Access chain into packed vectors as arrays.

Cleaner and should be more compatible with storing to packed vector
elements.

Fix CompositeExtract bug with packed vectors on MSL.

reference/opt/shaders-msl/flatten/swizzle.flatten.vert

@@ -39,7 +39,7 @@ vertex main0_out main0(constant UBO& _22 [[buffer(0)]])
     out.oA = _22.A;
     out.oB = float4(_22.B0, _22.B1);
     out.oC = float4(_22.C0, _22.C1) + float4(_22.C1.xy, _22.C1.z, _22.C0);
-    out.oD = float4(_22.D0, _22.D1) + float4(_22.D0.xy, float3(_22.D0).z, _22.D1);
+    out.oD = float4(_22.D0[0], _22.D0[1], _22.D0[2], _22.D1) + float4(_22.D0[0], _22.D0[1], _22.D0[2u], _22.D1);
     out.oE = float4(_22.E0, _22.E1, _22.E2, _22.E3);
     out.oF = float4(_22.F0, _22.F1, _22.F2);
     return out;

reference/opt/shaders-msl/frag/packing-test-3.frag

@@ -22,7 +22,7 @@ struct main0_out
 fragment main0_out main0(constant CB0& _26 [[buffer(0)]], float4 gl_FragCoord [[position]])
 {
     main0_out out = {};
-    out._entryPointOutput = float4(_26.CB0[1].position, _26.CB0[1].radius);
+    out._entryPointOutput = float4(_26.CB0[1].position[0], _26.CB0[1].position[1], _26.CB0[1].position[2], _26.CB0[1].radius);
     return out;
 }
 

reference/shaders-msl/flatten/swizzle.flatten.vert

@@ -39,7 +39,7 @@ vertex main0_out main0(constant UBO& _22 [[buffer(0)]])
     out.oA = _22.A;
     out.oB = float4(_22.B0, _22.B1);
     out.oC = float4(_22.C0, _22.C1) + float4(_22.C1.xy, _22.C1.z, _22.C0);
-    out.oD = float4(_22.D0, _22.D1) + float4(float3(_22.D0).xy, float3(_22.D0).z, _22.D1);
+    out.oD = float4(_22.D0[0], _22.D0[1], _22.D0[2], _22.D1) + float4(float3(_22.D0).xy, _22.D0[2u], _22.D1);
     out.oE = float4(_22.E0, _22.E1, _22.E2, _22.E3);
     out.oF = float4(_22.F0, _22.F1, _22.F2);
     return out;

spirv_glsl.cpp

@@ -4667,7 +4667,7 @@ string CompilerGLSL::access_chain_internal(uint32_t base, const uint32_t *indice
 
 	bool access_chain_is_arrayed = false;
 	bool row_major_matrix_needs_conversion = is_non_native_row_major_matrix(base);
-	bool is_packed = false;
+	bool is_packed = has_decoration(base, DecorationCPacked);
 	bool pending_array_enclose = false;
 	bool dimension_flatten = false;
 
@@ -4826,23 +4826,22 @@ string CompilerGLSL::access_chain_internal(uint32_t base, const uint32_t *indice
 		// Vector -> Scalar
 		else if (type->vecsize > 1)
 		{
-			if (is_packed)
-			{
-				expr = unpack_expression_type(expr, *type);
-				is_packed = false;
-			}
-
-			if (index_is_literal)
+			if (index_is_literal && !is_packed)
 			{
 				expr += ".";
 				expr += index_to_swizzle(index);
 			}
-			else if (ids[index].get_type() == TypeConstant)
+			else if (ids[index].get_type() == TypeConstant && !is_packed)
 			{
 				auto &c = get<SPIRConstant>(index);
 				expr += ".";
 				expr += index_to_swizzle(c.scalar());
 			}
+			else if (index_is_literal)
+			{
+				// For packed vectors, we can only access them as an array, not by swizzle.
+				expr += join("[", index, "]");
+			}
 			else
 			{
 				expr += "[";
@@ -4850,6 +4849,7 @@ string CompilerGLSL::access_chain_internal(uint32_t base, const uint32_t *indice
 				expr += "]";
 			}
 
+			is_packed = false;
 			type_id = type->parent_type;
 			type = &get<SPIRType>(type_id);
 		}
@@ -5993,6 +5993,10 @@ void CompilerGLSL::emit_instruction(const Instruction &instruction)
 		if (composite_type.basetype == SPIRType::Struct || !composite_type.array.empty())
 			allow_base_expression = false;
 
+		// Packed expressions cannot be split up.
+		if (has_decoration(ops[2], DecorationCPacked))
+			allow_base_expression = false;
+
 		// Only apply this optimization if result is scalar.
 		if (allow_base_expression && should_forward(ops[2]) && type.vecsize == 1 && type.columns == 1 && length == 1)
 		{