[Bug tree-optimization/82665] New: missing value range optimization for memchr

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82665

            Bug ID: 82665
           Summary: missing value range optimization for memchr
           Product: gcc
           Version: 8.0
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: msebor at gcc dot gnu.org
  Target Milestone: ---

VRP optimizes the first three out of the four equivalent functions below into
no-ops.  In f3() it's possible to make the same assumption about the argument
of memchr() (and thus the result) as for that of all the other calls: that it
contains a zero byte in the first PTRDIFF_MAX - 1 bytes, and thus that the
difference between a pointer to the zero byte and the beginning of the array is
less than PTRDIFF_MAX.

$ cat y.c && gcc -O2 -S -Wall -fdump-tree-vrp=/dev/stdout y.c

void f0 (char *p)
{
  unsigned long n = 0;

  while (*p)
    ++n;

  if (n >= __PTRDIFF_MAX__)
    __builtin_abort ();
}

void f1 (char *p)
{
  unsigned long n = __builtin_strlen (p);

  if (n >= __PTRDIFF_MAX__)
    __builtin_abort ();
}

void f2 (char *p)
{
  char *q = __builtin_strchr (p, 0);
  unsigned long n = q - p;

  if (n >= __PTRDIFF_MAX__)
    __builtin_abort ();
}

void f3 (char *p)
{
  char *q = __builtin_memchr (p, 0, __SIZE_MAX__);
  unsigned long n = q - p;

  if (n >= __PTRDIFF_MAX__)
    __builtin_abort ();
}
...
Value ranges after VRP:

q.2_1: VARYING
p.3_2: ~[0, 0]
_3: VARYING
p_5(D): VARYING
q_6: VARYING
n_7: [0, +INF]
p_9: ~[0B, 0B]  EQUIVALENCES: { p_5(D) } (1 elements)


f3 (char * p)
{
  long unsigned int n;
  char * q;
  long int q.2_1;
  long int p.3_2;
  long int _3;

  <bb 2> [100.00%] [count: INV]:
  q_6 = __builtin_memchr (p_5(D), 0, 18446744073709551615);
  q.2_1 = (long int) q_6;
  p.3_2 = (long int) p_5(D);
  _3 = q.2_1 - p.3_2;
  n_7 = (long unsigned int) _3;
  if (n_7 > 9223372036854775806)
    goto <bb 3>; [0.04%]
  else
    goto <bb 4>; [99.96%]

  <bb 3> [0.04%] [count: 0]:
  __builtin_abort ();

  <bb 4> [99.96%] [count: INV]:
  return;

}


y.c: In function ‘f3’:
y.c:31:13: warning: ‘__builtin_memchr’ specified size 18446744073709551615
exceeds maximum object size 9223372036854775807 [-Wstringop-overflow=]
   char *q = __builtin_memchr (p, 0, __SIZE_MAX__);
             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~