Re: [Xen-devel] [PATCH] i386 linux: make 32-bit PAE kernel work when built with newer gcc

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

On 14 Mar 2006, at 10:40, Jan Beulich wrote:

> > Won't this need a barrier() (compile barrier) between the updates of
> > low and high portions?
> No, I can't see why. The compiler isn't permitted to re-order separate 
> writes across
> sequence points (which is different from a single 64-bit write, where 
> the compiler is
> only expected to carry out the full 64-bit write prior to the next 
> sequence point, but
> nothing requires it to do this in any particular order).
The compiler is certainly allowed to reorder those updates. The 
constraints on a conforming implementation of C99 are pretty weak, and 
don't say anything about obeying the rules for access/update ordering 
on non-volatile objects. Whether gcc reorders such updates is another 
matter. :-)
If I build the following with -O2 on x86/64 gcc 4.1, the compiler 
removes the first update of x. If I take a signal after the update of 
y, the signal handler can see y==2, z!=2 but also x!=2, which disobeys 
the semantics of the C99 abstract machine semantics:
int x, y, z;
void foo(void) { x = 2; y = 2; z = 2; x = 0; }

Really it's safer just to include the barrier(), and makes our ordering 
requirement explicit in the code.
 -- Keir


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