[Bug tree-optimization/97605] New: unused conditionally freed allocation not eliminated

[msebor at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97605

            Bug ID: 97605
           Summary: unused conditionally freed allocation not eliminated
           Product: gcc
           Version: 11.0
            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: ---

In the test program below GCC eliminates the unnecessary pair of malloc/free in
f() but it doesn't perform the likely much more impactful optimization in g()
whose body could be transformed into:

  void* g (int i)
  {
    if (i)
      return 0;
    return malloc (1);
  }


$ cat q.c && gcc -O2 -S -Wall -fdump-tree-optimized=/dev/stdout q.c
typedef __SIZE_TYPE__   size_t;

extern void free (void*);
extern void* malloc (size_t);

void f (int i)
{ 
  void *p = malloc (1);   // eliminated
  free (p);               // ditto
}

void* g (int i)
{
  void *p = malloc (1);
  if (i)
    {
      free (p);
      return 0;
    }

  return p; 
}

;; Function f (f, funcdef_no=0, decl_uid=1936, cgraph_uid=1, symbol_order=0)

f (int i)
{
  <bb 2> [local count: 1073741824]:
  return;

}



;; Function g (g, funcdef_no=1, decl_uid=1940, cgraph_uid=2, symbol_order=1)

Removing basic block 5
g (int i)
{
  void * p;
  void * _1;

  <bb 2> [local count: 1073741824]:
  p_5 = malloc (1);
  if (i_6(D) != 0)
    goto <bb 3>; [9.39%]
  else
    goto <bb 4>; [90.61%]

  <bb 3> [local count: 100824360]:
  free (p_5);

  <bb 4> [local count: 1073741824]:
  # _1 = PHI <0B(3), p_5(2)>
  return _1;

}