On Mon, Jul 26, 2021 at 4:19 PM H.J. Lu <hjl.to...@gmail.com> wrote:
>
> On Mon, Jul 26, 2021 at 3:56 PM H.J. Lu <hjl.to...@gmail.com> wrote:
> >
> > On Mon, Jul 26, 2021 at 2:53 PM Richard Sandiford
> > <richard.sandif...@arm.com> wrote:
> > >
> > > "H.J. Lu via Gcc-patches" <gcc-patches@gcc.gnu.org> writes:
> > > > On Mon, Jul 26, 2021 at 11:42 AM Richard Sandiford
> > > > <richard.sandif...@arm.com> wrote:
> > > >>
> > > >> "H.J. Lu via Gcc-patches" <gcc-patches@gcc.gnu.org> writes:
> > > >> > +to avoid stack realignment when expanding memset. The default is
> > > >> > +@code{gen_reg_rtx}.
> > > >> > +@end deftypefn
> > > >> > +
> > > >> > @deftypefn {Target Hook} unsigned TARGET_LOOP_UNROLL_ADJUST
> > > >> > (unsigned @var{nunroll}, class loop *@var{loop})
> > > >> > This target hook returns a new value for the number of times
> > > >> > @var{loop}
> > > >> > should be unrolled. The parameter @var{nunroll} is the number of
> > > >> > times
> > > >> > […]
> > > >> > @@ -1446,7 +1511,10 @@ can_store_by_pieces (unsigned HOST_WIDE_INT
> > > >> > len,
> > > >> > max_size = STORE_MAX_PIECES + 1;
> > > >> > while (max_size > 1 && l > 0)
> > > >> > {
> > > >> > - scalar_int_mode mode = widest_int_mode_for_size (max_size);
> > > >> > + /* Since this can be called before virtual registers are
> > > >> > ready
> > > >> > + to use, avoid QI vector mode here. */
> > > >> > + fixed_size_mode mode
> > > >> > + = widest_fixed_size_mode_for_size (max_size, false);
> > > >>
> > > >> I think I might have asked this before, sorry, but: when is that true
> > > >> and why does it matter?
> > > >
> > > > can_store_by_pieces may be called:
> > > >
> > > > value-prof.c: if (!can_store_by_pieces (val,
> > > > builtin_memset_read_str,
> > > > value-prof.c: if (!can_store_by_pieces (val,
> > > > builtin_memset_read_str,
> > > >
> > > > before virtual registers can be used. When true is passed to
> > > > widest_fixed_size_mode_for_size, virtual registers may be used
> > > > to expand memset to broadcast, which leads to ICE. Since for the
> > > > purpose of can_store_by_pieces, we don't need to expand memset
> > > > to broadcast and pass false here can avoid ICE.
> > >
> > > Ah, I see, thanks.
> > >
> > > That sounds like a problem in the way that the memset const function is
> > > written though. can_store_by_pieces is just a query function, so I don't
> > > think it should be trying to create new registers for can_store_by_pieces,
> > > even if it could. At the same time, can_store_by_pieces should make the
> > > same choices as the real expander would.
> > >
> > > I think this means that:
> > >
> > > - gen_memset_broadcast should be inlined into its callers, with the
> > > builtin_memset_read_str getting the CONST_INT_P case and
> > > builtin_memset_gen_str getting the variable case.
> > >
> > > - builtin_memset_read_str should then stop at and return the
> > > gen_const_vec_duplicate when the prev argument is null.
>
> This doesn't work since can_store_by_pieces has
>
> cst = (*constfun) (constfundata, nullptr, offset, mode);
> if (!targetm.legitimate_constant_p (mode, cst))
We can add a target hook, targetm.legitimate_memset_constant_p,
which defaults to targetm.legitimate_constant_p. Will it be acceptable?
> ix86_legitimate_constant_p only allows 0 or -1 for CONST_VECTOR.
> can_store_by_pieces doesn't work well for vector modes.
>
> > > Only when prev is nonnull should it go on to call the hook
> > > and copy the constant to the register that the hook returns.
> >
> > How about keeping gen_memset_broadcast and passing PREV to it:
> >
> > rtx target;
> > if (CONST_INT_P (data))
> > {
> > rtx const_vec = gen_const_vec_duplicate (mode, data);
> > if (prev == NULL)
> > /* Return CONST_VECTOR when called by a query function. */
> > target = const_vec;
> > else
> > {
> > /* Use the move expander with CONST_VECTOR. */
> > target = targetm.gen_memset_scratch_rtx (mode);
> > emit_move_insn (target, const_vec);
> > }
> > }
> > else
> > {
> > target = targetm.gen_memset_scratch_rtx (mode);
> > class expand_operand ops[2];
> > create_output_operand (&ops[0], target, mode);
> > create_input_operand (&ops[1], data, QImode);
> > expand_insn (icode, 2, ops);
> > if (!rtx_equal_p (target, ops[0].value))
> > emit_move_insn (target, ops[0].value);
> > }
> >
> > > I admit that's uglier than the current version, but it looks like that's
> > > what the current interface expects.
> > >
> > > Thanks,
> > > Richard
> >
> >
> >
> > --
> > H.J.
>
>
>
> --
> H.J.
--
H.J.
