> 2020-12-10 Jakub Jelinek <ja...@redhat.com>
>
> PR middle-end/98190
> * expr.c (expand_assignment): If to_rtx is a promoted SUBREG,
> ensure sign or zero extension when the most significant bit has
> been overwritten, either through use of store_expr or by extending
> manually.
>
> * gcc.dg/pr98190.c: New test.
>
> --- gcc/expr.c.jj 2020-12-09 23:50:41.385776978 +0100
> +++ gcc/expr.c 2020-12-10 09:35:56.231058928 +0100
> @@ -5451,6 +5451,43 @@ 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, we need to zero or sign
> + extend the value afterwards. */
> + if (TREE_CODE (to) == MEM_REF
> + && !REF_REVERSE_STORAGE_ORDER (to)
> + && known_eq (bitpos, 0)
> + && known_eq (bitsize, GET_MODE_BITSIZE (GET_MODE
(to_rtx))))
> + result = store_expr (from, to_rtx, 0, nontemporal,
false);
> + else
> + {
> + rtx to_rtx1 = to_rtx;
> + /* Optimize by checking if the overwritten bits
> + include the most significant bit. */
> + if (BYTES_BIG_ENDIAN != WORDS_BIG_ENDIAN
> + || (BYTES_BIG_ENDIAN && known_eq (bitpos, 0))
> + || (!BYTES_BIG_ENDIAN
> + && known_eq (bitpos + bitsize,
> + GET_MODE_BITSIZE (GET_MODE
(to_rtx)))))
> + to_rtx1 = lowpart_subreg (subreg_unpromoted_mode
(to_rtx),
> + SUBREG_REG
(to_rtx),
> +
subreg_promoted_mode (to_rtx));
> + result = store_field (to_rtx1, bitsize, bitpos,
> + bitregion_start,
bitregion_end,
> + mode1, from,
get_alias_set (to),
> + nontemporal, reversep);
> + if (to_rtx1 != 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,
Are you sure that the optimization is worth the hassle (and maybe the risk,
i.e. can't store_field clobber the entire field)?
Any particular reason not to use the canonical idiom at the end, i.e. just
convert_move (SUBREG_REG (to_rtx), to_rtx1,
SUBREG_PROMOTED_SIGN (to_rtx));
--
Eric Botcazou
