Chris Lattner <clatt...@apple.com> writes:
> On Jan 27, 2009, at 5:10 PM, Ian Lance Taylor wrote:
>
>> Chris Lattner <clatt...@apple.com> writes:
>>
>>> On Jan 27, 2009, at 1:10 PM, Ian Lance Taylor wrote:
>>>
>>>> Laurent GUERBY <laur...@guerby.net> writes:
>>>>
>>>>> Just curious: is there a "portable" way to read from memory
>>>>> address zero in C code? "portable" here means likely to work
>>>>> on most compilers without exotic compile flags in 2009.
>>>>
>>>> char *my_null_pointer;
>>>> char fn() { return *my_null_pointer; }
>>>>
>>>> It will be quite a while before whole program optimization is good
>>>> enough to prove that my_null_pointer is not referenced by anything
>>>> else.
>>>
>>> Perhaps in GCC, but this is not a good solution if you care about
>>> other compilers.
>>
>> Is LLVM smart enough to optimize that away, even when using shared
>> libraries?
>
> Yes absolutely. Just build with -fvisibility-hidden or use an export
> map to say that my_null_pointer is not exported. If it is static, it
> will also do it at -O2. This is not even a particularly aggressive
> optimization.
Sure, but the whole idea here is for the compiler to *not* know that
the pointer has a zero value. What I was really asking was whether
LLVM would figure that out without taking any special action to
restrict the visibility of the pointer.
Ian
