Re: C++ Threads, what's the status quo?
Zeljko Vrba wrote:
int x;
void thread_i() {
while(1) {
// do something
if(--x == 0)
break;
// 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
transition.
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
primitives.
-Le Chaud Lapin-
--
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