[Bug middle-end/96564] [11/12/13/14 Regression] New maybe use of uninitialized variable warning since r11-959

["rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96564

--- Comment #16 from Richard Biener <rguenth at gcc dot gnu.org> ---
(In reply to Andrew Macleod from comment #15)
> (In reply to Richard Biener from comment #13)
> > (In reply to Jeffrey A. Law from comment #12)
> > > So I think we could solve this with a bit of help from the alias oracle.  We
> > > have  the routine ptrs_compare_unequal, but points-to-null is going to get
> > > in the way.
> > > 
> > > I think VRP and DOM have enough information to rule out NULL for both
> > > objects in question.  So if we could query the points-to information,
> > > ignoring NULL then we could likely solve this particular bug.
> > > 
> > > Essentially VRP or DOM would prove NULL isn't in the set of possible values
> > > at the comparison point.  Then we query the alias information ignoring NULL.
> > > Voila we compute a static result for the comparison of the two pointers and
> > > the problematical block becomes unreachable and the bogus warning goes away.
> > > 
> > > Richi, any thoughts in viability of such an API?
> > 
> > We now treat pt.null conservatively and track non-null-ness derived from
> > range-info in it.  That means when VRP/DOM can prove a pointer is always
> > not NULL they can do set_ptr_nonnull (p) on it.
> > 
> > This means the
> > 
> >   /* ???  We'd like to handle ptr1 != NULL and ptr1 != ptr2
> >      but those require pt.null to be conservatively correct.  */
> > 
> > is no longer true and we could finally implement it, like with
> > 
> > diff --git a/gcc/tree-ssa-alias.cc b/gcc/tree-ssa-alias.cc
> > index e7c1c1aa624..5b6d9e0aa6a 100644
> > --- a/gcc/tree-ssa-alias.cc
> > +++ b/gcc/tree-ssa-alias.cc
> > @@ -479,9 +479,25 @@ ptrs_compare_unequal (tree ptr1, tree ptr2)
> >         }
> >        return !pt_solution_includes (&pi->pt, obj1);
> >      }
> > -
> > -  /* ???  We'd like to handle ptr1 != NULL and ptr1 != ptr2
> > -     but those require pt.null to be conservatively correct.  */
> > +  else if (TREE_CODE (ptr1) == SSA_NAME)
> > +    {
> > +      struct ptr_info_def *pi1 = SSA_NAME_PTR_INFO (ptr1);
> > +      if (!pi1
> > +         || pi1->pt.vars_contains_restrict
> > +         || pi1->pt.vars_contains_interposable)
> > +       return false;
> > +      if (integer_zerop (ptr2) && !pi1->pt.null)
> > +       return true;
> > +      if (TREE_CODE (ptr2) == SSA_NAME)
> > +       {
> > +         struct ptr_info_def *pi2 = SSA_NAME_PTR_INFO (ptr2);
> > +         if (!pi2
> > +             || pi2->pt.vars_contains_restrict
> > +             || pi2->pt.vars_contains_interposable)
> > +         if (!pi1->pt.null || !pi2->pt.null)
> > +           return !pt_solutions_intersect (&pi1->pt, &pi2->pt);
> > +       }
> > +    }
> >  
> >    return false;
> >  }
> > 
> > 
> > but the testcase shows the non-null-ness is only conditional which means
> > we'd have to use a range query above which necessarily falls back to
> > the global ranges given we don't have any context available here.  The
> > old EVRP adjusted global ranges during the walk but this is no longer done.
> > 
> You mean it lied?  because x_1 is not NULL until after _8 = *x_1(D);
> executes.  It can still be NULL on that stmt can it not?   Did it reset the
> global value afterwards?

Yes and yes, old EVRP turned global ranges into "ranges at the point of
stmt evaluation/folding" (and restored them to the global values later).
Note that EVRP didn't do any sort of iteration for loop handling and it
folded stmts as it analyzed them.  Using the global ranges as "lattice"
had the advantage that all folding utilities picked up "local" ranges
for free.  IMO it was quite elegant and fast what EVRP did (with it's
obvious limitations of course).

> Contextually ranger knows both are non-null at EVRP time:
> a.0_27 : [irange] int[0:D.xxxx] * [1, +INF]
> 2->3  (T) x_1(D) :     [irange] int * [1, +INF]
> 2->3  (T) a.0_27 :      [irange] int[0:D.xxxx] * [1, +INF]
> 2->4  (F) x_1(D) :     [irange] int * [1, +INF]
> 2->4  (F) a.0_27 :      [irange] int[0:D.xxxx] * [1, +INF]
> 
> So we know x_1 is non-NULL after the de-reference for the rest of the block
> (and function).  It also sets a.0_27 globally to be [1, +INF].
> 
> > Note it's enough that one pointer is nonnull, so for your idea the
> > API could be extended with a bool one_ptr_nonnull parameter.
> 
> ranger currently sets a.0 globally to be non-null in EVRP.

After EVRP I see

  # PT = nonlocal null
  unsigned int * x_8(D) = x;
...
  <bb 2> :
  _1 = *x_8(D);
  # RANGE [irange] long unsigned int [0, 4294967295] MASK 0xffffffff VALUE 0x0
  _2 = (long unsigned int) _1;
  # PT = null { D.2781 }
  # ALIGN = 8, MISALIGN = 0
  # USE = nonlocal escaped
  # CLB = nonlocal escaped
  a_10 = malloc (_2);
  if (a_10 == 0B)
...
  <bb 4> :
  if (x_8(D) != a_10)

the last test is the one we want to eliminate.  The proposed change (with
a missing 'return false' fixed) isn't enough since it just looks at
global ranges where both x_8 and a_10 can be null.  In the
ptrs_compare_unequal function I could at most use range_of_expr without
a stmt context (as I don't have that) which wouldn't help.  EVRP with
the trick to adjust global ranges effectively had a "global context"
it would use.  I suppose that one could have something like that for
ranger as well, add ranger::set_context (gimple *) which EVRP could set
when folding a stmt (set it to right before 'stmt' execution) and which
would be the context to fall back to when a folding dependent utility
didn't specify one?

Adding a global (not ranger specific) "folding context stack" might do
the trick as well.  Any utility that could take advantage of a context
could look at the stack top (which might be NULL).  That would be less
churn than wrapping each and every folding function inside a "folder"
class containing a context.

Of course one has to be careful with such a thing, like with recursively
invoking number of iteration or SCEV analysis which work on a more fuzzy
context than a specific stmt contained in a loop.