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

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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