https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77686
Richard Biener <rguenth at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |jwakely.gcc at gmail dot com
Component|target |libstdc++
--- Comment #10 from Richard Biener <rguenth at gcc dot gnu.org> ---
So it's
@(insn:TI 21 24 15 (parallel [
@ (set (reg:SI 0 r0)
@ (mem/c:SI (reg/f:SI 3 r3 [120]) [23 MEM[(union _Any_data
&)&D.50945]+0 S4 A64]))
@ (set (reg:SI 1 r1)
@ (mem/c:SI (plus:SI (reg/f:SI 3 r3 [120])
@ (const_int 4 [0x4])) [23 MEM[(union _Any_data
&)&D.50945]+4 S4 A32]))
@ ]) t.ii:1926 383 {*ldm2_}
@ (nil))
ldm r3, {r0, r1} @ 21 *ldm2_ [length = 4]
vs.
@(insn:TI 17 25 18 (set (mem/f/c:SI (plus:SI (reg/f:SI 13 sp)
@ (const_int 32 [0x20])) [30 MEM[(struct __lambda1 *)&D.50945]+0
S4 A64])
@ (reg/f:SI 5 r5 [orig:114 this ] [114])) 630 {*arm_movsi_vfp}
@ (nil))
str r5, [sp, #32] @ 17 *arm_movsi_vfp/6 [length = 4]
@(insn:TI 18 17 118 (set (mem/f/c:SI (plus:SI (reg/f:SI 13 sp)
@ (const_int 36 [0x24])) [30 MEM[(struct __lambda1 *)&D.50945 +
4B]+0 S4 A32])
@ (reg/f:SI 5 r5 [orig:114 this ] [114])) 630 {*arm_movsi_vfp}
@ (nil))
str r5, [sp, #36] @ 18 *arm_movsi_vfp/6 [length = 4]
and obviously r3 == sp + 32 and thus this is a must-alias. But the alias sets
are 30 vs. 23 here.
At RTL expansion time:
;; MEM[(struct __lambda1 *)&D.50945] = this_4(D);
(insn 17 16 0 (set (mem/f/c:SI (plus:SI (reg/f:SI 105 virtual-stack-vars)
(const_int -16 [0xfffffffffffffff0])) [30 MEM[(struct __lambda1
*)&D.50945]+0 S4 A64])
(reg/f:SI 114 [ this ])) -1
(nil))
;; MEM[(struct __lambda1 *)&D.50945 + 4B] = this_4(D);
(insn 18 17 0 (set (mem/f/c:SI (plus:SI (reg/f:SI 105 virtual-stack-vars)
(const_int -12 [0xfffffffffffffff4])) [30 MEM[(struct __lambda1
*)&D.50945 + 4B]+0 S4 A32])
(reg/f:SI 114 [ this ])) -1
(nil))
...
;; __tmp = MEM[(union _Any_data &)&D.50945];
(insn 19 18 20 (set (reg:SI 119)
(plus:SI (reg/f:SI 105 virtual-stack-vars)
(const_int -40 [0xffffffffffffffd8]))) t.ii:1926 -1
(nil))
(insn 20 19 21 (set (reg:SI 120)
(plus:SI (reg/f:SI 105 virtual-stack-vars)
(const_int -16 [0xfffffffffffffff0]))) t.ii:1926 -1
(nil))
(insn 21 20 22 (parallel [
(set (reg:SI 0 r0)
(mem/c:SI (reg:SI 120) [23 MEM[(union _Any_data &)&D.50945]+0
S4 A64]))
(set (reg:SI 1 r1)
(mem/c:SI (plus:SI (reg:SI 120)
(const_int 4 [0x4])) [23 MEM[(union _Any_data
&)&D.50945]+4 S4 A32]))
]) t.ii:1926 -1
(nil))
(insn 22 21 0 (parallel [
(set (mem/c:SI (reg:SI 119) [23 __tmp+0 S4 A64])
(reg:SI 0 r0))
(set (mem/c:SI (plus:SI (reg:SI 119)
(const_int 4 [0x4])) [23 __tmp+4 S4 A32])
(reg:SI 1 r1))
]) t.ii:1926 -1
(nil))
From GIMPLE with more context:
;; basic block 2, loop depth 0
;; pred: ENTRY
dummy_a = 1;
std::__ostream_insert<char, std::char_traits<char> > (&cout, "", 0);
MEM[(struct &)&f] ={v} {CLOBBER};
MEM[(struct &)&f] ={v} {CLOBBER};
MEM[(union _Any_data *)&f] = this_4(D);
MEM[(union _Any_data *)&f + 4B] = &dummy_a;
MEM[(struct &)&D.50945] ={v} {CLOBBER};
MEM[(struct &)&D.50945] ={v} {CLOBBER};
MEM[(struct __lambda1 *)&D.50945] = this_4(D); <---
MEM[(struct __lambda1 *)&D.50945 + 4B] = this_4(D); <---
__tmp = MEM[(union _Any_data &)&D.50945]; <---
MEM[(union _Any_data *)&D.50945] = MEM[(union _Any_data &)&f];
MEM[(union _Any_data *)&f] = __tmp;
__tmp ={v} {CLOBBER};
...
not very well optimized either, the copy to __tmp could be elided.
What SRA does is obviously correct now (and incorrect before):
- MEM[(struct __lambda0 *)&D.47785] = __f;
+ MEM[(struct __lambda0 *)&D.47785] = __f$__this_17;
+ MEM[(struct __lambda0 *)&D.47785 + 4B] = __f$__dummy_a_19;
...
- MEM[(struct __lambda1 *)&D.47815] = __f;
+ MEM[(struct __lambda1 *)&D.47815] = __f$__this_17;
+ MEM[(struct __lambda1 *)&D.47815 + 4B] = __f$__tmp_19;
so you see it now preserves the alias sets for the stores.
That _Any_data identifier makes me suspicious of the testcase invoking
undefined behavior:
union _Any_data
{
void* _M_access() { return &_M_pod_data[0]; }
const void* _M_access() const { return &_M_pod_data[0]; }
template<typename _Tp>
_Tp&
_M_access()
{ return *static_cast<_Tp*>(_M_access()); }
template<typename _Tp>
const _Tp&
_M_access() const
{ return *static_cast<const _Tp*>(_M_access()); }
_Nocopy_types _M_unused;
char _M_pod_data[sizeof(_Nocopy_types)];
};
it seems to fall foul of the common misconception that you can do aggregate
copies of type _Any_data and that this will properly transfer objects
constructed into _Any_data::_M_pod_data. That is obviously not the case.
-> latent libstd++ issue unless proved otherwise.
