I think the point is we fail to represent

Analyzing # of iterations of loop 1
  exit condition [i_5(D) + 1, + , 1] < n_11(D)
  bounds on difference of bases: -18446744073709551615 ... 18446744073709551615
  result:
    zero if i_5(D) + 1 > n_11(D)
    # of iterations (n_11(D) - i_5(D)) + 18446744073709551615, bounded by 18446744073709551615
  number of iterations (n_11(D) - i_5(D)) + 18446744073709551615; zero if i_5(D) + 1 > n_11(D)

specifically the 'zero if i_5(D) + 1 > n_11(D)'

I think may_eliminate_iv is wrong here, maybe not considering overflow
of the niter expression?

I wonder if it is possible to write a runtime testcase that FAILs with
reasonable memory requirement/layout.

Here is a runtime testcase:

#include <sys/mman.h>
#include <unistd.h>
#include <stdint.h>

__attribute__((noipa))
void f1 (char *p, uintptr_t i, uintptr_t n)
{
  p += i;
  do
    {
      *p = '\0';
      p += 1;
      i++;
    }
  while (i < n);
}

int main()
{
  long pgsz = sysconf (_SC_PAGESIZE);
  void *p = mmap (NULL, pgsz * 2, PROT_READ|PROT_WRITE,
     MAP_ANONYMOUS|MAP_PRIVATE, 0, 0);
  if (p == MAP_FAILED)
    return 0;
  mprotect (p+pgsz, pgsz, PROT_NONE);
  uintptr_t n = -3 - (uintptr_t)p;
  f1 (p+2, -2, n);
  return 0;
}

Oops. I messed up the test case...  It "works", but the actual values does not make sense...

The following is better:

int main()
{
  long pgsz = sysconf (_SC_PAGESIZE);
  void *p = mmap (NULL, pgsz * 2, PROT_READ|PROT_WRITE,
     MAP_ANONYMOUS|MAP_PRIVATE, 0, 0);
  if (p == MAP_FAILED)
    return 0;
  mprotect (p+pgsz, pgsz, PROT_NONE);
  uintptr_t n = -2 - (uintptr_t)(p+pgsz);
  f1 (p+pgsz, -2, n);
  return 0;
}

Confirmed.

It's going wrong in iv_elimination_compare_lt which tries to exactly handle this kind of loop:

   We aim to handle the following situation:

   sometype *base, *p;
   int a, b, i;
  
   i = a;
   p = p_0 = base + a;
  
   do
     {
       bla (*p);
       p++;
       i++;
     }
   while (i < b);
    
   Here, the number of iterations of the loop is (a + 1 > b) ? 0 : b - a - 1.
   We aim to optimize this to

   p = p_0 = base + a;
   do
     {
       bla (*p);
       p++;
     }
   while (p < p_0 - a + b);
    
   This preserves the correctness, since the pointer arithmetics does not
   overflow.  More precisely:
  
   1) if a + 1 <= b, then p_0 - a + b is the final value of p, hence there is no
      overflow in computing it or the values of p.
   2) if a + 1 > b, then we need to verify that the expression p_0 - a does not
      overflow.  To prove this, we use the fact that p_0 = base + a.

there's either a hole in that logic or the implementation is off.

  /* Finally, check that CAND->IV->BASE - CAND->IV->STEP * A does not
     overflow.  */
  offset = fold_build2 (MULT_EXPR, TREE_TYPE (cand->iv->step),
                        cand->iv->step,
                        fold_convert (TREE_TYPE (cand->iv->step), a));
  if (!difference_cannot_overflow_p (data, cand->iv->base, offset))
    return false;

where 'A' is 'i', CAND->IV->BASE is 'p + i' and CAND->IV->STEP is 1
as 'sizetype'.

That just checks that (p + i) - i doesn't overflow.

Somehow it misses to prove p + b doesn't overflow since we end up with
p' < (p + i) + (n - i) aka p' < p + n.