25 Jul
2003
25 Jul
'03
5:05 p.m.
On Fri, 2003-07-25 at 16:58, Juhana Sadeharju wrote:
So, there is no "do not optimize away apperent
volatile variables"?
Volatile variables in shmclient are easily recognized -- at least
I recognized them. It is the array variable which content is only
read but never written.
There is reason why optimizer chooses to keep variables in CPU registers
instead of flushing all the stuff at every assignment operator.
With current processors you would get _really_ slow code without careful
optimizer use of register variables (btw. x86 arch has way too few
registers available, compared to RISCs) and to be careful _not_ to trash
the cache.
Few points to keep in mind:
1) compiler doesn't know anything about threads (except those which
support OpenMP), so any non-volatile variable is potential register
variable which update time to other threads is unspecified. All
inter-thread variables should be defined volatile.
2) compiler doesn't know anything about processes or IPC, so same as
above. All inter-process variables should be defined volatile.
3) on SMP systems each CPU still has it's own dcache
--
Jussi Laako <jussi.laako(a)pp.inet.fi>