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From: CTips <ctips@bestweb.net>
Newsgroups: comp.lang.ada,comp.lang.c++,comp.realtime,comp.software-eng
Subject: Re: Teaching new tricks to an old dog (C++ -->Ada)
Date: Sat, 12 Mar 2005 12:36:30 -0500
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Xref: g2news1.google.com comp.lang.ada:9241 comp.lang.c++:45306
 comp.realtime:1373 comp.software-eng:4933
Date: 2005-03-12T12:36:30-05:00
List-Id: <comp.lang.ada>

Simon Wright wrote:

> Pascal Obry <pascal@obry.org> writes:
> 
> 
>>CTips <ctips@bestweb.net> writes:
>>
>>
>>>>protected type Atomic_Integer is
>>>>   procedure Increment;
>>>>private
>>>>   Value : Integer;
>>>>end Atomic_Integer;
>>>>-- Implementation
>>>>protected body Atomic_Integer is
>>>>   procedure Increment is
>>>>   begin
>>>>      Value := Value + 1;
>>>>   end Increment;
>>>>end Atomic_Integer;
>>>>
>>>
>>>Will that generate:
>>>   L0:
>>>       lwlock temp,&Value
>>>       add    temp,temp,1
>>>       stwcond temp,&Value
>>>       if( failed ) goto L0;
>>>or will it generate something much more heavy-weight.
> 
> 
> Google shows me lwlock as a C function in the PostGres support groups;
> I don't know what machine architecture has these instructions? PowerPC?

Yes, and alpha.

> 
> It could be used by any compiler as part of a low-level
> implementation, of course, but it's hardly likely that a C compiler
> could use it off its own bat. Why whould it want to unless you the
> programmer tells it to?
> 
> If you're talking about assembler inserts in C -- for Pete's sake, we
> can insert assembler in Ada as a last resort if it's needed.

Can you do the following in Ada: use an assembler inset for just stwcond 
and lwlock, and use C for everything else, and then get the expected 
assembly code.

> What happens with lwlock about priority inversion?

You don't need it. Thats one of the big advantages about lock free schemes.

> 
>>No protected type implementations will be so light.

Umm ... look at the set of lock-free data-structures out there. Quite 
heavy-weight structures can be implemented, including a queue where a 
thread can be adding elements while other threads are removing elements.