Re: [Boost-bugs] [Boost C++ Libraries] #3341: sp_counted_base_gcc_sparc.hpp compile failure on gcc.4.2.3 w/-O2

Subject: Re: [Boost-bugs] [Boost C++ Libraries] #3341: sp_counted_base_gcc_sparc.hpp compile failure on gcc.4.2.3 w/-O2
From: Boost C++ Libraries (noreply_at_[hidden])
Date: 2009-08-15 18:14:40

#3341: sp_counted_base_gcc_sparc.hpp compile failure on gcc.4.2.3 w/-O2
---------------------------------------+------------------------------------
 Reporter:  jon_p_griffiths@…          |       Owner:  pdimov   
     Type:  Bugs                       |      Status:  new      
Milestone:  Boost 1.40.0               |   Component:  smart_ptr
  Version:  Boost 1.39.0               |    Severity:  Problem  
 Keywords:  gcc, CAS, sp_counted_base  |  
---------------------------------------+------------------------------------

Comment(by jon_p_griffiths@…):

 > the proper constraint could be 'V' instead of 'm'.

 I tried +V before reporting but it also failed (with 'inconsistent asm
 contraints' IIRC). I found two sparc CAS implementations by googling
 (GASNet and the Linux kernel) - both use the following formulation:

 {{{
 __asm__ __volatile("cas [%2], %3, %0\n\t"
     : "=&r" (swap_)
     : "0" (swap_), "r" (dest_), "r" (compare_)
     : "memory");
 }}}

 This forces the address to be stored in a register, which avoids any
 offset addition in the emitted instruction. However, the Linux kernel
 implementation adds memory barriers before and after the CAS; it would be
 interesting to know whether they are really needed in this case.

 For reference, I've tested the following version; it seems to be running
 without issues here:

 {{{
 inline int32_t compare_and_swap( volatile int32_t * dest_, int32_t
 compare_, int32_t swap_ )
 {
     __asm__ __volatile__("membar #StoreLoad|#LoadLoad\n"
                          "cas [%2], %3, %0\n"
                          "membar #StoreLoad|#StoreStore\n"
                          : "=&r" (swap_)
                          : "0" (swap_), "r" (dest_), "r" (compare_)
                          : "memory");
     return swap_;
 }
 }}}

 Cheers,
 Jon

-- 
Ticket URL: <https://svn.boost.org/trac/boost/ticket/3341#comment:2>
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