Target Environment - Cross-compilation

[Doug Simon]

> On 20 Jan 2017, at 10:31, Andrew Haley <aph at redhat.com> wrote:
> 
> On 19/01/17 23:55, David Clarke wrote:
> 
>> So on x86 we want to optimise away a full fence that immediately
>> follows a weakCompareAndSet and on ARM, we don’t.  This could be
>> delayed until the code generation phase, but I would like to keep
>> the optimisations at the IR or LIR level so that we retain the
>> greatest generality for as long as possible and avoid repeating
>> similar code in several distinct code generation phases.
> 
> There's a related problem on ARMv8.  At the present time we emit
> fences very early, which means that we don't generate the STLR/LDAR
> instructions we really want for Java volailes.  At the very least this
> is inefficient.
> 
>> So my questions are “How can the code within a Node find out the
>> target environment?" and “Can we fiddle this so that the target
>> environment is different from the actual host environment?”
> 
> I think that the real answer is to handle all of this properly in the
> Graal IR by lowering the nodes in a way that is controlled by the back
> end.  This will mean that the compiler will have to be taught about
> how to handle the nodes appropriately.

Yes, you will need something like the AddressLowering/AddressLoweringPhase mechanism Graal currently has.

-Doug