Fwd: Problem report: Struct aliasing problem causes Thread_Ready_Chain corruption in

Peer Stritzinger peerst at gmail.com
Tue Nov 21 16:39:45 UTC 2006


On 11/21/06, Joel Sherrill <joel.sherrill at oarcorp.com> wrote:
> I am more concerned about the _Chain_Head and _Chain_Tail routines.
> I don't see how to clean them up without the union.  And I believe (but
> can't prove) that they are the root of gcc's confusion.

>From what I learned during the hunt this might very well be.

Nevertheless the question is if the complete code behaves correctly with
strict-aliasing.  I mean if you are strict about strict-aliasing you
may have no overlapping structs at all not depending if you use tricks
to tell gcc what to do (or confuse the optimizer so it will work ...
until it gets a bit smarter in the next release).

But overlapping structs are used all over the code as it is very often
especially in operating system code.  Thats actually the reason for
the existence of
-fno-strict-aliasing.  Its telling the compiler that you intend to
overlap structs and do some punning now and then.  Looks like a
(manual) space against time optimization(supposing the code will run
faster when we assume strict-aliasing).

The -Wstrict-aliasing warnings (included in -Wall) do not report all cases
that might break the strict-aliasing rule.

You might work with -Wstrict-aliasing=2 additionally while trying to
root out more
cases.  BTW they both probably only work when -On n > 2 is also on.

None of these warning switches claim to get all strict-aliasing violations.

-- Peer

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