Re: [boost] Solaris versions of sp_counted_base and atomic_count

From: Peter Dimov (pdimov_at_[hidden])
Date: 2006-07-08 13:14:46

Michael van der Westhuizen wrote:

> Ok, it looks like I spoke too soon. I was getting core dumps on
> weak_ptr_mt_test until I changed compare_and_swap to this:
>
> inline int32_t compare_and_swap( int32_t * dest_, int32_t compare_,
> int32_t swap_ )
> {
>    __asm__ __volatile__( "cas [%2], %3, %0"
>                         : "=&r"(compare_), "=m"(*dest_)
>                         : "r"(dest_), "r"(compare_), "0"(swap_),
>                         "m"(*dest_) : "memory" );
>    return compare_;
> }

It's better if we can get an +m(*p)/=m(*p) formulation to work, instead of 
passing p in a register with r(p). It makes a difference in efficiency on 
platforms that can address an expression in addition to a single register, 
and on some architectures/modes when a C++ pointer is 32 bit but the 
hardware address is 64 bit, using r(p) truncates p to 32 bits and crashes. 
:-)

Actually, the bug in the original seems to be that CAS returns the old value 
in swap_ and not compare_, so how about:

inline int32_t compare_and_swap( int32_t * dest_, int32_t compare_, int32_t 
swap_ )
{
    __asm__ __volatile__( "cas %0, %2, %1"
        : "+m" (*dest_), "+r" (swap_)
        : "r" (compare_)
        : "memory" );

    return swap_;
}