[LLVMdev] volatiles (was comparison of correctness of llvm and gcc)

[John Regehr]

Hi Duncan,

We currently check that every byte of a volatile is accessed the same 
number of times, and that this number doesn't change across optimization 
levels.

If LLVM wants to make stronger guarantees that's great, I suspect this is 
not hard to all to check.  Can you provide a list of types that should be 
atomic for the x86 target?  For example would we expect a struct of size 4 
to be atomically accessed?  How about pointers?  Bitfields?  Actually I 
think the interaction of volatile and bitfields is murky so maybe we don't 
want to go there.

Also, right now we don't check that the order in which volatiles are 
accessed is the same across all optimization levels.  Fixing this is on my 
list.  As long as we generate code with at most one volatile access 
between any given pair of sequence points (which we do) the compiler is 
not free to reorder.

John