IR: add support for `discard` statement (#261)

- Add definition of `discard` instruction
  - A `discard` is a terminator instruction, just like `returnVoid`

- Lower `DiscardStmt` in AST to a `discard` instruction in the IR

- Emit `discard` instruction as a `discard;` statement when emitting HLSL/GLSL

- Add a test case using the "graphics compute" mode that tests discard. The test writes to one entry in a UAV before doing a conditional (always true at runtime) discard, and then writes to another entry; we expect to see the results of the first write, but not the second.

2 files changed, 93 insertions, 0 deletions

diff --git a/tests/compute/discard-stmt.slang b/tests/compute/discard-stmt.slang
new file mode 100644
index 000000000..18ffc39e2
--- /dev/null
+++ b/tests/compute/discard-stmt.slang
@@ -0,0 +1,91 @@
+//TEST(compute):COMPARE_RENDER_COMPUTE:
+//TEST_INPUT: Texture2D(size=4, content = one) : dxbinding(0),glbinding(0)
+//TEST_INPUT: Sampler : dxbinding(0),glbinding(0)
+//TEST_INPUT: ubuffer(data=[0 0], stride=4):dxbinding(1),glbinding(0),out
+
+
+Texture2D tex;
+SamplerState samp;
+RWStructuredBuffer<float> outputBuffer;
+
+cbuffer Uniforms
+{
+	float4x4 modelViewProjection;
+}
+
+struct AssembledVertex
+{
+	float3	position;
+	float3	color;
+    float2  uv;
+};
+
+struct CoarseVertex
+{
+	float3	color;
+    float2  uv;
+};
+
+struct Fragment
+{
+	float4 color;
+};
+
+
+// Vertex  Shader
+
+struct VertexStageInput
+{
+	AssembledVertex assembledVertex	: A;
+};
+
+struct VertexStageOutput
+{
+	CoarseVertex	coarseVertex	: CoarseVertex;
+	float4			sv_position		: SV_Position;
+};
+
+VertexStageOutput vertexMain(VertexStageInput input)
+{
+	VertexStageOutput output;
+
+	float3 position = input.assembledVertex.position;
+	float3 color	= input.assembledVertex.color;
+
+	output.coarseVertex.color = color;
+	output.sv_position = mul(modelViewProjection, float4(position, 1.0));
+    output.coarseVertex.uv = input.assembledVertex.uv;
+	return output;
+}
+
+// Fragment Shader
+
+struct FragmentStageInput
+{
+	CoarseVertex	coarseVertex	: CoarseVertex;
+};
+
+struct FragmentStageOutput
+{
+	Fragment fragment	: SV_Target;
+};
+
+FragmentStageOutput fragmentMain(FragmentStageInput input)
+{
+	FragmentStageOutput output;
+
+	float3 color = input.coarseVertex.color;
+    float2 uv = input.coarseVertex.uv;
+	output.fragment.color = float4(color, 1.0);
+
+	float4 val = float4(color, 1.0);
+	val = val - 16*tex.Sample(samp, uv);
+
+	outputBuffer[0] = 1;
+
+	if(val.x < 0)
+		discard;
+
+	outputBuffer[1] = 1;
+	return output;
+}
\ No newline at end of file
diff --git a/tests/compute/discard-stmt.slang.expected.txt b/tests/compute/discard-stmt.slang.expected.txt
new file mode 100644
index 000000000..e0e43c4e8
--- /dev/null
+++ b/tests/compute/discard-stmt.slang.expected.txt
@@ -0,0 +1,2 @@
+3F800000
+0