GLSL: Improve support for GL_ARB_shader_draw_parameters in desktop GLSL.

Opt-in to using the extension to support gl_InstanceIndex.

reference/opt/shaders/asm/vert/extract-transposed-matrix-from-struct.asm.vert

@@ -1,4 +1,7 @@
 #version 450
+#ifdef GL_ARB_shader_draw_parameters
+#extension GL_ARB_shader_draw_parameters : enable
+#endif
 
 struct InstanceData
 {
@@ -12,7 +15,11 @@ layout(binding = 0, std430) readonly buffer gInstanceData
 } gInstanceData_1;
 
 layout(location = 0) in vec3 PosL;
+#ifdef GL_ARB_shader_draw_parameters
+#define SPIRV_Cross_BaseInstance gl_BaseInstanceARB
+#else
 uniform int SPIRV_Cross_BaseInstance;
+#endif
 layout(location = 0) out vec4 _entryPointOutput_Color;
 
 void main()

reference/opt/shaders/asm/vert/uint-vertex-id-instance-id.asm.vert

@@ -1,6 +1,13 @@
 #version 450
+#ifdef GL_ARB_shader_draw_parameters
+#extension GL_ARB_shader_draw_parameters : enable
+#endif
 
+#ifdef GL_ARB_shader_draw_parameters
+#define SPIRV_Cross_BaseInstance gl_BaseInstanceARB
+#else
 uniform int SPIRV_Cross_BaseInstance;
+#endif
 
 void main()
 {

reference/opt/shaders/desktop-only/vert/shader-draw-parameters-450.desktop.vk.vert

@@ -0,0 +1,24 @@
+#version 450
+#ifdef GL_ARB_shader_draw_parameters
+#extension GL_ARB_shader_draw_parameters : enable
+#endif
+
+#ifdef GL_ARB_shader_draw_parameters
+#define SPIRV_Cross_BaseVertex gl_BaseVertexARB
+#else
+uniform int SPIRV_Cross_BaseVertex;
+#endif
+#ifdef GL_ARB_shader_draw_parameters
+#define SPIRV_Cross_BaseInstance gl_BaseInstanceARB
+#else
+uniform int SPIRV_Cross_BaseInstance;
+#endif
+#ifndef GL_ARB_shader_draw_parameters
+#error GL_ARB_shader_draw_parameters is not supported.
+#endif
+
+void main()
+{
+    gl_Position = vec4(float(SPIRV_Cross_BaseVertex), float(SPIRV_Cross_BaseInstance), float(gl_DrawIDARB), 1.0);
+}
+

reference/opt/shaders/desktop-only/vert/shader-draw-parameters.desktop.vk.vert

@@ -0,0 +1,24 @@
+#version 460
+#ifdef GL_ARB_shader_draw_parameters
+#extension GL_ARB_shader_draw_parameters : enable
+#endif
+
+#ifdef GL_ARB_shader_draw_parameters
+#define SPIRV_Cross_BaseVertex gl_BaseVertexARB
+#else
+uniform int SPIRV_Cross_BaseVertex;
+#endif
+#ifdef GL_ARB_shader_draw_parameters
+#define SPIRV_Cross_BaseInstance gl_BaseInstanceARB
+#else
+uniform int SPIRV_Cross_BaseInstance;
+#endif
+#ifndef GL_ARB_shader_draw_parameters
+#error GL_ARB_shader_draw_parameters is not supported.
+#endif
+
+void main()
+{
+    gl_Position = vec4(float(SPIRV_Cross_BaseVertex), float(SPIRV_Cross_BaseInstance), float(gl_DrawIDARB), 1.0);
+}
+

reference/opt/shaders/vert/ground.vert

@@ -1,4 +1,7 @@
 #version 310 es
+#ifdef GL_ARB_shader_draw_parameters
+#extension GL_ARB_shader_draw_parameters : enable
+#endif
 
 struct PatchData
 {
@@ -44,7 +47,11 @@ layout(binding = 1) uniform mediump sampler2D TexLOD;
 layout(binding = 0) uniform mediump sampler2D TexHeightmap;
 
 layout(location = 1) in vec4 LODWeights;
+#ifdef GL_ARB_shader_draw_parameters
+#define SPIRV_Cross_BaseInstance gl_BaseInstanceARB
+#else
 uniform int SPIRV_Cross_BaseInstance;
+#endif
 layout(location = 0) in vec2 Position;
 layout(location = 1) out vec3 EyeVec;
 layout(location = 0) out vec2 TexCoord;

reference/opt/shaders/vert/ocean.vert

@@ -1,4 +1,7 @@
 #version 310 es
+#ifdef GL_ARB_shader_draw_parameters
+#extension GL_ARB_shader_draw_parameters : enable
+#endif
 
 struct PatchData
 {
@@ -45,7 +48,11 @@ layout(binding = 1) uniform mediump sampler2D TexLOD;
 layout(binding = 0) uniform mediump sampler2D TexDisplacement;
 
 layout(location = 1) in vec4 LODWeights;
+#ifdef GL_ARB_shader_draw_parameters
+#define SPIRV_Cross_BaseInstance gl_BaseInstanceARB
+#else
 uniform int SPIRV_Cross_BaseInstance;
+#endif
 layout(location = 0) in vec4 Position;
 layout(location = 0) out vec3 EyeVec;
 layout(location = 1) out vec4 TexCoord;

reference/opt/shaders/vulkan/vert/vulkan-vertex.vk.vert

@@ -1,6 +1,13 @@
 #version 310 es
+#ifdef GL_ARB_shader_draw_parameters
+#extension GL_ARB_shader_draw_parameters : enable
+#endif
 
+#ifdef GL_ARB_shader_draw_parameters
+#define SPIRV_Cross_BaseInstance gl_BaseInstanceARB
+#else
 uniform int SPIRV_Cross_BaseInstance;
+#endif
 
 void main()
 {

reference/shaders/asm/vert/extract-transposed-matrix-from-struct.asm.vert

@@ -1,4 +1,7 @@
 #version 450
+#ifdef GL_ARB_shader_draw_parameters
+#extension GL_ARB_shader_draw_parameters : enable
+#endif
 
 struct V2F
 {
@@ -18,7 +21,11 @@ layout(binding = 0, std430) readonly buffer gInstanceData
 } gInstanceData_1;
 
 layout(location = 0) in vec3 PosL;
+#ifdef GL_ARB_shader_draw_parameters
+#define SPIRV_Cross_BaseInstance gl_BaseInstanceARB
+#else
 uniform int SPIRV_Cross_BaseInstance;
+#endif
 layout(location = 0) out vec4 _entryPointOutput_Color;
 
 V2F _VS(vec3 PosL_1, uint instanceID)

reference/shaders/asm/vert/uint-vertex-id-instance-id.asm.vert

@@ -1,6 +1,13 @@
 #version 450
+#ifdef GL_ARB_shader_draw_parameters
+#extension GL_ARB_shader_draw_parameters : enable
+#endif
 
+#ifdef GL_ARB_shader_draw_parameters
+#define SPIRV_Cross_BaseInstance gl_BaseInstanceARB
+#else
 uniform int SPIRV_Cross_BaseInstance;
+#endif
 
 vec4 _main(uint vid, uint iid)
 {

reference/shaders/desktop-only/vert/shader-draw-parameters-450.desktop.vk.vert

@@ -0,0 +1,24 @@
+#version 450
+#ifdef GL_ARB_shader_draw_parameters
+#extension GL_ARB_shader_draw_parameters : enable
+#endif
+
+#ifdef GL_ARB_shader_draw_parameters
+#define SPIRV_Cross_BaseVertex gl_BaseVertexARB
+#else
+uniform int SPIRV_Cross_BaseVertex;
+#endif
+#ifdef GL_ARB_shader_draw_parameters
+#define SPIRV_Cross_BaseInstance gl_BaseInstanceARB
+#else
+uniform int SPIRV_Cross_BaseInstance;
+#endif
+#ifndef GL_ARB_shader_draw_parameters
+#error GL_ARB_shader_draw_parameters is not supported.
+#endif
+
+void main()
+{
+    gl_Position = vec4(float(SPIRV_Cross_BaseVertex), float(SPIRV_Cross_BaseInstance), float(gl_DrawIDARB), 1.0);
+}
+

reference/shaders/desktop-only/vert/shader-draw-parameters.desktop.vk.vert

@@ -0,0 +1,24 @@
+#version 460
+#ifdef GL_ARB_shader_draw_parameters
+#extension GL_ARB_shader_draw_parameters : enable
+#endif
+
+#ifdef GL_ARB_shader_draw_parameters
+#define SPIRV_Cross_BaseVertex gl_BaseVertexARB
+#else
+uniform int SPIRV_Cross_BaseVertex;
+#endif
+#ifdef GL_ARB_shader_draw_parameters
+#define SPIRV_Cross_BaseInstance gl_BaseInstanceARB
+#else
+uniform int SPIRV_Cross_BaseInstance;
+#endif
+#ifndef GL_ARB_shader_draw_parameters
+#error GL_ARB_shader_draw_parameters is not supported.
+#endif
+
+void main()
+{
+    gl_Position = vec4(float(SPIRV_Cross_BaseVertex), float(SPIRV_Cross_BaseInstance), float(gl_DrawIDARB), 1.0);
+}
+

reference/shaders/vert/ground.vert

@@ -1,4 +1,7 @@
 #version 310 es
+#ifdef GL_ARB_shader_draw_parameters
+#extension GL_ARB_shader_draw_parameters : enable
+#endif
 
 struct PatchData
 {
@@ -44,7 +47,11 @@ layout(binding = 1) uniform mediump sampler2D TexLOD;
 layout(binding = 0) uniform mediump sampler2D TexHeightmap;
 
 layout(location = 1) in vec4 LODWeights;
+#ifdef GL_ARB_shader_draw_parameters
+#define SPIRV_Cross_BaseInstance gl_BaseInstanceARB
+#else
 uniform int SPIRV_Cross_BaseInstance;
+#endif
 layout(location = 0) in vec2 Position;
 layout(location = 1) out vec3 EyeVec;
 layout(location = 0) out vec2 TexCoord;

reference/shaders/vert/ocean.vert

@@ -1,4 +1,7 @@
 #version 310 es
+#ifdef GL_ARB_shader_draw_parameters
+#extension GL_ARB_shader_draw_parameters : enable
+#endif
 
 struct PatchData
 {
@@ -45,7 +48,11 @@ layout(binding = 1) uniform mediump sampler2D TexLOD;
 layout(binding = 0) uniform mediump sampler2D TexDisplacement;
 
 layout(location = 1) in vec4 LODWeights;
+#ifdef GL_ARB_shader_draw_parameters
+#define SPIRV_Cross_BaseInstance gl_BaseInstanceARB
+#else
 uniform int SPIRV_Cross_BaseInstance;
+#endif
 layout(location = 0) in vec4 Position;
 layout(location = 0) out vec3 EyeVec;
 layout(location = 1) out vec4 TexCoord;

reference/shaders/vulkan/vert/vulkan-vertex.vk.vert

@@ -1,6 +1,13 @@
 #version 310 es
+#ifdef GL_ARB_shader_draw_parameters
+#extension GL_ARB_shader_draw_parameters : enable
+#endif
 
+#ifdef GL_ARB_shader_draw_parameters
+#define SPIRV_Cross_BaseInstance gl_BaseInstanceARB
+#else
 uniform int SPIRV_Cross_BaseInstance;
+#endif
 
 void main()
 {

spirv_glsl.cpp

@@ -731,6 +731,13 @@ void CompilerGLSL::emit_header()
 				statement("#endif");
 			}
 		}
+		else if (!options.vulkan_semantics && ext == "GL_ARB_shader_draw_parameters")
+		{
+			// Soft-enable this extension on plain GLSL.
+			statement("#ifdef ", ext);
+			statement("#extension ", ext, " : enable");
+			statement("#endif");
+		}
 		else
 			statement("#extension ", ext, " : require");
 	}
@@ -3082,6 +3089,8 @@ void CompilerGLSL::emit_resources()
 		statement("");
 	emitted = false;
 
+	bool emitted_base_instance = false;
+
 	// Output in/out interfaces.
 	ir.for_each_typed_id<SPIRVariable>([&](uint32_t, SPIRVariable &var) {
 		auto &type = this->get<SPIRType>(var.basetype);
@@ -3104,13 +3113,42 @@ void CompilerGLSL::emit_resources()
 		}
 		else if (is_builtin_variable(var))
 		{
+			auto builtin = BuiltIn(get_decoration(var.self, DecorationBuiltIn));
 			// For gl_InstanceIndex emulation on GLES, the API user needs to
 			// supply this uniform.
-			if (options.vertex.support_nonzero_base_instance &&
-			    ir.meta[var.self].decoration.builtin_type == BuiltInInstanceIndex && !options.vulkan_semantics)
+
+			// The draw parameter extension is soft-enabled on GL with some fallbacks.
+			if (!options.vulkan_semantics)
 			{
-				statement("uniform int SPIRV_Cross_BaseInstance;");
-				emitted = true;
+				if (!emitted_base_instance &&
+				    ((options.vertex.support_nonzero_base_instance && builtin == BuiltInInstanceIndex) ||
+				     (builtin == BuiltInBaseInstance)))
+				{
+					statement("#ifdef GL_ARB_shader_draw_parameters");
+					statement("#define SPIRV_Cross_BaseInstance gl_BaseInstanceARB");
+					statement("#else");
+					// A crude, but simple workaround which should be good enough for non-indirect draws.
+					statement("uniform int SPIRV_Cross_BaseInstance;");
+					statement("#endif");
+					emitted = true;
+					emitted_base_instance = true;
+				}
+				else if (builtin == BuiltInBaseVertex)
+				{
+					statement("#ifdef GL_ARB_shader_draw_parameters");
+					statement("#define SPIRV_Cross_BaseVertex gl_BaseVertexARB");
+					statement("#else");
+					// A crude, but simple workaround which should be good enough for non-indirect draws.
+					statement("uniform int SPIRV_Cross_BaseVertex;");
+					statement("#endif");
+				}
+				else if (builtin == BuiltInDrawIndex)
+				{
+					statement("#ifndef GL_ARB_shader_draw_parameters");
+					// Cannot really be worked around.
+					statement("#error GL_ARB_shader_draw_parameters is not supported.");
+					statement("#endif");
+				}
 			}
 		}
 	});
@@ -6829,7 +6867,14 @@ string CompilerGLSL::builtin_to_glsl(BuiltIn builtin, StorageClass storage)
 		if (options.vulkan_semantics)
 			return "gl_InstanceIndex";
 		else if (options.vertex.support_nonzero_base_instance)
+		{
+			if (!options.vulkan_semantics)
+			{
+				// This is a soft-enable. We will opt-in to using gl_BaseInstanceARB if supported.
+				require_extension_internal("GL_ARB_shader_draw_parameters");
+			}
 			return "(gl_InstanceID + SPIRV_Cross_BaseInstance)"; // ... but not gl_InstanceID.
+		}
 		else
 			return "gl_InstanceID";
 	case BuiltInPrimitiveId:
@@ -6871,33 +6916,69 @@ string CompilerGLSL::builtin_to_glsl(BuiltIn builtin, StorageClass storage)
 		return "gl_LocalInvocationIndex";
 	case BuiltInHelperInvocation:
 		return "gl_HelperInvocation";
+
 	case BuiltInBaseVertex:
 		if (options.es)
 			SPIRV_CROSS_THROW("BaseVertex not supported in ES profile.");
-		if (options.version < 460)
+
+		if (options.vulkan_semantics)
 		{
-			require_extension_internal("GL_ARB_shader_draw_parameters");
-			return "gl_BaseVertexARB";
+			if (options.version < 460)
+			{
+				require_extension_internal("GL_ARB_shader_draw_parameters");
+				return "gl_BaseVertexARB";
+			}
+			return "gl_BaseVertex";
 		}
-		return "gl_BaseVertex";
+		else
+		{
+			// On regular GL, this is soft-enabled and we emit ifdefs in code.
+			require_extension_internal("GL_ARB_shader_draw_parameters");
+			return "SPIRV_Cross_BaseVertex";
+		}
+		break;
+
 	case BuiltInBaseInstance:
 		if (options.es)
 			SPIRV_CROSS_THROW("BaseInstance not supported in ES profile.");
-		if (options.version < 460)
+
+		if (options.vulkan_semantics)
 		{
-			require_extension_internal("GL_ARB_shader_draw_parameters");
-			return "gl_BaseInstanceARB";
+			if (options.version < 460)
+			{
+				require_extension_internal("GL_ARB_shader_draw_parameters");
+				return "gl_BaseInstanceARB";
+			}
+			return "gl_BaseInstance";
 		}
-		return "gl_BaseInstance";
+		else
+		{
+			// On regular GL, this is soft-enabled and we emit ifdefs in code.
+			require_extension_internal("GL_ARB_shader_draw_parameters");
+			return "SPIRV_Cross_BaseInstance";
+		}
+		break;
+
 	case BuiltInDrawIndex:
 		if (options.es)
 			SPIRV_CROSS_THROW("DrawIndex not supported in ES profile.");
-		if (options.version < 460)
+
+		if (options.vulkan_semantics)
 		{
+			if (options.version < 460)
+			{
+				require_extension_internal("GL_ARB_shader_draw_parameters");
+				return "gl_DrawIDARB";
+			}
+			return "gl_DrawID";
+		}
+		else
+		{
+			// On regular GL, this is soft-enabled and we emit ifdefs in code.
 			require_extension_internal("GL_ARB_shader_draw_parameters");
 			return "gl_DrawIDARB";
 		}
-		return "gl_DrawID";
+		break;
 
 	case BuiltInSampleId:
 		if (options.es && options.version < 320)