On 04/01/2015 07:15 PM, Martin Sebor wrote:
> 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;
This is a common misconception.
It's wrong because a->b is identical to (*(a)).b and a[i] is identical
to (*((a)+(i))) .
Therefore, in the section above, the expression
&((twobytes*)ar + ii)->a
is the same as
&(((twobytes*)ar)[ii].b)
and the illegal access is more obvious.
GCC helpfully wants about this, but unfortunately only with -Wall:
z.c: In function 'main':
z.c:16: warning: dereferencing pointer '({anonymous})' does break strict-aliasing rules
Andrew.
