[Bug tree-optimization/102556] New: equality comparison of a [static N] parameter to null not folded

[Bug tree-optimization/102556] New: equality comparison of a [static N] parameter to null not folded

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102556

            Bug ID: 102556
           Summary: equality comparison of a [static N] parameter to null
                    not folded
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: msebor at gcc dot gnu.org
  Target Milestone: ---

The C99 [static N] array notation in a function parameter indicates that the
caller must provide as an argument an array with at least N element. 
Therefore, in the body of the function, the parameter may be assumed to be
nonnull, the same way as if it had been declared with attribute nonnull.

The test case below shows that GCC fails to take advantage of this to fold
pointless comparisons of such parameters to null, even though it does make use
of the equivalent guarantee provided by the attribute.

In contrast, Clang folds the expression in both functions to false.

$ cat z.c && gcc -O2 -S -Wall -fdump-tree-optimized=/dev/stdout z.c
__attribute__ ((nonnull)) int f (int *a)
{
  return a == 0;   // folded to false with a warning (good)
}

int g (int a[static 1])
{
  return a == 0;   // not folded, missing warning
}
z.c: In function ‘f’:
z.c:3:12: warning: ‘nonnull’ argument ‘a’ compared to NULL [-Wnonnull-compare]
    3 |   return a == 0;   // folded to false with a warning (good)
      |          ~~^~~~

;; Function f (f, funcdef_no=0, decl_uid=1978, cgraph_uid=1, symbol_order=0)

__attribute__((nonnull))
int f (int * a)
{
  <bb 2> [local count: 1073741824]:
  return 0;

}



;; Function g (g, funcdef_no=1, decl_uid=1981, cgraph_uid=2, symbol_order=1)

__attribute__((access ("^0[s1]", )))
int g (int * a)
{
  _Bool _1;
  int _3;

  <bb 2> [local count: 1073741824]:
  _1 = a_2(D) == 0B;
  _3 = (int) _1;
  return _3;

}

[Bug tree-optimization/102556] equality comparison of a [static N] parameter to null not folded

[roman.zilka at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102556

Roman Žilka <roman.zilka at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |roman.zilka at gmail dot com

--- Comment #1 from Roman Žilka <roman.zilka at gmail dot com> ---
This'd be helpful.

[Bug tree-optimization/102556] equality comparison of a [static N] parameter to null not folded

[muecker at gwdg dot de]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102556

Martin Uecker <muecker at gwdg dot de> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |muecker at gwdg dot de

--- Comment #2 from Martin Uecker <muecker at gwdg dot de> ---
PATCH

https://gcc.gnu.org/pipermail/gcc-patches/2023-July/625559.html