> On Sep 19, 2015, at 11:00 AM, Sören Brinkmann <[email protected]>
> 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.
Thanks,
Andrew
>
> int main(void)
> {
> unsigned int *vector_base = &_vector_table;
>
> if (vector_base == 0) {
> return 1;
> } else {
> return 2;
> }
> }
>
> The code generated for this function is (I tested a couple of different
> compilers and architectures (4.7, 4.9 and 5.2)):
> 0000000000000000 <main>:
> 0: b8 02 00 00 00 mov $0x2,%eax
> 5: c3 retq
>
> If my understanding is correct, this is a legal optimization under the
> assumption that no object may reside at address 0, resulting in the
> optimized out if branch.
> Though, my understanding is that the switch
> '-fno-delete-null-pointer-checks' would remove that assumption allowing
> such code to be compiled "correctly" (I'm on an embedded system and code
> at 0 is actually important).
>
> Could somebody tell me what the correct way for implementing something
> like that would be? Or whether this is actually a gcc issue?
>
> Thanks,
> Sören
> <main.c>
> <Makefile>
