Re: How to make this program more efficient?
On Sep 24, 8:19 pm, courp...@gmail.com wrote:
On 24 sep, 17:39, James Kanze <james.ka...@gmail.com> wrote:
On Sep 23, 9:40 pm, gpderetta <gpdere...@gmail.com> wrote:
Also, IIRC the standard prohibits only reordering of
accesses to volatile objects. Reads and writes to non
volatile objects can still be reordered even across a
volatile.
Not really. Almost all of the actual specification of
volatile is implementation defined, so an implementation
pretty much do what it wants. The expressed intent in C++
is that it should work as in C, and the expressed intent in
C corresponds pretty much to what you say, but it remains
intent, and not a formal requirement.
It is implementation defined, but there is one formal requirement :
[1.9/11] :
The least requirements on a conforming implementation are:
? At sequence points, volatile objects are stable in the sense that
previous evaluations are complete and subsequent evaluations have not
yet occurred.
Which means? (For starters, a sequence point is a compile time
characterization, and doesn't exist at runtime.)
That means volatile prevents static reordering of volatile
variables accesses, and just that (we were dissecting all the
effects of volatile).
But only if there is a sequence point between them. And only
for whatever the implementation defines "access" to mean: as I
think I said, the Sparc compilers I have access to consider
accessing the value in the memory pipeline sufficient, even
though the memory pipeline is local to each CPU (core, not
visible except from the CPU, and not synchronized with anything
outside the CPU.
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