On 01/10/11 16:19, Somebody in the thread at some point said:
> Quoting Andy Green<andy@xxxxxxxxxxx>:
>
>> On 01/10/11 12:23, Somebody in the thread at some point said:
>>
>>>>> Even on x86 it may not always generate code that does what you want:
>>>>> gcc is perfectly entitled to assume that an unaligned pointer is
>>>>> never used. And even now, x86 has a big performance hit for word
>>>>> accesses that straddle a cache line boundary.
>>>>
>>>> I don't know about gcc being able to discard unaligned pointers, and
>>>> have never observed it to my knowledge. That's just a "C lawyer"
>>>> possibility or actually can happen?
>>>
>>> Both. It doesn't happen, but according to the language specification
>>> it could. However, we've seen quite enough of "this is only a
>>> language lawyer's theoretical possibility" bugs turning into real
>>> bugs.
>>
>> Well, with --pedantic -Wall -Werror gcc doesn't care to warn about the
>> unaligned pointer test x.c I posted the other day (not even with a
>> static byte buffer) and produces code using the pointer correctly. So I
>> think you're right with gcc at least silently dropping a misaligned
>> pointer "doesn't happen" and I still don't know what the basis is that
>> it could happen.
>>
>> I agree though having been bitten by it myself with mIsdn a guy just
>> casting things unaligned is at least writing nonportable code and should
>> be able to be written better. There're benefits even on x86 and ARM
>> with hardware fixup so it's hard to argue against.
>
> I think you will find this interesting.
>
> on fc14 i586 gcc (GCC) 4.5.1 20100924 (Red Hat 4.5.1-4)
> [so@bk14 ~]$ gcc --pedantic -o x x.c
> x.c: In function ?main?:
> x.c:9:2: warning: pointer targets in passing argument 1 of ?strcpy?
> differ in signedness
> /usr/include/string.h:128:14: note: expected ?char * __restrict__? but
> argument is of type ?unsigned char *?
It's just noticing that we passed as unsigned char * when strcpy is
built to want a char *, because of pedantic.
That's nothing to do with alignment, and if you edit the buffer to be
char [] instead of unsigned char [] there are no warnings, despite the
alignment problem is still there the same.
> Also if you add -Wcast-align gcc version 4.3.2 (Debian 4.3.2-1.1) on
> the guruplug, you actually get
> x.c:7: warning: cast increases required alignment of target type
That is interesting... I also can see that if I give -Wcast-align on
4.4.2 on F12 Arm Fedora despite it doesn't appear with -Wall.
However with this -->
#include <stdio.h>
#include <string.h>
int main(int argc, char * argv[])
{
char buf[8];
void *v = &buf[1];
unsigned int *p = (unsigned int *)v;
strcpy(buf, "abcdefg");
printf("0x%08x\n", *p);
return 0;
}
which is a very common construct that an unknown-typed pointer is passed
into a callback API as a void *, but the callback knows its type and
casts it, you still don't get any warning even with:
gcc --pedantic -Wall -Werror -Wcast-align /tmp/x.c
So it's not going to sniff out all the problems. Still, thanks for
pointing that out.
-Andy
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