std::optional defaut constructor

[Marc Glisse]

On Thu, 4 Jun 2020, Richard Biener wrote:

> On Thu, Jun 4, 2020 at 11:34 AM Ville Voutilainen via Gcc-patches
> <gcc-patches@gcc.gnu.org> wrote:
>>
>> On Thu, 4 Jun 2020 at 11:53, Marc Glisse <marc.glisse@inria.fr> wrote:
>>>
>>> On Thu, 4 Jun 2020, Ville Voutilainen wrote:
>>>
>>>> On Thu, 4 Jun 2020 at 11:00, Marc Glisse <marc.glisse@inria.fr> wrote:
>>>>> Maybe create a buffer, fill it with some non-zero values (-1?), then call
>>>>> placement new, and read some value in the middle of the buffer, possibly
>>>>> with some protection against optimizations? Ah, no, actual constructors
>>>>> are fine, it is only the inlined initialization that happens with the
>>>>> defaulted constructor that zeroes things.
>>>>
>>>> The zero-init is part of value-initialization of a class type with a
>>>> defaulted default constructor, so value-initialization with placement
>>>> new should indeed show us whether the target buffer is zeroed.
>>>
>>> Ah, yes, I had forgotten the empty () at the end of the operator new line
>>> when testing. Now the patch makes this runtime test go from abort to
>>> success at -O0 (with optimizations, the memset is removed as dead code). I
>>> am still not sure we want this kind of test though. And I added launder
>>> more to quiet a warning than with confidence that it does the right thing.
>>>
>>> #include <optional>
>>> struct A {
>>>    int a[1024];
>>> };
>>> typedef std::optional<A> O;
>>> int main(){
>>>    unsigned char t[sizeof(O)];
>>>    __builtin_memset(t, -1, sizeof(t));
>>>    new(t)O();
>>>    if(std::launder(t)[512] != (unsigned char)(-1)) __builtin_abort();
>>> }
>>
>> Yeah, I think the patch is OK with or without the test. As a side
>> note, you don't need the launder
>> if the check uses the pointer value returned by placement-new.
>
> Doesn't the placement new make the memory state of anything
> not explicitely initialized indeterminate?  That is, isn't the
> testcase broken anyways since GCC can elide the memset
> when seeing the placement new?

Ah, I was just replying to that in parallel. Yes it is broken, that's why 
I don't really like adding it. But -fno-lifetime-dse may be enough to make 
it work if we really want to.

-- 
Marc Glisse