[Bug tree-optimization/80576] dead strcpy and strncpy followed by memset not eliminated

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80576

--- Comment #4 from Martin Sebor <msebor at gcc dot gnu.org> ---
I've seen code similar to it but I don't remember how close they were.  I
imagine it comes up quite a bit as GCC itself transforms code in various ways.

DSE already eliminates stores with constant sizes but it doesn't handle the
non-constant ones:

$ cat pr80576.c && gcc -O2 -S -Wall -Wextra -Wpedantic
-fdump-tree-optimized=/dev/stdout pr80576.c
extern char a[32];

void f (const char *s)
{
  __builtin_memcpy (a, s, 7);        // eliminated
  __builtin_memset (a, 0, sizeof a);
}

void g (const char *s, unsigned n)
{
  __builtin_memcpy (a, s, n);        // not eliminated
  __builtin_memset (a, 0, sizeof a); 
}


;; Function f (f, funcdef_no=0, decl_uid=1909, cgraph_uid=1, symbol_order=0)

f (const char * s)
{
  <bb 2> [local count: 1073741824]:
  __builtin_memset (&a, 0, 32); [tail call]
  return;

}



;; Function g (g, funcdef_no=1, decl_uid=1913, cgraph_uid=2, symbol_order=1)

g (const char * s, unsigned int n)
{
  long unsigned int _1;

  <bb 2> [local count: 1073741824]:
  _1 = (long unsigned int) n_2(D);
  __builtin_memcpy (&a, s_4(D), _1);
  __builtin_memset (&a, 0, 32); [tail call]
  return;

}