GCC 4.8.4 4.9.2 nifti_swap_2bytes optimization bug

[Andrew Haley]

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.