Optimization bug?

[Martin Sebor]

On 09/20/2015 12:38 AM, Richard Biener wrote:
> On September 20, 2015 1:40:12 AM GMT+02:00, Martin Sebor <msebor@gmail.com> wrote:
>> On 09/19/2015 03:32 PM, Sören Brinkmann wrote:
>>> Hi Andrew,
>>>
>>> On Sat, 2015-09-19 at 11:34AM -0700, pinskia@gmail.com wrote:
>>>>
>>>>
>>>>> On Sep 19, 2015, at 11:00 AM, Sören Brinkmann
>> <soren.brinkmann@xilinx.com> wrote:
>>>>>
>>>>> Hi,
>>>>>
>>>>> I recently came across some bug in my program that I could narrow
>> down
>>>>> to the snipped below (also attached with a Makefile).
>>>>>
>>>>>    extern unsigned int _vector_table;
>>>>
>>>> You need the attribute weak here if the location of _vector_table
>> can be a null pointer.
>>>
>>> Indeed, that fixes the problem. Would you have any additional
>> background
>>> why the 'weak' is required in this case? To me it sounded like the
>>> '-fno-delete-null-pointer-checks' should be sufficient.
>>
>> -fno-delete-null-pointer-checks prevents GCC from removing tests
>> for null pointers after such pointers have been dereferenced. For
>> example, in the following function, GCC normally eliminates the
>> second if statement because the first statement lets it assume
>> that p is not null (otherwise dereferencing it would be undefined).
>>
>>    int foo (int *p) {
>>      if (*p == 0) return 0;
>>      if (p == 0) return 1;
>>      return 2;
>>    }
>>
>> The option doesn't affect other assumptions such as that every
>> object and function in a program lives at a valid, non-null
>> address.
>
> Actually we abuse this flag for exactly this purpose so I think the testcase shows a bug.  Unless the particular target cannot have global vars at address zero.
>
> Would you mind opening a bugreport?

I'm not sure I follow. I've tested the original example with 6.0,
5.1, 4.9, 4.8.3, and 4.8.1. All of these eliminate the test with
-fno-delete-null-pointer-checks. Are you saying that's a bug? If
so, what version retains the test?

Martin