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Re: Optimization bug?



> On Sep 19, 2015, at 11:00 AM, SÃren 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. 

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,
>    SÃren
> <main.c>
> <Makefile>


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