Re: C++ Threads, what's the status quo?

"Le Chaud Lapin" <>
14 Jan 2007 08:39:08 -0500
Zeljko Vrba wrote:

int x;

void thread_i() {
   while(1) {
   // do something
   if(--x == 0)
   // do something else

Now, in the situation as it is now, the compiler may generate code for
'--x == 0' as [for those who know x86 ASM, I write it by the side]

   1. load x into register (movl x, %eax)
   2. decrement register (decl %eax)
   3. store register into x (movl %eax, x)
   4. if result == 0 goto exit (jz exit)
Now 1. is single atomic operation, where the hardware serializes parallel
accesses to memory. No race conditions are possible. Yes, wrapping x
into something Volatile<int> x _would_ work with the disadvantages that

   - implicit wrapping in a mutex is suboptimal if the architecture
     supports atomic operation

I like your creativity with new keywords, but note that a user-mode
atomic operation will often obviate a full user-to-kernel-mode

Also, as hinted by John Q, there will be many cases (probably most)
where there is no choice but to make one thread wait while another
thread operates on a shared global variable, so a compiler-supplied
atomic operation will only get you up to the spin-lock, and after that,
it is back to kernel-mode synchronization primitives.

This is actually the motivation of most of my posts: to remind us that
all trickery will ultimately lead to kernel-mode synchronization

-Le Chaud Lapin-

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