On Tue, Jun 7, 2011 at 4:05 PM, Herman, Geza <g...@bitquad.com> wrote:
> On 06/07/2011 03:02 PM, Richard Guenther wrote:
>>
>> On Tue, Jun 7, 2011 at 2:58 PM, Herman, Geza<g...@bitquad.com> wrote:
>>>
>>> On 06/07/2011 12:27 PM, Richard Guenther wrote:
>>>>
>>>> On Tue, Jun 7, 2011 at 5:51 AM, Herman, Geza<g...@bitquad.com> wrote:
>>>>>
>>>>> Hi,
>>>>>
>>>>> Sorry, if it has been discussed before, I found a lot of information on
>>>>> strict aliasing in gcc, but nothing about this particular case. I'd
>>>>> like
>>>>> to
>>>>> code a custom container class: it has a char[] (or dynamically
>>>>> allocated
>>>>> "char *") for storage, putting an element is done with placement new,
>>>>> getting an element is done with reinterpret_cast'ing the storage, like
>>>>> this:
>>>>>
>>>>> #include<new>
>>>>>
>>>>> struct Something {
>>>>> int value;
>>>>>
>>>>> Something() { value = 42; }
>>>>> };
>>>>>
>>>>> template<typename OBJECT, int MAX_SIZE>
>>>>> struct Container {
>>>>> union {
>>>>> char data[sizeof(OBJECT)*MAX_SIZE];
>>>>> // here goes a member which ensures proper alignment
>>>>> };
>>>>> int currentSize;
>>>>>
>>>>> Container() { currentSize=0; }
>>>>>
>>>>> void add() {
>>>>> new(data+sizeof(OBJECT)*currentSize) OBJECT;
>>>>> currentSize++;
>>>>> }
>>>>>
>>>>> OBJECT&operator[](int index) {
>>>>> // assert(index<currentSize);
>>>>> return reinterpret_cast<OBJECT*>(data)[index]; // gcc warns here
>>>>> }
>>>>> };
>>>>>
>>>>> void bar() {
>>>>> Container<Something, 5> c;
>>>>> c.add();
>>>>> c[0].value = 41;
>>>>> }
>>>>>
>>>>> Here, gcc warns on the reinterpret_cast line. However, as I
>>>>> understand,
>>>>> this usage cannot cause any harm. If I always access "data" as
>>>>> "OBJECT",
>>>>> nothing can go wrong, right? Even, if I access "data" as char
>>>>> sometimes,
>>>>> gcc should handle this case, I think. Of course, if "data" would be a
>>>>> non-char array, the situation would be different. Is the warning
>>>>> appropriate here? Can a cast from char[] cause anything bad? If it's
>>>>> difficult to detect this usage method on the compiler side, is there
>>>>> any
>>>>> way
>>>>> to avoid this warning just for this line (maybe rephrasing this line
>>>>> somehow
>>>>> to tell gcc that everything is OK)?
>>>>
>>>> The code looks ok and should work fine with GCC 4.5 and newer. No
>>>> guarantees for older versions though, if it works there it certainly
>>>> isn't
>>>> by design.
>>>
>>> Thanks for the answer!
>>>
>>> You're right, this example compiles without warnings with GCC 4.5. My
>>> mistake, I copied a version which warns only with GCC 4.4 in my previous
>>> email. But, if I add another member function, like:
>>>
>>> OBJECT&first() {
>>> return reinterpret_cast<OBJECT*>(data)[0]; // gcc warns here
>>> }
>>>
>>> and call it from "bar()":
>>>
>>> c.first().value = 41;
>>>
>>> I tried both GCC 4.5, and the latest available in my system (gcc version
>>> 4.7.0 20110531 (experimental) [trunk revision 174470]), both produces a
>>> warning. Is it a false positive warning?
>>
>> Yes.
>
> Okay :)
>
> Here's my last question (I hope). As GCC cannot know the usage pattern of
> char[], it's good that it emits a warning. For example, if we'd remove the
> warning (just for the case where char[] is casted), this case wouldn't get a
> warning, and would generate incorrect code:
>
> char data[...];
> reinterpret_cast<int*>(data) = 1;
> printf("%d\n", reinterpret_cast<short*>(data));
>
> However, for my construct, which appears to be completely legal, I get a
> warning, which I'd like to disable. How can I do that? Currently I'm using
> -Wno-strict-aliasing, but I'd like to have a better solution. I tried to
> cast (void*) before the cast to (OBJECT*), it didn't help. Is it possible
> to disable this warning for this line only (maybe with some GCC specific
> tricks)?
Try
void *temp = (void *)data;
reinterpret_cast<int *>(temp)
Richard.
> Thanks,
> Geza
>
>>
>> Richard.
>>
>>> Luckily, generated code works, even with previous versions of GCC.
>>>
>>>>
>>>> Richard.
>>>>
>>>>> Thanks,
>>>>> Geza
>>>>>
>>>
>>>
>
>
