On 04/01/2015 08:49 AM, Andrew Haley 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.)
The C requirement is that "An object shall have its stored value
accessed only by an lvalue expression of ... a character type."
The original program accesses the value object via the tb[ii].a
and tb[ii].b expressions, which are both unsigned char lvalues.
That the program derives those lvalues from pointers to some
other type (*) doesn't violate the aliasing rule.
[*] As if by:
unsigned char *b0 = &((twobytes*)ar + ii)->a;
unsigned char *b1 = &((twobytes*)ar + ii)->b;
tt = *b0;
*b0 = *b1;
*b1 = tt;
Martin
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.
