Messages in this thread | | | Date | Thu, 27 Feb 2003 14:45:22 -0500 | From | Daniel Jacobowitz <> | Subject | Re: Invalid compilation without -fno-strict-aliasing |
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On Thu, Feb 27, 2003 at 07:30:03PM +0000, Linus Torvalds wrote: > In article <20030226172213.O3910@devserv.devel.redhat.com>, > Jakub Jelinek <jakub@redhat.com> wrote: > > > >To fix that, __constant_memcpy would have to access the data through > >union, > > Which is impossible, since memcpy _fundamentally_ cannot know what the > different types are.. > > > or you could as well forget about __constant_memcpy and use > >__builtin_memcpy where gcc will take care about the constant copying. > > Which is impossible because (a) historically __builtin_memcpy does a bad > job and (b) it doesn't solve the generic case anyway, ie for other > non-memcpy things. > > The fact is, for type-based alias analysis gcc needs a way to tell it > "this can alias", which it doesn't have. Unions are _not_ useful, > _regardless_ of what silly language lawyers say, since they are not a > generic method. Unions only work for trivial and largely uninteresting > cases, and it doesn't _matter_ what C99 says about the issue, since that > nasty thing called "real life" interferes. > > Until we get some non-union way to say "this can alias", that > -fno-strict-alias has to stay because gcc is too broken to allow us > doing interesting stuff in-line without it. > > My personal opinion is (and was several years ago when this started > coming up) that a cast (any cast) should do it. But I don't are _what_ > it is, as long as it is syntactically sane and isn't limited to special > cases like unions.
Well, if that's all you're asking for, it's easy - I don't know if you'll agree that the syntax is sane, but it's there. From the GCC 3.3 manual:
`may_alias' Accesses to objects with types with this attribute are not subjected to type-based alias analysis, but are instead assumed to be able to alias any other type of objects, just like the `char' type. See `-fstrict-aliasing' for more information on aliasing issues.
Example of use:
typedef short __attribute__((__may_alias__)) short_a;
int main (void) { int a = 0x12345678; short_a *b = (short_a *) &a;
b[1] = 0;
if (a == 0x12345678) abort();
exit(0); }
If you replaced `short_a' with `short' in the variable declaration, the above program would abort when compiled with `-fstrict-aliasing', which is on by default at `-O2' or above in recent GCC versions.
So you define a typedef for unsigned long which has the __may_alias__ attribute, and you go to town writing memcpy inline with that type instead of a normal unsigned long.
-- Daniel Jacobowitz MontaVista Software Debian GNU/Linux Developer - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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