Support conditional copy-by-reference in local memory

[mairooni]

Description

This PR adds support for conditional copy-by-reference in local memory.

Problem description

If a plain copy-by-reference occurs in local memory, graal updates the references automatically. For example, the code below is legal and functionally correct:

public static void intPrivateCopyNoCondition(IntArray a, IntArray b) {
        for (@Parallel int i = 0; i < a.getSize(); i++) {
            int[] arrayA = new int[128];
            int[] arrayB = new int[128];
            for (int j = 0; j < 128; j++) {
                arrayA[j] = j;
                arrayB[j] = 2;
            }

            // copy arrayA to arrayB
            arrayB = arrayA;

            b.set(i, arrayB[i]);
        }
}

However, if the copy takes place depending on a condition, e.g.:

 if ((a.get(i) % 2) == 0 ) {
       // if condition is true, copy arrayA to arrayB
        arrayB = arrayA;
 }

Tornado produces a compiler error. This PR adds the necessary functionality to make this conditional copy-by-reference legal. This patch is for OpenCL and PTX, SPIR-V needs further investigation.

Backend/s tested

Mark the backends affected by this PR.

• [x] OpenCL
• [x] PTX
• [ ] SPIRV

OS tested

Mark the OS where this PR is tested.

• [x] Linux
• [ ] OSx
• [ ] Windows

Did you check on FPGAs?

If it is applicable, check your changes on FPGAs.

• [ ] Yes
• [x] No

How to test the new patch?

Run tornado-test -V --fast uk.ac.manchester.tornado.unittests.arrays.TestArrayCopies

[jjfumero]

[Review in progress] Test are passing for OpenCL and PTX.

[jjfumero]

Throw exception when this type of code is executed for the SPIR-V backend. This can be done within the a compilation phase.

[mairooni]

The comments have been addressed. @jjfumero please have another look.

[jjfumero]

Great. Thanks Mary!

[mairooni]

The refactoring has been applied, thanks @mikepapadim!

[jjfumero]

Great, so we merge this.