LIRGenerator.emitMove()

[Roland Schatz]

Hi Andrew,

On 10/27/2016 05:29 PM, Andrew Haley wrote:
>      @Override
>      public Variable emitMove(Value input) {
>          assert !(input instanceof Variable) : "Creating a copy of a variable via this method is not supported (and potentially a bug): " + input;
>          Variable result = newVariable(input.getValueKind());
>          emitMove(result, input);
>          return result;
>      }
>
> So how am I supposed to copy a variable?
In Graal LIR, there are no destructive updates, so there should never be 
a need to copy a variable (it's basically SSA-form before register 
allocation).

If you want to "modify" a variable (e.g. two-operand assembler 
instruction), you should still create a new variable for the result, and 
put a HINT on the LIR instruction that does the modification. The 
register allocator will try to allocate the input and the result to the 
same register. If it's not possible to do so (e.g. because the input has 
another use later), you have to do the move from the input register to 
the result register manually.

See for example the class AMD64Binary.TwoOp. That's used for 
implementing the x86 two-operand arithmetic instructions that have a 
common register for the result and the first input.
(the "AMD64Move.move(crb, masm, result, x);" call emits a MOV 
instruction if `result` and `x` are different registers, and nothing if 
they are the same)

- Roland
>
> Thanks,
>
> Andrew.