[Bug tree-optimization/79715] New: hand-rolled strdup with unused result not eliminated

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79715

            Bug ID: 79715
           Summary: hand-rolled strdup with unused result not eliminated
           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: ---

While looking for other opportunities similar to those noted in bug 79697 and
the discussion of the patch for it
(https://gcc.gnu.org/ml/gcc-patches/2017-02/msg01566.html) I noticed another
one.  The two functions f and g below are equivalent.  GCC successfully
transforms f into a return statement but it misses the same opportunity in g. 
Clang 5.0 generates the same optimal code for both functions.

$ cat t.c && gcc -O2 -S -Wall -Wextra -Wpedantic -xc
-fdump-tree-printf-return-value=/dev/stdout t.c
void f (const char *s)
{
  unsigned n = __builtin_strlen (s) + 1;
  char *p = __builtin_malloc (n);
  __builtin_memcpy (p, s, n);
  __builtin_free (p);
}

void g (const char *s)
{
  unsigned n = __builtin_strlen (s) + 1;
  char *p = __builtin_malloc (n);
  __builtin_strcpy (p, s);
  __builtin_free (p);
}


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

f (const char * s)
{
  <bb 2> [100.00%]:
  return;

}



;; Function g (g, funcdef_no=1, decl_uid=1800, cgraph_uid=1, symbol_order=1)

g (const char * s)
{
  char * p;
  unsigned int n;
  long unsigned int _1;
  unsigned int _2;
  long unsigned int _3;

  <bb 2> [100.00%]:
  _1 = __builtin_strlen (s_5(D));
  _2 = (unsigned int) _1;
  n_6 = _2 + 1;
  _3 = (long unsigned int) n_6;
  p_8 = __builtin_malloc (_3);
  __builtin_strcpy (p_8, s_5(D));
  __builtin_free (p_8);
  return;

}