[lkml]   [2003]   [Feb]   [27]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
Messages in this thread
SubjectRe: Invalid compilation without -fno-strict-aliasing
On Thu, Feb 27, 2003 at 07:30:03PM +0000, Linus Torvalds wrote:
> In article <>,
> Jakub Jelinek <> 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

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

Example of use:

typedef short __attribute__((__may_alias__)) short_a;

main (void)
int a = 0x12345678;
short_a *b = (short_a *) &a;

b[1] = 0;

if (a == 0x12345678)


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
More majordomo info at
Please read the FAQ at

 \ /
  Last update: 2005-03-22 13:33    [W:0.040 / U:5.388 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site