On 12/01/2012 14:45, Michal Soltys wrote:
On 12.01.2012 08:14, David Brown wrote:
On 12/01/12 00:43, NeilBrown wrote:
On Thu, 5 Jan 2012 12:16:41 +0100 Jes.Sorensen@xxxxxxxxxx wrote:
From: Jes Sorensen<Jes.Sorensen@xxxxxxxxxx>
- info->array.ctime = DECADE + __be32_to_cpu(*(__u32*)
- (ddf->anchor.guid+16));
+ cptr = (__u32 *)(ddf->anchor.guid + 16);
+ info->array.ctime = DECADE + __be32_to_cpu(*cptr);
But going through intermediate (yet incompatible) pointer still breaks
the aliasing rules - unless the above is related to how 4.7+ in
particular do things.
Somewhat similar code - on 4.5.2 and 4.4.1 though, of which the former
will warn about possible break only with the most aggressive (broad)
-Wstrict-aliasing=1 (and the latter at all levels):
int main(void)
{
struct s {
uint16_t d[128];
} tab;
uint32_t *p32;
tab.d[8] = 0x5678;
tab.d[9] = 0x1234;
p32 = (uint32_t *)&tab.d[8];
printf("%X\n", *p32);
return 0;
}
Will output 0 on my machines when compiled with -O2 or higher (or -O1
-fstrict-aliasing) - unless -fno-strict-aliasing is added.
The code clearly breaks the rules, so no wonder optimizations relying on
them messed up. I'm not sure if/how much changed since then, so perhaps
it would behave fine on 4.7+.
The basic rule is that the compiler can assume that objects whose type
has different sizes, cannot appear at the same address. Unions are one
way to avoid this.
Following this lengthy (but quite interesting) thread it's not (was not
?) allowed either:
http://thread.gmane.org/gmane.comp.gcc.devel/111111
It's 2 years old thread, still the rules and interpretation seem pretty
well entrenched.
OTOH: __attribute__((__may_alias__)) should(?) always give consistent
(expected) results with type definitions that are used as aliases. Maybe it
would be better to go by this route.
I couldn't find that attribute in the gcc manual.
In context of mdadm - I wonder if simply -fno-strict-aliasing wouldn't
be the overall best thing to do.
I don't know much about the style requirements for kernel code, but it
might be best to specify this using "#pragma GCC optimize
("no-strict-aliasing")" to force it into the source code independently
of any global compiler options.
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