On Fri, 26 Oct 2007, Andi Kleen wrote:
>
> The conditional add/sub using carry trick is not generally bogus.
> But for registers it's a fine optimization.
For registers it's fine. For memory, it's a disaster. It's more than just
dirty cachelines and introducing race conditions, it's also about
protection and dirty pages.
So even in user space, to even be correct in the first place, the compiler
would need to make sure that the variable is writable at all (or you might
take a SIGSEGV), but I guess that gcc just assumes it is, at least for
globals (or gcc could depend on seeing *other* writes that are done
unconditionally).
More likely, the compiler people don't even care, because "the C standard
doesn't specify that behaviour" - ie things like write-protected memory or
garbage collection based on dirty/accessed bits are outside the scope of
what the language specifies. Much less things like pthreads or other
synchronization primitives in threads.
Linus
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