Re: Simplify more EXACT_DIV_EXPR comparisons

[Marc Glisse <marc.glisse@inria.fr>]

On Mon, 20 May 2019, Richard Biener wrote:

> On Mon, May 20, 2019 at 10:16 AM Marc Glisse <marc.glisse@inria.fr> wrote:
>>
>> On Mon, 20 May 2019, Richard Biener wrote:
>>
>>> On Sun, May 19, 2019 at 6:16 PM Marc Glisse <marc.glisse@inria.fr> wrote:
>>>>
>>>> Hello,
>>>>
>>>> 2 pieces:
>>>>
>>>> - the first one handles the case where the denominator is negative. It
>>>> doesn't happen often with exact_div, so I don't handle it everywhere, but
>>>> this one looked trivial
>>>>
>>>> - handle the case where a pointer difference is cast to an unsigned type
>>>> before being compared to a constant (I hit this in std::vector). With some
>>>> range info we could probably handle some non-constant cases as well...
>>>>
>>>> The second piece breaks Walloca-13.c (-Walloca-larger-than=100 -O2)
>>>>
>>>> void f (void*);
>>>> void g (int *p, int *q)
>>>> {
>>>>    __SIZE_TYPE__ n = (__SIZE_TYPE__)(p - q);
>>>>    if (n < 100)
>>>>      f (__builtin_alloca (n));
>>>> }
>>>>
>>>> At the time of walloca2, we have
>>>>
>>>>    _1 = p_5(D) - q_6(D);
>>>>    # RANGE [-2305843009213693952, 2305843009213693951]
>>>>    _2 = _1 /[ex] 4;
>>>>    # RANGE ~[2305843009213693952, 16140901064495857663]
>>>>    n_7 = (long unsigned intD.10) _2;
>>>>    _11 = (long unsigned intD.10) _1;
>>>>    if (_11 <= 396)
>>>> [...]
>>>>    _3 = allocaD.1059 (n_7);
>>>>
>>>> and warn.
>>>
>>> That's indeed to complicated relation of _11 to n_7 for
>>> VRP predicate discovery.
>>>
>>>> However, DOM3 later produces
>>>>
>>>>    _1 = p_5(D) - q_6(D);
>>>>    _11 = (long unsigned intD.10) _1;
>>>>    if (_11 <= 396)
>>>
>>> while _11 vs. _1 works fine.
>>>
>>>> [...]
>>>>    # RANGE [0, 99] NONZERO 127
>>>>    _2 = _1 /[ex] 4;
>>>>    # RANGE [0, 99] NONZERO 127
>>>>    n_7 = (long unsigned intD.10) _2;
>>>>    _3 = allocaD.1059 (n_7);
>>>>
>>>> so I am tempted to say that the walloca2 pass is too early, xfail the
>>>> testcase and file an issue...
>>>
>>> Hmm, there's a DOM pass before walloca2 already and moving
>>> walloca2 after loop opts doesn't look like the best thing to do?
>>> I suppose it's not DOM but sinking that does the important transform
>>> here?  That is,
>>>
>>> Index: gcc/passes.def
>>> ===================================================================
>>> --- gcc/passes.def      (revision 271395)
>>> +++ gcc/passes.def      (working copy)
>>> @@ -241,9 +241,9 @@ along with GCC; see the file COPYING3.
>>>       NEXT_PASS (pass_optimize_bswap);
>>>       NEXT_PASS (pass_laddress);
>>>       NEXT_PASS (pass_lim);
>>> -      NEXT_PASS (pass_walloca, false);
>>>       NEXT_PASS (pass_pre);
>>>       NEXT_PASS (pass_sink_code);
>>> +      NEXT_PASS (pass_walloca, false);
>>>       NEXT_PASS (pass_sancov);
>>>       NEXT_PASS (pass_asan);
>>>       NEXT_PASS (pass_tsan);
>>>
>>> fixes it?
>>
>> I will check, but I don't think walloca uses any kind of on-demand VRP, so
>> we still need some pass to update the ranges after sinking, which doesn't
>> seem to happen until the next DOM pass.
>
> Oh, ok...  Aldy, why's this a separate pass anyways?  I think similar
> other warnigns are emitted from RTL expansion?  So maybe we can
> indeed move the pass towards warn_restrict or late_warn_uninit.

I tried moving it after 'sink' and that didn't help. Moving it next to 
warn_restrict works for this test but breaks 2 others that currently 
"work" by accident (+ one where the message changes between "unbounded" 
and "too large", it isn't clear what the difference is between those 
messages).

My suggestion, in addition to the original patch, is

 	* gcc.dg/Walloca-13.c: Xfail.

--- Walloca-13.c	(revision 271742)
+++ Walloca-13.c	(working copy)
@@ -1,12 +1,12 @@
  /* { dg-do compile } */
  /* { dg-require-effective-target alloca } */
  /* { dg-options "-Walloca-larger-than=100 -O2" } */

  void f (void*);

  void g (int *p, int *q)
  {
    __SIZE_TYPE__ n = (__SIZE_TYPE__)(p - q);
    if (n < 100)
-    f (__builtin_alloca (n));
+    f (__builtin_alloca (n)); // { dg-bogus "may be too large due to conversion" "" { xfail { *-*-* } } }
  }

Is that ok?

> I also see that the Og pass pipeline misses the second walloca pass
> completely (and also the warn_restrict pass).
>
> Given code sinkings obvious effects on SSA value-range representation
> it may make sense to add another instance of that pass earlier.

-- 
Marc Glisse