Hi Andrew,
On Sat, 2015-09-19 at 11:34AM -0700, [email protected] wrote:
>
>
> > 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.
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.
Thanks,
Sören
>
> 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>
