GLSL: Add option to force flattening IO blocks.

It is not always desirable to use actual blocks.
A prime example in the case where EXT_shader_io_blocks is not supported
on the target implementation.

CMakeLists.txt

@@ -323,7 +323,7 @@ if (SPIRV_CROSS_STATIC)
 endif()
 
 set(spirv-cross-abi-major 0)
-set(spirv-cross-abi-minor 36)
+set(spirv-cross-abi-minor 37)
 set(spirv-cross-abi-patch 0)
 
 if (SPIRV_CROSS_SHARED)

main.cpp

@@ -563,6 +563,7 @@ struct CLIArguments
 	uint32_t msl_additional_fixed_sample_mask = 0xffffffff;
 	bool glsl_emit_push_constant_as_ubo = false;
 	bool glsl_emit_ubo_as_plain_uniforms = false;
+	bool glsl_force_flattened_io_blocks = false;
 	SmallVector<pair<uint32_t, uint32_t>> glsl_ext_framebuffer_fetch;
 	bool vulkan_glsl_disable_ext_samplerless_texture_functions = false;
 	bool emit_line_directives = false;
@@ -674,6 +675,7 @@ static void print_help_glsl()
 	                "\t[--no-420pack-extension]:\n\t\tDo not make use of GL_ARB_shading_language_420pack in older GL targets to support layout(binding).\n"
 	                "\t[--remap-variable-type <variable_name> <new_variable_type>]:\n\t\tRemaps a variable type based on name.\n"
 	                "\t\tPrimary use case is supporting external samplers in ESSL for video rendering on Android where you could remap a texture to a YUV one.\n"
+	                "\t[--glsl-force-flattened-io-blocks]:\n\t\tAlways flatten I/O blocks and structs.\n"
 	);
 	// clang-format on
 }
@@ -1131,6 +1133,7 @@ static string compile_iteration(const CLIArguments &args, std::vector<uint32_t>
 	opts.vertex.support_nonzero_base_instance = args.support_nonzero_baseinstance;
 	opts.emit_push_constant_as_uniform_buffer = args.glsl_emit_push_constant_as_ubo;
 	opts.emit_uniform_buffer_as_plain_uniforms = args.glsl_emit_ubo_as_plain_uniforms;
+	opts.force_flattened_io_blocks = args.glsl_force_flattened_io_blocks;
 	opts.emit_line_directives = args.emit_line_directives;
 	opts.enable_storage_image_qualifier_deduction = args.enable_storage_image_qualifier_deduction;
 	opts.force_zero_initialized_variables = args.force_zero_initialized_variables;
@@ -1319,6 +1322,7 @@ static int main_inner(int argc, char *argv[])
 	cbs.add("--metal", [&args](CLIParser &) { args.msl = true; }); // Legacy compatibility
 	cbs.add("--glsl-emit-push-constant-as-ubo", [&args](CLIParser &) { args.glsl_emit_push_constant_as_ubo = true; });
 	cbs.add("--glsl-emit-ubo-as-plain-uniforms", [&args](CLIParser &) { args.glsl_emit_ubo_as_plain_uniforms = true; });
+	cbs.add("--glsl-force-flattened-io-blocks", [&args](CLIParser &) { args.glsl_force_flattened_io_blocks = true; });
 	cbs.add("--glsl-remap-ext-framebuffer-fetch", [&args](CLIParser &parser) {
 		uint32_t input_index = parser.next_uint();
 		uint32_t color_attachment = parser.next_uint();

reference/shaders-no-opt/vert/io-blocks.force-flattened-io.vert

@@ -0,0 +1,25 @@
+#version 450
+
+struct Foo
+{
+    vec4 bar[2];
+    vec4 baz[2];
+};
+
+out vec4 _14_foo_bar[2];
+out vec4 _14_foo_baz[2];
+out vec4 _14_foo2_bar[2];
+out vec4 _14_foo2_baz[2];
+out vec4 foo3_bar[2];
+out vec4 foo3_baz[2];
+
+void main()
+{
+    _14_foo_bar[0] = vec4(1.0);
+    _14_foo_baz[1] = vec4(2.0);
+    _14_foo2_bar[0] = vec4(3.0);
+    _14_foo2_baz[1] = vec4(4.0);
+    foo3_bar[0] = vec4(5.0);
+    foo3_baz[1] = vec4(6.0);
+}
+

shaders-no-opt/vert/io-blocks.force-flattened-io.vert

@@ -0,0 +1,25 @@
+#version 450
+
+struct Foo
+{
+	vec4 bar[2];
+	vec4 baz[2];
+};
+
+layout(location = 0) out Vertex
+{
+	Foo foo;
+	Foo foo2;
+};
+
+layout(location = 8) out Foo foo3;
+
+void main()
+{
+	foo.bar[0] = vec4(1.0);
+	foo.baz[1] = vec4(2.0);
+	foo2.bar[0] = vec4(3.0);
+	foo2.baz[1] = vec4(4.0);
+	foo3.bar[0] = vec4(5.0);
+	foo3.baz[1] = vec4(6.0);
+}

spirv_cross_c.cpp

@@ -459,6 +459,9 @@ spvc_result spvc_compiler_options_set_uint(spvc_compiler_options options, spvc_c
 	case SPVC_COMPILER_OPTION_GLSL_EMIT_UNIFORM_BUFFER_AS_PLAIN_UNIFORMS:
 		options->glsl.emit_uniform_buffer_as_plain_uniforms = value != 0;
 		break;
+	case SPVC_COMPILER_OPTION_GLSL_FORCE_FLATTENED_IO_BLOCKS:
+		options->glsl.force_flattened_io_blocks = value != 0;
+		break;
 #endif
 
 #if SPIRV_CROSS_C_API_HLSL

spirv_cross_c.h

@@ -33,7 +33,7 @@ extern "C" {
 /* Bumped if ABI or API breaks backwards compatibility. */
 #define SPVC_C_API_VERSION_MAJOR 0
 /* Bumped if APIs or enumerations are added in a backwards compatible way. */
-#define SPVC_C_API_VERSION_MINOR 36
+#define SPVC_C_API_VERSION_MINOR 37
 /* Bumped if internal implementation details change. */
 #define SPVC_C_API_VERSION_PATCH 0
 
@@ -634,6 +634,8 @@ typedef enum spvc_compiler_option
 	SPVC_COMPILER_OPTION_MSL_VERTEX_FOR_TESSELLATION = 64 | SPVC_COMPILER_OPTION_MSL_BIT,
 	SPVC_COMPILER_OPTION_MSL_VERTEX_INDEX_TYPE = 65 | SPVC_COMPILER_OPTION_MSL_BIT,
 
+	SPVC_COMPILER_OPTION_GLSL_FORCE_FLATTENED_IO_BLOCKS = 66 | SPVC_COMPILER_OPTION_GLSL_BIT,
+
 	SPVC_COMPILER_OPTION_INT_MAX = 0x7fffffff
 } spvc_compiler_option;
 

spirv_glsl.cpp

@@ -2232,7 +2232,8 @@ void CompilerGLSL::emit_interface_block(const SPIRVariable &var)
 		// ESSL earlier than 310 and GLSL earlier than 150 did not support
 		// I/O variables which are struct types.
 		// To support this, flatten the struct into separate varyings instead.
-		if ((options.es && options.version < 310) || (!options.es && options.version < 150))
+		if (options.force_flattened_io_blocks || (options.es && options.version < 310) ||
+		    (!options.es && options.version < 150))
 		{
 			// I/O blocks on ES require version 310 with Android Extension Pack extensions, or core version 320.
 			// On desktop, I/O blocks were introduced with geometry shaders in GL 3.2 (GLSL 150).
@@ -2292,7 +2293,8 @@ void CompilerGLSL::emit_interface_block(const SPIRVariable &var)
 		// I/O variables which are struct types.
 		// To support this, flatten the struct into separate varyings instead.
 		if (type.basetype == SPIRType::Struct &&
-		    ((options.es && options.version < 310) || (!options.es && options.version < 150)))
+		    (options.force_flattened_io_blocks || (options.es && options.version < 310) ||
+		     (!options.es && options.version < 150)))
 		{
 			emit_flattened_io_block(var, qual);
 		}

spirv_glsl.hpp

@@ -121,6 +121,10 @@ public:
 		// which would otherwise be uninitialized will now be initialized to 0 instead.
 		bool force_zero_initialized_variables = false;
 
+		// In GLSL, force use of I/O block flattening, similar to
+		// what happens on legacy GLSL targets for blocks and structs.
+		bool force_flattened_io_blocks = false;
+
 		enum Precision
 		{
 			DontCare,

test_shaders.py

@@ -501,6 +501,8 @@ def cross_compile(shader, vulkan, spirv, invalid_spirv, eliminate, is_legacy, fl
         extra_args += ['--glsl-remap-ext-framebuffer-fetch', '3', '3']
     if '.zero-initialize.' in shader:
         extra_args += ['--force-zero-initialized-variables']
+    if '.force-flattened-io.' in shader:
+        extra_args += ['--glsl-force-flattened-io-blocks']
 
     spirv_cross_path = paths.spirv_cross