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
...
