https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94541
--- Comment #12 from H.J. Lu <hjl.tools at gmail dot com> ---
(In reply to Andrew Pinski from comment #11)
> (In reply to H.J. Lu from comment #10)
> > (In reply to Andrew Pinski from comment #8)
> > > (In reply to H.J. Lu from comment #7)
> > > > After 284r.ira:
> > >
> > > That is fine according to the rules as long as
> > > TARGET_TRULY_NOOP_TRUNCATION
> > > is true.
> >
> > We can't turn
> >
> > (insn 13 12 14 3 (set (reg:SI 4 si)
> > (subreg:SI (reg/v:DI 85 [ b ]) 0)) "y.i":14:14 67 {*movsi_internal}
> > (expr_list:REG_DEAD (reg/v:DI 85 [ b ])
> > (nil)))
> >
> > into
> >
> >
> > (insn 13 12 14 3 (set (reg:SI 4 si)
> > (reg:SI 4 si [orig:85 b ] [85])) "y.i":14:14 67 {*movsi_internal}
> > (expr_list:REG_DEAD (reg/v:DI 4 si [orig:85 b ] [85])
> > (nil)))
> >
> > This is simple wrong.
>
> How is that wrong? What register was 85 assigned to? 4?
> Then is fully correct as TARGET_TRULY_NOOP_TRUNCATION is true for those
> sizes.
>
> On MIPS, TARGET_TRULY_NOOP_TRUNCATION is false for SI/DI truncation.
> You need TARGET_TRULY_NOOP_TRUNCATION to be false for those sizes and you
> get a truncate RTL instead of the subreg. subreg means the upper bits of
> the register are undefined.
In this case, upper bits in 85 are undefined:
(insn 13 12 14 3 (set (reg:SI 4 si)
(subreg:SI (reg/v:DI 4 si [orig:85 b ] [85]) 0)) "y.i":14:14 67
{*movsi_internal}
(expr_list:REG_DEAD (reg/v:DI 4 si [orig:85 b ] [85])
(nil)))
LRA has:
for (i = id->insn_static_data->n_operands - 1; i >= 0; i--)
if ((DEBUG_INSN_P (insn) || ! static_id->operand[i].is_operator)
&& alter_subregs (id->operand_loc[i], ! DEBUG_INSN_P (insn)))
{
lra_update_dup (id, i);
insn_change_p = true;
}
if (insn_change_p)
lra_update_operator_dups (id);
if ((set = single_set (insn)) != NULL
&& REG_P (SET_SRC (set)) && REG_P (SET_DEST (set))
&& REGNO (SET_SRC (set)) == REGNO (SET_DEST (set)))
{
/* Remove an useless move insn. IRA can generate move
insns involving pseudos. It is better remove them
earlier to speed up compiler a bit. It is also
better to do it here as they might not pass final RTL
check in LRA, (e.g. insn moving a control register
into itself). */
lra_invalidate_insn_data (insn);
delete_insn (insn);
}
But
(insn 13 12 14 3 (set (reg:SI 4 si)
(subreg:SI (reg/v:DI 4 si [orig:85 b ] [85]) 0)) "y.i":14:14 67
{*movsi_internal}
(expr_list:REG_DEAD (reg/v:DI 4 si [orig:85 b ] [85])
(nil)))
isn't useless and shouldn't be removed.
