Re: Am I or Alexandrescu wrong about singletons?
On Mar 23, 1:42 pm, Michael Doubez <michael.dou...@free.fr> wrote:
On 23 mar, 00:22, "Bo Persson" <b...@gmb.dk> wrote:
[...]
Still it does say something of the semantic of the memory
location. In practice the compiler will cut the optimizations
regarding the volatile location; I don't see a compiler
ignoring this kind of notification.
Not really. It makes some vague statements concerning "access",
while not defining what it really means by access. And "memory
location", without further qualifiers, has no real meaning on
modern processors, with their five or six levels of memory---is
the memory the core specific cache, the memory shared by all the
cores, or the virtual backup store (which maintains its values
even after the machine has been shut down)?
And of course, what really counts is what the compilers
implement: neither g++, nor Sun CC, nor VC++ (at least through
8.0) give volatile any more semantics that issuing a load or
store instruction---which the hardware will execute when it gets
around to it. Maybe.
Which means that the memory value will eventually (after an
undetermined amount of time) be flushed to the location and
not kept around in the stack or somewhere else for
optimization reasons.
Sorry, but executing a store instruction (or a mov with a
destination in memory) does NOT guarantee that there will be a
write cycle in main memory, ever. At least not on modern Sparc
and Intel architectures. (I'm less familiar with others, but
from what I've heard, Sparc and Intel are among the most strict
in this regard.)
My understanding is that it is the amount of optimization
cutting that is implementation dependent.
The issue isn't compiler optimization. If that were the
problem, just turn off the optimizer: all of the compilers that
I know will then follow the rules of the abstract machine very
rigorously.
Still, I have not understodd how this can be useful for
multithreading with the next-to-be standard, AFAIS atomic
types gives better guarantees and better optimization
possibilities.
C++0x addresses threading. And the people who worked on that
part of it have learned from earlier attempts. (Java had to
rewrite their memory model at least once in order to provide
adequate guarantees without totally breaking performance.)
--
James Kanze
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