[Bug middle-end/78153] New: strlen return value can be assumed to be less than PTRDIFF_MAX

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78153

            Bug ID: 78153
           Summary: strlen return value can be assumed to be less than
                    PTRDIFF_MAX
           Product: gcc
           Version: 7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: middle-end
          Assignee: unassigned at gcc dot gnu.org
          Reporter: msebor at gcc dot gnu.org
  Target Milestone: ---

Due to the limits of ptrdiff_t the size of no object can exceed that of
PTRDIFF_MAX.  As a result, the return value of any valid strlen call can be
assumed to be less than PTRDIFF_MAX.  This might open up some optimization
opportunities down the line, such as that after conversion to a signed type of
the same size the result is non-negative.  The test case below shows that GCC
currently doesn't take advantage of this assumption.

$ cat b.c && /build/gcc-git/gcc/xgcc -B/build/gcc-git/gcc -O2 -S -Wall -Wextra
-Wpedantic -fdump-tree-optimized=/dev/stdout b.c
extern __SIZE_TYPE__ strlen (const char*);

void f (const char *s)
{
  if (__PTRDIFF_MAX__ <= strlen (s))
    __builtin_abort ();
}

void g (const char *s)
{
  __PTRDIFF_TYPE__ n = strlen (s);
  if (n < 0)
    __builtin_abort ();
}


;; Function f (f, funcdef_no=0, decl_uid=1795, cgraph_uid=0, symbol_order=0)

f (const char * s)
{
  long unsigned int _1;

  <bb 2>:
  _1 = strlen (s_3(D));
  if (_1 > 9223372036854775806)
    goto <bb 3>;
  else
    goto <bb 4>;

  <bb 3>:
  __builtin_abort ();

  <bb 4>:
  return;

}



;; Function g (g, funcdef_no=1, decl_uid=1798, cgraph_uid=1, symbol_order=1)

g (const char * s)
{
  long int n;
  long unsigned int _1;

  <bb 2>:
  _1 = strlen (s_3(D));
  n_4 = (long int) _1;
  if (n_4 < 0)
    goto <bb 3>;
  else
    goto <bb 4>;

  <bb 3>:
  __builtin_abort ();

  <bb 4>:
  return;

}