Placement new is specified to end life-time of the object starting life-time of
a new one. The current implementation messes up with aliasing though, so
stores are re-ordered:
#include <new>
struct Foo { long i; };
struct Bar { void *p; };
long foo(int n)
{
Foo *f = new Foo;
f->i = 1;
for (int i=0; i<n; ++i)
{
Bar *b = new (f) Bar;
b->p = 0;
f = new (f) Foo;
f->i = i;
}
return f->i;
}
Produces .099t.optimized:
long int foo(int) (n)
{
struct Foo * f;
void * D.2530;
<bb 2>:
D.2530 = operator new (8);
f = (struct Foo *) D.2530;
f->i = 1;
if (n > 0) goto <L7>; else goto <L2>;
<L7>:;
f->i = (long int) ((unsigned int) n - 1);
((struct Bar *) f)->p = 0B;
<L2>:;
return f->i;
}
where the stores to Bar::p and Foo:i are reordered. Until .147r.bbro
everything looks like above on RTL level, too:
...
;; Start of basic block 3, registers live: 1 [dx] 3 [bx] 7 [sp]
(note:HI 21 19 22 3 [bb 3] NOTE_INSN_BASIC_BLOCK)
(note:HI 22 21 48 3 NOTE_INSN_DELETED)
(insn 48 22 24 3 (set (reg:DI 0 ax [63])
(zero_extend:DI (subreg:SI (plus:DI (reg:DI 3 bx [orig:61 n ] [61])
(const_int -1 [0xffffffffffffffff])) 0))) 195 {*lea_1_zext}
(nil)
(expr_list:REG_DEAD (reg:DI 3 bx [orig:61 n ] [61])
(nil)))
(insn:HI 24 48 25 3 (set (mem/s:DI (reg/v/f:DI 1 dx [orig:58 f ] [58]) [3
<variable>.i+0 S8 A64])
(reg:DI 0 ax [63])) 81 {*movdi_1_rex64} (insn_list:REG_DEP_TRUE 23
(nil))
(expr_list:REG_DEAD (reg:DI 0 ax [63])
(nil)))
(insn:HI 25 24 26 3 (set (mem/s/f:DI (reg/v/f:DI 1 dx [orig:58 f ] [58]) [5
<variable>.p+0 S8 A64])
(const_int 0 [0x0])) 81 {*movdi_1_rex64} (nil)
(nil))
;; End of basic block 3, registers live:
1 [dx] 7 [sp]
...
but now (unfortunately) scheduling reorders them back (on x86_64), so the
assembly looks good again:
_Z3fooi:
.LFB15:
pushq %rbx
.LCFI0:
movl %edi, %ebx
movl $8, %edi
call _Znwm
testl %ebx, %ebx
movq %rax, %rdx
movq $1, (%rax)
jle .L2
leal -1(%rbx), %eax
movq $0, (%rdx)
movq %rax, (%rdx)
.L2:
popq %rbx
movq (%rdx), %rax
ret
But with -O2 -fno-schedule-insns2 we finally get wrong-code for this:
_Z3fooi:
.LFB15:
pushq %rbx
.LCFI0:
movl %edi, %ebx
movl $8, %edi
call _Znwm
movq %rax, %rdx
movq $1, (%rax)
testl %ebx, %ebx
jle .L2
leal -1(%rbx), %eax
movq %rax, (%rdx)
movq $0, (%rdx)
.L2:
movq (%rdx), %rax
popq %rbx
ret
--
Summary: placement new does not provide required side-effects
Product: gcc
Version: 4.2.0
Status: UNCONFIRMED
Keywords: wrong-code, alias
Severity: normal
Priority: P3
Component: libstdc++
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: rguenth at gcc dot gnu dot org
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=29286
