HLSL: Support BaseVertex/BaseInstance offsets.

Opt-in, since user need to know about a cbuffer.
Conflicts a bit with the GLSL option for base instance,
since that one is enabled by default, but the HLSL one isn't (because
user needs to know about a magic cbuffer, whereas GLSL can only get
default initialized uniform).

main.cpp

@@ -516,6 +516,7 @@ struct CLIArguments
 	bool msl = false;
 	bool hlsl = false;
 	bool hlsl_compat = false;
+	bool hlsl_support_nonzero_base = false;
 	bool vulkan_semantics = false;
 	bool flatten_multidimensional_arrays = false;
 	bool use_420pack_extension = true;
@@ -549,6 +550,7 @@ static void print_help()
 	                "\t[--reflect]\n"
 	                "\t[--shader-model]\n"
 	                "\t[--hlsl-enable-compat]\n"
+	                "\t[--hlsl-support-nonzero-basevertex-baseinstance]\n"
 	                "\t[--separate-shader-objects]\n"
 	                "\t[--pls-in format input-name]\n"
 	                "\t[--pls-out format output-name]\n"
@@ -705,6 +707,7 @@ static int main_inner(int argc, char *argv[])
 	cbs.add("--msl", [&args](CLIParser &) { args.msl = true; });
 	cbs.add("--hlsl", [&args](CLIParser &) { args.hlsl = true; });
 	cbs.add("--hlsl-enable-compat", [&args](CLIParser &) { args.hlsl_compat = true; });
+	cbs.add("--hlsl-support-nonzero-basevertex-baseinstance", [&args](CLIParser &) { args.hlsl_support_nonzero_base = true; });
 	cbs.add("--vulkan-semantics", [&args](CLIParser &) { args.vulkan_semantics = true; });
 	cbs.add("--flatten-multidimensional-arrays", [&args](CLIParser &) { args.flatten_multidimensional_arrays = true; });
 	cbs.add("--no-420pack-extension", [&args](CLIParser &) { args.use_420pack_extension = false; });
@@ -991,6 +994,7 @@ static int main_inner(int argc, char *argv[])
 			build_dummy_sampler = true;
 		}
 
+		hlsl_opts.support_nonzero_base_vertex_base_instance = args.hlsl_support_nonzero_base;
 		hlsl->set_hlsl_options(hlsl_opts);
 	}
 

spirv_glsl.hpp

@@ -105,6 +105,7 @@ public:
 			// Inverts gl_Position.y or equivalent.
 			bool flip_vert_y = false;
 
+			// GLSL only, for HLSL version of this option, see CompilerHLSL.
 			// If true, the backend will assume that InstanceIndex will need to apply
 			// a base instance offset. Set to false if you know you will never use base instance
 			// functionality as it might remove some internal uniforms.

spirv_hlsl.cpp

@@ -902,6 +902,8 @@ void CompilerHLSL::emit_builtin_variables()
 	Bitset builtins = active_input_builtins;
 	builtins.merge_or(active_output_builtins);
 
+	bool need_base_vertex_info = false;
+
 	// Emit global variables for the interface variables which are statically used by the shader.
 	builtins.for_each_bit([&](uint32_t i) {
 		const char *type = nullptr;
@@ -921,8 +923,13 @@ void CompilerHLSL::emit_builtin_variables()
 
 		case BuiltInVertexId:
 		case BuiltInVertexIndex:
-		case BuiltInInstanceId:
 		case BuiltInInstanceIndex:
+			type = "int";
+			if (hlsl_options.support_nonzero_base_vertex_base_instance)
+				need_base_vertex_info = true;
+			break;
+
+		case BuiltInInstanceId:
 		case BuiltInSampleId:
 			type = "int";
 			break;
@@ -998,6 +1005,16 @@ void CompilerHLSL::emit_builtin_variables()
 				statement("static ", type, " ", builtin_to_glsl(builtin, storage), ";");
 		}
 	});
+
+	if (need_base_vertex_info)
+	{
+		statement("cbuffer SPIRV_Cross_VertexInfo");
+		begin_scope();
+		statement("int SPIRV_Cross_BaseVertex;");
+		statement("int SPIRV_Cross_BaseInstance;");
+		end_scope_decl();
+		statement("");
+	}
 }
 
 void CompilerHLSL::emit_composite_constants()
@@ -2243,8 +2260,20 @@ void CompilerHLSL::emit_hlsl_entry_point()
 
 		case BuiltInVertexId:
 		case BuiltInVertexIndex:
-		case BuiltInInstanceId:
 		case BuiltInInstanceIndex:
+			// D3D semantics are uint, but shader wants int.
+			if (hlsl_options.support_nonzero_base_vertex_base_instance)
+			{
+				if (static_cast<BuiltIn>(i) == BuiltInInstanceIndex)
+					statement(builtin, " = int(stage_input.", builtin, ") + SPIRV_Cross_BaseInstance;");
+				else
+					statement(builtin, " = int(stage_input.", builtin, ") + SPIRV_Cross_BaseVertex;");
+			}
+			else
+				statement(builtin, " = int(stage_input.", builtin, ");");
+			break;
+
+		case BuiltInInstanceId:
 			// D3D semantics are uint, but shader wants int.
 			statement(builtin, " = int(stage_input.", builtin, ");");
 			break;

spirv_hlsl.hpp

@@ -54,6 +54,12 @@ public:
 
 		// Allows the PointCoord builtin, returns float2(0.5, 0.5), as PointCoord is not supported in HLSL.
 		bool point_coord_compat = false;
+
+		// If true, the backend will assume that VertexIndex and InstanceIndex will need to apply
+		// a base offset, and you will need to fill in a cbuffer with offsets.
+		// Set to false if you know you will never use base instance or base vertex
+		// functionality as it might remove an internal cbuffer.
+		bool support_nonzero_base_vertex_base_instance = false;
 	};
 
 	explicit CompilerHLSL(std::vector<uint32_t> spirv_)