[cpp-threads] A question about N2153

[Chris Thomasson]

----- Original Message ----- 
From: "Chris Thomasson" <cristom at comcast.net>
To: "C++ threads standardisation" <cpp-threads at decadentplace.org.uk>; 
<paulmck at linux.vnet.ibm.com>
Sent: Wednesday, January 17, 2007 4:09 PM
Subject: Re: [cpp-threads] A question about N2153


> [...]
>
>>> load_depends == *#LoadDepends | #LoadLoad
>
>> Ummm...  On all CPUs other than Alpha, you don't need -any- fencing,
>> barriers, or special instructions to cause the CPU to respect
>> ordering on data dependencies.
>
> I know... Of course load_depends would be a NOP on everything except 
> Alpha.

Okay... Let me just sum up how I would like the new and improved version of 
C++, or whatever...

To do RCU, well, you do can do the barriers like this:

<pseudo c++ code>

// shared stack
class node;
static mystack_t gs;

// writer side - mutex-based
void writer_thread(...) {
  node *n = node_cache::pop(...);
  lock_guard lock(gs);
    n:(#StoreStore)->next = gs:(#Naked).front;
    gs:(#Naked).front = n;
}

// reader side
void reader_thread(...) {
  // nop on most systems
  node *n:(#LoadDepends | #LoadLoad) = gs:(#Naked).front;
  while(n) {
    // again, its a nop
    node *nx:(#LoadDepends | #LoadLoad) = n:(#Naked)->next;
    n->const_function(...);
    n = nx;
  }
}


so, the reader-side has exactly 0 memory barriers on every current system 
out there except the alpha. Also, its weak enough to express just a normal 
#StoreStore inside the writers critical section that is guarded by the stack 
objects associated mutex... I would kind of like it if C++ would copy from 
the SPARC model... Just my humble opinion of course...

;^)