[Bug tree-optimization/51491] ccp when converting from alloca should add a CLOBBER to right before __builtin_stack_restore

["vries at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51491

--- Comment #4 from vries at gcc dot gnu.org 2011-12-12 21:25:58 UTC ---
f1.c vs. f.c:
...
int g(int*);            int g(int*);

int f1(void)             |    int f(void)
{                {
  int tt = 0;              int tt = 0;
  int t = 4;              int t = 4;
  {                  {
    int a[4];             |        int a[t];
    tt = g(a);                tt = g(a);
    tt += a[0];                tt += a[0];
  }                  }
  {                  {
    int a[4];                int a[4];
    tt += g(a);                tt += g(a);
    tt += a[0];                tt += a[0];
  }                  }
  return tt;              return tt;
}                }
...

f1.c and f.c are compiled like this (for x86_64):
...
$ gcc -O2 -S f.c f1.c -fdump-tree-all
...

f1.c.149t.optimized has 2 clobbers:
...
f1 ()
{
  int a[4];
  int a[4];
  int tt;
  int D.1712;
  int D.1711;
  int D.1710;

<bb 2>:
  tt_3 = g (&a);
  D.1710_4 = a[0];
  tt_5 = D.1710_4 + tt_3;
  a ={v} {CLOBBER};
  D.1711_6 = g (&a);
  tt_7 = D.1711_6 + tt_5;
  D.1712_8 = a[0];
  tt_9 = D.1712_8 + tt_7;
  a ={v} {CLOBBER};
  return tt_9;

}
...

f.c.149t.optimized has only 1 clobbers:
...
f ()
{
  <unnamed-unsigned:8> D.1726[16];
  int a[4];
  int tt;
  int D.1722;
  int D.1721;
  void * saved_stack.2;
  int D.1719;

<bb 2>:
  saved_stack.2_3 = __builtin_stack_save ();
  tt_18 = g (&D.1726);
  D.1719_19 = MEM[(int[0:D.1713] *)&D.1726][0];
  tt_20 = D.1719_19 + tt_18;
  __builtin_stack_restore (saved_stack.2_3);
  D.1721_21 = g (&a);
  tt_22 = D.1721_21 + tt_20;
  D.1722_23 = a[0];
  tt_24 = D.1722_23 + tt_22;
  a ={v} {CLOBBER};
  return tt_24;

}
...

f1 uses only 16 bytes stack space, since the 2 'a[4]' arrays share the stack
space:
...
grep 'subq.*rsp' f1.s f.s
f1.s:    subq    $16, %rsp
f.s:    subq    $32, %rsp
...