On 2021-07-12 16:57, Richard Biener wrote:
On Mon, 12 Jul 2021, guojiufu wrote:
On 2021-07-12 14:20, Richard Biener wrote:
> On Fri, 9 Jul 2021, Segher Boessenkool wrote:
>
>> On Fri, Jul 09, 2021 at 08:43:59AM +0200, Richard Biener wrote:
>> > I wonder if there's a way to query the target what modes the doloop
>> > pattern can handle (not being too familiar with the doloop code).
>>
>> You can look what modes are allowed for operand 0 of doloop_end,
>> perhaps? Although that is a define_expand, not a define_insn, so it is
>> hard to introspect.
>>
>> > Why do you need to do any checks besides the new type being able to
>> > represent all IV values? The original doloop IV will never wrap
>> > (OTOH if niter is U*_MAX then we compute niter + 1 which will become
>> > zero ... I suppose the doloop might still do the correct thing here
>> > but it also still will with a IV with larger type).
The issue comes from U*_MAX (original short MAX), as you said: on
which
niter + 1 becomes zero. And because the step for doloop is -1; then,
on
larger type 'zero - 1' will be a very large number on larger type
(e.g. 0xff...ff); but on the original short type 'zero - 1' is a small
value
(e.g. "0xff").
But for the larger type the small type MAX + 1 fits and does not yield
zero so it should still work exactly as before, no? Of course you
have to compute the + 1 in the larger type.
You are right, if compute the "+ 1" in the larger type it is ok, as
below code:
```
/* Use type in word size may fast. */
if (TYPE_PRECISION (ntype) < BITS_PER_WORD)
{
ntype = lang_hooks.types.type_for_size (BITS_PER_WORD, 1);
niter = fold_convert (ntype, niter);
}
tree base = fold_build2 (PLUS_EXPR, ntype, unshare_expr (niter),
build_int_cst (ntype, 1));
add_candidate (data, base, build_int_cst (ntype, -1), true, NULL,
NULL, true);
```
The issue of this is, this code generates more stmt for doloop.xxx:
_12 = (unsigned int) xx(D);
_10 = _12 + 4294967295;
_24 = (long unsigned int) _10;
doloop.6_8 = _24 + 1;
if use previous patch, "+ 1" on original type, then the stmts will looks
like:
_12 = (unsigned int) xx(D);
doloop.6_8 = (long unsigned int) _12;
This is the reason for checking
wi::ltu_p (niter_desc->max, wi::to_widest (TYPE_MAX_VALUE (ntype)))
>>
>> doloop_valid_p guarantees it is simple and doesn't wrap.
>>
>> > I'd have expected sth like
>> >
>> > ntype = lang_hooks.types.type_for_mode (word_mode, TYPE_UNSIGNED
>> > (ntype));
>> >
>> > thus the decision made using a mode - which is also why I wonder
>> > if there's a way to query the target for this. As you say,
>> > it _may_ be fast, so better check (somehow).
I was also thinking of using hooks like type_for_size/type_for_mode.
/* Use type in word size may fast. */
if (TYPE_PRECISION (ntype) < BITS_PER_WORD
&& Wi::ltu_p (niter_desc->max, wi::to_widest (TYPE_MAX_VALUE
(ntype))))
{
ntype = lang_hooks.types.type_for_size (BITS_PER_WORD, 1);
base = fold_convert (ntype, base);
}
As you pointed out, this does not query the mode from targets.
As Segher pointed out "doloop_end" checks unsupported mode, while it
seems
not easy to use it in tree-ssa-loop-ivopts.c.
For implementations of doloop_end, tartgets like rs6000/aarch64/ia64
requires
Pmode/DImode; while there are other targets that work on other 'mode'
(e.g.
SI).
In doloop_optimize, there is code:
```
mode = desc->mode;
.....
doloop_reg = gen_reg_rtx (mode);
rtx_insn *doloop_seq = targetm.gen_doloop_end (doloop_reg,
start_label);
word_mode_size = GET_MODE_PRECISION (word_mode);
word_mode_max = (HOST_WIDE_INT_1U << (word_mode_size - 1) << 1) -
1;
if (! doloop_seq
&& mode != word_mode
/* Before trying mode different from the one in that # of
iterations is
computed, we must be sure that the number of iterations
fits into
the new mode. */
&& (word_mode_size >= GET_MODE_PRECISION (mode)
|| wi::leu_p (iterations_max, word_mode_max)))
{
if (word_mode_size > GET_MODE_PRECISION (mode))
count = simplify_gen_unary (ZERO_EXTEND, word_mode, count,
mode);
else
count = lowpart_subreg (word_mode, count, mode);
PUT_MODE (doloop_reg, word_mode);
doloop_seq = targetm.gen_doloop_end (doloop_reg, start_label);
}
if (! doloop_seq)
{
if (dump_file)
fprintf (dump_file,
"Doloop: Target unwilling to use doloop
pattern!\n");
return false;
}
```
The above code first tries the mode of niter_desc by call
targetm.gen_doloop_end
to see if the target can generate doloop insns, if fail, then try to
use
'word_mode' against gen_doloop_end.
>>
>> Almost all targets just use Pmode, but there is no such guarantee I
>> think, and esp. some targets that do not have machine insns for this
>> (but want to generate different code for this anyway) can do pretty much
>> anything.
>>
>> Maybe using just Pmode here is good enough though?
>
> I think Pmode is a particularly bad choice and I'd prefer word_mode
> if we go for any hardcoded mode. s390x for example seems to handle
> both SImode and DImode (but names the helper gen_doloop_si64
> for SImode?!). But indeed it looks like somehow querying doloop_end
> is going to be difficult since the expander doesn't have any mode,
> so we'd have to actually try emit RTL here.
Instead of using hardcode mode, maybe we could add a hook for targets
to
return
the preferred mode.
That's a possiblity of course. Like the following which just shows the
default implementation then (pass in current mode, return a more
preferred
mode or the mode itself)
enum machine_mode
prefered_doloop_mode (enum machine_mode mode)
{
return mode;
}
Yes, thanks!
Checking current do_loop_end in targets, in general, when do_loop_end
requires
SI mode, the target is defining Pmode as SImode and word_mode (from
BITS_PER_WORD
which defaults from UNITS_PER_WORD) is also defined to align with SI
mode.
When do_loop_end requires DI mode, the target is defining Pmode as
DImode
and word_mode/UNITS_PER_WORD is also defined to align with DI mode.
So, if aggressively, then by default we may just return word_mode.
BR,
Jiufu Guo.
Thanks for those valuable comments!
Jiufu Guo
>
> Richard.
