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

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

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").

>> 
>> 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.


Thanks for those valuable comments!

Jiufu Guo



> 
> Richard.