Support the SPV_EXT_demote_to_helper_invocation extension.

This extension provides a new operation which causes a fragment to be
discarded without terminating the fragment shader invocation. The
invocation for the discarded fragment becomes a helper invocation, so
that derivatives will remain defined. The old `HelperInvocation` builtin
becomes undefined when this occurs, so a second new instruction queries
the current helper invocation status.

This is only fully supported for GLSL. HLSL doesn't support the
`IsHelperInvocation` operation and MSL doesn't support the
`DemoteToHelperInvocation` op.

Fixes #1052.

reference/opt/shaders-hlsl/frag/demote-to-helper.frag

@@ -0,0 +1,9 @@
+void frag_main()
+{
+    discard;
+}
+
+void main()
+{
+    frag_main();
+}

reference/opt/shaders-msl/vulkan/frag/demote-to-helper.vk.nocompat.msl21.invalid.frag

@@ -0,0 +1,9 @@
+#include <metal_stdlib>
+#include <simd/simd.h>
+
+using namespace metal;
+
+fragment void main0()
+{
+}
+

reference/opt/shaders/vulkan/frag/demote-to-helper.vk.nocompat.frag.vk

@@ -0,0 +1,9 @@
+#version 450
+#extension GL_EXT_demote_to_helper_invocation : require
+
+void main()
+{
+    demote;
+    bool helper = helperInvocationEXT();
+}
+

reference/shaders-hlsl/frag/demote-to-helper.frag

@@ -0,0 +1,9 @@
+void frag_main()
+{
+    discard;
+}
+
+void main()
+{
+    frag_main();
+}

reference/shaders-msl/vulkan/frag/demote-to-helper.vk.nocompat.msl21.invalid.frag

@@ -0,0 +1,10 @@
+#include <metal_stdlib>
+#include <simd/simd.h>
+
+using namespace metal;
+
+fragment void main0()
+{
+    bool helper = simd_is_helper_thread();
+}
+

reference/shaders/vulkan/frag/demote-to-helper.vk.nocompat.frag.vk

@@ -0,0 +1,9 @@
+#version 450
+#extension GL_EXT_demote_to_helper_invocation : require
+
+void main()
+{
+    demote;
+    bool helper = helperInvocationEXT();
+}
+

shaders-hlsl/frag/demote-to-helper.frag

@@ -0,0 +1,7 @@
+#version 450
+#extension GL_EXT_demote_to_helper_invocation : require
+
+void main()
+{
+	demote;
+}

shaders-msl/vulkan/frag/demote-to-helper.vk.nocompat.msl21.invalid.frag

@@ -0,0 +1,8 @@
+#version 450
+#extension GL_EXT_demote_to_helper_invocation : require
+
+void main()
+{
+	//demote;	// FIXME: Not implemented for MSL
+	bool helper = helperInvocationEXT();
+}

shaders/vulkan/frag/demote-to-helper.vk.nocompat.frag

@@ -0,0 +1,8 @@
+#version 450
+#extension GL_EXT_demote_to_helper_invocation : require
+
+void main()
+{
+	demote;
+	bool helper = helperInvocationEXT();
+}

spirv_glsl.cpp

@@ -9778,6 +9778,20 @@ void CompilerGLSL::emit_instruction(const Instruction &instruction)
 	case OpNoLine:
 		break;
 
+	case OpDemoteToHelperInvocationEXT:
+		if (!options.vulkan_semantics)
+			SPIRV_CROSS_THROW("GL_EXT_demote_to_helper_invocation is only supported in Vulkan GLSL.");
+		require_extension_internal("GL_EXT_demote_to_helper_invocation");
+		statement(backend.demote_literal, ";");
+		break;
+
+	case OpIsHelperInvocationEXT:
+		if (!options.vulkan_semantics)
+			SPIRV_CROSS_THROW("GL_EXT_demote_to_helper_invocation is only supported in Vulkan GLSL.");
+		require_extension_internal("GL_EXT_demote_to_helper_invocation");
+		emit_op(ops[0], ops[1], "helperInvocationEXT()", true);
+		break;
+
 	default:
 		statement("// unimplemented op ", instruction.op);
 		break;

spirv_glsl.hpp

@@ -369,6 +369,7 @@ protected:
 	struct BackendVariations
 	{
 		std::string discard_literal = "discard";
+		std::string demote_literal = "demote";
 		std::string null_pointer_literal = "";
 		bool float_literal_suffix = false;
 		bool double_literal_suffix = true;

spirv_hlsl.cpp

@@ -4663,6 +4663,9 @@ void CompilerHLSL::emit_instruction(const Instruction &instruction)
 		break;
 	}
 
+	case OpIsHelperInvocationEXT:
+		SPIRV_CROSS_THROW("helperInvocationEXT() is not supported in HLSL.");
+
 	default:
 		CompilerGLSL::emit_instruction(instruction);
 		break;
@@ -4819,6 +4822,7 @@ string CompilerHLSL::compile()
 	backend.uint16_t_literal_suffix = "u";
 	backend.basic_int_type = "int";
 	backend.basic_uint_type = "uint";
+	backend.demote_literal = "discard";
 	backend.boolean_mix_function = "";
 	backend.swizzle_is_function = false;
 	backend.shared_is_implied = true;

spirv_msl.cpp

@@ -757,6 +757,7 @@ string CompilerMSL::compile()
 	backend.basic_int16_type = "short";
 	backend.basic_uint16_type = "ushort";
 	backend.discard_literal = "discard_fragment()";
+	backend.demote_literal = "unsupported-demote";
 	backend.boolean_mix_function = "select";
 	backend.swizzle_is_function = false;
 	backend.shared_is_implied = false;
@@ -4467,6 +4468,14 @@ void CompilerMSL::emit_instruction(const Instruction &instruction)
 		break;
 	}
 
+	case OpIsHelperInvocationEXT:
+		if (msl_options.is_ios())
+			SPIRV_CROSS_THROW("simd_is_helper_thread() is only supported on macOS.");
+		else if (msl_options.is_macos() && !msl_options.supports_msl_version(2, 1))
+			SPIRV_CROSS_THROW("simd_is_helper_thread() requires version 2.1 on macOS.");
+		emit_op(ops[0], ops[1], "simd_is_helper_thread()", true);
+		break;
+
 	default:
 		CompilerGLSL::emit_instruction(instruction);
 		break;