On Wed, Sep 26, 2012 at 11:14:41AM +0300, Andriy Gapon wrote: > > Typical x86 MONITOR+MWAIT is like this (taken from Intel manual): > > EAX = Logical Address(Trigger) > ECX = 0 (*Hints *) > EDX = 0 (* Hints *) > IF ( !trigger_store_happened) { > MONITOR EAX, ECX, EDX > IF ( !trigger_store_happened ) { > MWAIT EAX, ECX > } > } > > In FreeBSD we have this helper function for MONITOR: > static __inline void > cpu_monitor(const void *addr, u_long extensions, u_int hints) > { > > __asm __volatile("monitor" > : : "a" (addr), "c" (extensions), "d" (hints)); > } > > Now, let's assume that 'Trigger' is a global variable and > 'trigger_store_happened' is a check for a particular value in this variable. > > Then, with current state of matters, we must either declare the global > variable > to be volatile or use a volatile cast in the second IF. Otherwise, a compiler > is free to assume that the variable doesn't change between the first and the > second IF and drop the second IF altogether. And that would make > MONITOR+MWAIT > to miss an event if it happens "between" the first check and MONITOR. > > So what's my point. > - using volatile variable with cpu_monitor requires DEVOLATILE to silence > compiler warning about discarding volatile; this is unnecessary code bloat > - adding volatile cast in the checks is easy to forget and adds code bloat > > Possible improvements: > - make the argument of cpu_monitor be 'const volatile void *', the most > permissive type; this would also be a hint that variable should be volatile > - add some magic dust to cpu_monitor that would tell compiler to not cache > the variable; right now I can only think of the "memory" constraint, but it > seems to be too big of a hummer > > What do you think about this?
You might claim that the asm writes to *addr by specifying it in the output constraint. This should fool the compiler into reload *addr after the monitor execution.
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