
> + rtx to_rtx1
> + = lowpart_subreg (subreg_unpromoted_mode (to_rtx),
> + SUBREG_REG (to_rtx),
> + subreg_promoted_mode (to_rtx));
> + to_rtx1 = convert_to_mode (subreg_promoted_mode (to_rtx),
> + to_rtx1,
> + SUBREG_PROMOTED_SIGN (to_rtx));
> + emit_move_insn (SUBREG_REG (to_rtx), to_rtx1);
> + }
> + }
> else
> result = store_field (to_rtx, bitsize, bitpos,
> bitregion_start, bitregion_end,
>
> then? As in, if store_expr can handle it, use that, otherwise perform the
> extension at the end.
LGTM FWIW, although I shouldn't be the one to review. I'm not
sure off-hand whether it's OK to store an unpromoted value to
a promoted subreg lhs, with the promoted subreg being temporarily
invalid. If that's a problem, it might be safer to pass to_rtx1
to store_field instead.
