On 10/19/2011 09:26 AM, Andrew MacLeod wrote:
> well, they are explicitly 'compare_exchange_strong' and
> 'compare_exchange_weak' calls... so yes, they have 'hardcoded' one or the
> other :-)
> we could alternatively do 2 separate builtins, but I didn't see the point.
> But Im ambivalent.
I'm just not keen on issuing errors this late. If we 'know' it's always going
to
be 0/1 because of other things the compiler does, we might as well simply do
weak = (opN == const1_rtx)
and leave it at that.
> I clearly misunderstood what you said then, I thought you said use
> QImode :-P. So, look at target and use that mode? target may not
> be set sometimes tho right? what do you use then? QImode?
You never look at target, you look at
insn_data[icode].operand[opno].mode
if target is set and doesn't match that mode, then create a new pseudo
of the proper mode and let your caller do the conversion.
> If I understand my previous conversation with Lawrence, we need to
> know what mode the native CAS is.
We don't have that knowledge for any of the other rtl patterns; we
ask the rtl pattern to enforce a given mode. We should do the same
for CAS.
> Worst case, if the native CAS is relaxed, the fences need to be
> issued as such to implement
>
> compare_exchange (T* ptr, T* old, new, success, failure)
>
> memory_fence (success) // release modes
> val = *old;
> res = relaxed_CAS (ptr, val, new)
> if (success)
> {
> memory_fence (success); // acquire modes
> return true;
> }
> *old = res;
> memory_fence (failure)
> return false;
If the target uses LL/SC, then native CAS is likely relaxed. However,
we will already be emitting a number of jumps in order to implement
the LL/SC sequence and stuffing the fences into those jumps is going
to be much better than you trying to do it in the middle-end outside
of that sequence. Really.
> So if the CAS can handle it all, why does it matter if the pattern
> has a single "compressed" parameter for the 3 values, or whether it
> simply explicitly lists all three?
*shrug* Just for less memory consumption. It *could* remain 3 values.
r~
