Re: [PATCH] Check type size for doloop iv on BITS_PER_WORD [PR61837]

[guojiufu <guojiufu@linux.ibm.com>]

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.
>> 
>>