[LLVMdev] LLVMdev Digest, Vol 76, Issue 43

[Duncan Sands]

Hi Torok,

>>> Well...strictly as LLVM IR I find externally visible incorrect
>>> behavior unlikely, it's just a "different definition".  For C and
>>> C++, I'd be looking at more complicated variations of
>>>
>>> int main()
>>> {
>>>       volatile int i = 1;
>>>       return 0;
>>> }
>>>
>>> It's clear that the LLVM IR representation of i cannot be simply
>>> IR-volatile qualified, as that load gets optimized out while C and
>>> C++ won't optimize it out.  I'd *hope* that DragonEgg and llvm-gcc
>>> both leave the load of i in, when in --pedantic mode.  [That is, I
>>> expect it to take something more intricate than this elementary
>>> test case to trigger any bugs here.]
>>
>> both dragonegg and llvm-gcc remove the volatile.  I don't see why they
>> shouldn't, since the program behaves exactly the same (as far as
>> anyone external can tell) as if it had been left there.
>
> What if its address is taken, or it is a global variable?

the volatile is only removed if it can be proved that doing so is harmless.
It is not removed in the cases you mention.

Ciao,

Duncan.