Re: GCC 4.8.4 4.9.2 nifti_swap_2bytes optimization bug

[Andreas Schuh <andreas dot schuh dot 84 at gmail dot com>]

Thanks for the quick reply. As I just noticed, this problem has been fixed already in the latest NIfTI C library v2.0. We just still used an old copy it :S.

> On 1 Apr 2015, at 15:49, Andrew Haley <aph@redhat.com> wrote:
> 
> On 04/01/2015 03:36 PM, Andreas Schuh wrote:
> 
>> I believe I have encountered a bug in the optimization at least in
>> versions 4.8.4 and 4.9.2. The problem is not present in version
>> 4.6.3. The attached complete example code can be used to reproduce
>> the error (my system is Ubuntu 12.04). This code is crucial in
>> medical image processing where the NIfTI-1 file format is commonly
>> used. The respective function that is optimized incorrectly by most
>> recent C/C++ compilers of GCC was copied from the NIfTI-1 C library
>> and is used by many medical image processing tools. It is
>> responsible for swapping the bytes if necessary when the image data
>> was written on a system with differing endianness.
> 
> Sure, but it's not standard C.  You really don't need to alias a
> struct of two bytes and a short, and these are not compatible types.
> (I can give you chapter and verse of the C standard if you really want
> it, but the rule is simple: don't expect a cast between pointers to
> different types to work.  Doing so via a hidden void* doesn't help.)
> 
> It'd be perfectly legal to do this swap with an array of bytes,
> because byte arrays can alias anything.
> 
> Like this:
> 
>   char *p = ar;
>   for( ii=0 ; ii < 2*n ; ii +=2 ){
>     tt = p[ii]; p[ii] = p[ii+1] ; p[ii+1] = tt ;
>   }
> 
> If you really need to compile stuff like this you can use
> -fno-strict-aliasing, but I'd fix the code if I were you.
> 
> Andrew.