IR: pass through [unroll] attribute (shader-slang#284)

The initial lowering was adding an `IRLoopControlDecoration` to the instruction at the head of a loop, but this was getting dropped when the IR gets cloned for a particular entry point.
The fix was simply to add a case for loop-control decorations to `cloneDecoration`.

Author: Tim Foley

source/slang/ir.cpp

@@ -3063,6 +3063,14 @@ namespace Slang
                 }
                 break;
 
+            case kIRDecorationOp_LoopControl:
+                {
+                    auto originalDecoration = (IRLoopControlDecoration*)dd;
+                    auto newDecoration = context->builder->addDecoration<IRLoopControlDecoration>(clonedValue);
+                    newDecoration->mode = originalDecoration->mode;
+                }
+                break;
+
             default:
                 // Don't clone any decorations we don't understand.
                 break;

tests/compute/loop-unroll.slang

@@ -0,0 +1,29 @@
+//TEST(compute):COMPARE_COMPUTE:-xslang -use-ir
+
+//TEST_INPUT:ubuffer(data=[0 1 2 3], stride=4):dxbinding(0),glbinding(0),out
+//TEST_INPUT:ubuffer(data=[1 2 3 0], stride=4):dxbinding(1),glbinding(1)
+
+// Check that we propagate the `[unroll]` attribute
+// through to HLSL output correctly.
+//
+// If we neglect to generate the attribute in the output,
+// it will generate a warning output from fxc, and the
+// test will fail to match the expected output.
+
+RWStructuredBuffer<int> buffers[2];
+
+[numthreads(4, 1, 1)]
+void computeMain(uint3 dispatchThreadID : SV_DispatchThreadID)
+{
+	uint tid = dispatchThreadID.x;
+
+	int val = buffers[1][tid];
+
+	[unroll]
+	for(int ii = 0; ii < 2; ii++)
+	{
+		val = buffers[ii][val];
+	}
+
+	buffers[0][tid] = val;
+}

tests/compute/loop-unroll.slang.expected.txt

@@ -0,0 +1,4 @@
+2
+3
+0
+1