https://gcc.gnu.org/bugzilla/show_bug.cgi?id=125988
--- Comment #5 from Drea Pinski <pinskia at gcc dot gnu.org> ---
(In reply to Konstantinos Eleftheriou from comment #3)
> `store_bit_field_1` in `gcc/expmed.cc` may take the `gen_lowpart`
> narrowing branch when its destination register's mode has no equal
> integral mode (e.g. a multi-register vector tuple such as `V4x4BF`):
>
> ```
> opt_scalar_int_mode op0_mode = int_mode_for_mode (GET_MODE (op0));
> scalar_int_mode imode;
> if (!op0_mode.exists (&imode) || imode != GET_MODE (op0))
> {
> if (MEM_P (op0))
> op0 = adjust_bitfield_address_size (op0, op0_mode.else_blk (),
> 0, MEM_SIZE (op0));
> else if (!op0_mode.exists ())
> {
> ...
>
> /* Stack-temp branch: result is copied back into op0. */
> rtx temp = assign_stack_temp (GET_MODE (op0),
> GET_MODE_SIZE (GET_MODE (op0)));
> emit_move_insn (temp, op0);
> store_bit_field_1 (temp, bitsize, bitnum, 0, 0, fieldmode, value,
> reverse, fallback_p, undefined_p);
> emit_move_insn (op0, temp); /* <-- copy back */
> return true;
> }
> else
> op0 = gen_lowpart (op0_mode.require (), op0); /* <-- no copy back */
> }
>
> return store_integral_bit_field (op0, op0_mode, ibitsize, ibitnum,
> bitregion_start, bitregion_end,
> fieldmode, value, reverse, fallback_p);
> ```
>
> When `gen_lowpart` returns a fresh register (because the source mode
> cannot be reinterpreted into the chosen integral mode via a valid
> subreg), `store_integral_bit_field` writes the inserted value into
> that fresh register. The caller's `str_rtx` is never updated, so the
> insertion is silently dropped.
>
> The sibling `!op0_mode.exists()` branch already performs the
> analogous copy-back via the stack-temp pattern (the final
> `emit_move_insn (op0, temp)`). The `gen_lowpart` branch is missing
> that copy-back.
>
> Suggested fix:
>
> Mirror the stack-temp branch's copy-back at the end of
> `store_bit_field_1`: after `store_integral_bit_field` returns, if the
> register reached via `op0` differs from the one reached via
> `str_rtx`, copy the value back into `str_rtx`.
This smells of LLM usage, please discuss such issue if you are using LLMs.
My bet the problem is the caller to favoid-store-forwarding is doing something
which store_bit_field is not expected to handle and the code calling
store_bit_field* in asf should change.
