Re: [boost] missing __volatile__ tags in sp_counted_base_gcc_xxx.hpp

From: Howard Hinnant (howard.hinnant_at_[hidden])
Date: 2006-03-13 15:55:01

On Mar 13, 2006, at 3:37 PM, Peter Dimov wrote:

> Howard Hinnant wrote:
>> Hello,
>>
>> I've been investigating some failures on Mac OS X, ppc concerning
>> shared_ptr.  One of the things that has struck me is that in boost/
>> detail/sp_counted_base_gcc_ppc.hpp, the __asm__ block in
>> atomic_decrement is decorated with __volatile__, but the __asm__
>> blocks in atomic_increment and atomic_conditional_increment are not.
>> The failure I'm seeing disappears when the __volatile__ attribute is
>> added in the two missing places.
>
> This is, I believe, intentional; the increments shouldn't be
> reorder-sensitive, at least in theory. Why/how does it fail?

I'm seeing (only with inlining/optimizations on) that the reference  
count is not properly incremented.  This is with a single-threaded  
test case.  I haven't fully understood the code differences yet, but  
it appears that code is being reordered across these assembly blocks  
in such a way as to change the logic.  I'm not familiar enough with  
gcc assembly semantics to know if the volatile attribute (on the  
assembly block itself, not any data) is required to make inline  
assembly behave itself.

Again, this is not a multithread issue that I can see, as the  
failures I'm seeing are in single threaded code.

Thanks for your comments.

-Howard