[Bug tree-optimization/79191] New: potentially truncating unsigned conversion defeats range propagation

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79191

            Bug ID: 79191
           Summary: potentially truncating unsigned conversion defeats
                    range propagation
           Product: gcc
           Version: 7.0
            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: ---

As mentioned in a recent discussion
(https://gcc.gnu.org/ml/gcc-patches/2017-01/msg01617.html), a truncating
unsigned conversion such as from unsigned long to unsigned in LP64 appears to
defeat VRP and prevent GCC from emitting optimal code.

In the test case below, the indirect call to abort() in g() is optimized away
because n is known to be less than 3.  However, the same indirect call to
abort() is not optimized away in h().

$ cat t.c && gcc -O2 -S -Wall -fdump-tree-optimized=/dev/stdout t.c
void f (unsigned long n)
{
  if (n > 3)
    __builtin_abort ();
}

void g (unsigned n)
{
  if (n < 3)
    f (n);
}

void h (unsigned long m)
{
  unsigned n = m;

  if (n < 3)
    f (n);
}

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

f (long unsigned int n)
{
  <bb 2> [100.00%]:
  if (n_1(D) > 3)
    goto <bb 3>; [0.04%]
  else
    goto <bb 4>; [99.96%]

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

  <bb 4> [99.96%]:
  return;

}



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

g (unsigned int n)
{
  <bb 2> [100.00%]:
  return;

}



;; Function h (h, funcdef_no=2, decl_uid=1801, cgraph_uid=2, symbol_order=2)

Removing basic block 6
Removing basic block 7
h (long unsigned int m)
{
  unsigned int n;
  long unsigned int _5;

  <bb 2> [100.00%]:
  n_2 = (unsigned int) m_1(D);
  if (n_2 <= 2)
    goto <bb 3>; [54.00%]
  else
    goto <bb 5>; [46.00%]

  <bb 3> [54.00%]:
  _5 = m_1(D) & 4294967295;
  if (_5 > 3)
    goto <bb 4>; [0.04%]
  else
    goto <bb 5>; [99.96%]

  <bb 4> [0.02%]:
  __builtin_abort ();

  <bb 5> [99.98%]:
  return;

}