[Bug tree-optimization/80487] New: redundant memset/memcpy/strcpy calls not eliminated

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80487

            Bug ID: 80487
           Summary: redundant memset/memcpy/strcpy calls 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: ---

Somewhat similar to bug 79716 but (IIUC) with a different root cause, the
redundant clearing of the array prior to the call to strncpy in the following
function is not eliminated as it could be.  (Strncpy is specified to append
NULs to fill the destination.)

$ cat t.c && gcc -O2 -S -Wall -Wextra -fdump-tree-optimized=/dev/stdout t.c

void sink (void*);

void f (const char *s)
{
  char a[256] = "";   // redundant initialization

  __builtin_strncpy (a, s, sizeof a);

  sink (a);
}

void g (const char *s)
{
  char a[256];

  __builtin_memset (a, 0, sizeof a);   // redundant memset
  __builtin_strncpy (a, s, sizeof a);

  sink (a);
}

;; Function f (f, funcdef_no=0, decl_uid=1797, cgraph_uid=0, symbol_order=0)

f (const char * s)
{
  char a[256];

  <bb 2> [100.00%]:
  a = "";
  __builtin_strncpy (&a, s_3(D), 256);
  sink (&a);
  a ={v} {CLOBBER};
  return;

}



;; Function g (g, funcdef_no=1, decl_uid=1801, cgraph_uid=1, symbol_order=1)

g (const char * s)
{
  char a[256];

  <bb 2> [100.00%]:
  __builtin_memset (&a, 0, 256);
  __builtin_strncpy (&a, s_3(D), 256);
  sink (&a);
  a ={v} {CLOBBER};
  return;

}