On Fri, Apr 25, 2014 at 1:18 PM, Jonathan Wakely <jwakely....@gmail.com> wrote:
> On 25 April 2014 11:22, Andrew Haley wrote:
>> Summary: Devirtualization uses type information to determine if a
>> virtual method is reachable from a call site. If type information
>> indicates that it is not, devirt marks the site as unreachable. I
>> think this is wrong, and it breaks some programs. At least, it should
>> not do this if the user specifies -fno-strict-aliasing.
>>
>> Consider this class:
>>
>> class Container {
>> void *buffer[5];
>> public:
>> EmbeddedObject *obj() { return (EmbeddedObject*)buffer; }
>> Container() { new (buffer) EmbeddedObject(); }
>> };
>>
>> Placement new is used to embed an object in a buffer inside another
>> object. Its address can be retrieved. This usage of placement new is
>> common, and it even appears as the canonical use of placement new in
>> the in the C++ FAQ at
>> http://www.parashift.com/c++-faq/placement-new.html. (I am aware that
>> this is not strictly legal. For one thing, the memory at buffer may
>> not be suitably aligned. Please bear with me.)
>
> I think the program is strictly legal if you define Container like this:
>
> class Container {
> alignas(EmbeddedObject) char buffer[sizeof(EmbeddedObject)];
> public:
> EmbeddedObject *obj() { return (EmbeddedObject*)buffer; }
> Container() { new (buffer) EmbeddedObject(); }
> };
>
> But GCC still does the same transformation and prints nothing, which I
> agree is wrong (even with -fstrict-aliasing).
I agree, -fstrict-aliasing has nothing to do with this. Sounds simply like
a genuine bug (please open a bugzilla).
Thanks,
Richard.
> With ubsan that program gives:
>
> sa.cc:9:24: runtime error: load of null pointer of type '<unknown> *'
> Segmentation fault (core dumped)
