On Sat, Oct 27, 2007 at 03:06:21 +1000, skaller wrote:
> err .. what about the heap??
The heap are objects for which the addresses were taken. So they can
be shared. But I haven't yet seen that the optimization we discuss is
being applied to the object accessed though the pointer (see my reply
to Michael Matz). Maybe this is just a coincidence.
I was beaten already for repeating myself, but please let me do that
once more :). First, I have a strong believe (though I didn't test
it) that
if (C)
val->mem;
runs faster than
mem->reg;
if (C)
val->reg;
reg->mem;
(short) jump will cost less then unconditional load/store when they
are not needed (especially the store).
BTW, it would be interesting to measure if short jumps are as bad as
long jumps, i.e. whether CPU pipeline is flushed when jump target is
already in it.
Second, in situation like
loop
if (C)
val->mem;
i.e. when there are lots of conditional stores, only one final store
matters. And current optimization employs this:
mem->reg;
loop
if (C)
val->reg;
reg->mem; // One final store.
But at the cost of additional register this final store can be made
conditional (there are cases when even that register is not needed,
but that requires thorough analysis of val's possible values, i.e. reg
could be initialized to some "invalid" value and then checked for it).
Registers are a valuable resource, yes. But so is the correct program
result. Since GCC is correct wrt all standards, next comes its
usability in not-yet-standardized domains.
> And what do you do if you do not KNOW what the storage class is,
> which is the case 99.99% of the time in C++ member functions?
I'm not quite sure what you mean here. If extern vs static---that's
of no concern. What matters is whether the object can possibly be
accessed from another thread, and this has nothing specific to C++.
--
Tomash Brechko
