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+SP #004: Initialization
+=================
+
+This proposal documents the desired behavior of initialization related language semantics, including default constructor, initialization list and variable initialization.
+
+Status
+------
+
+Status: Design Approved, implementation in-progress.
+
+Implemtation: N/A
+
+Author: Yong He
+
+Reviewer: Theresa Foley, Kai Zhang
+
+Background
+----------
+
+Slang has introduced several different syntax around initialization to provide syntactic compatibility with HLSL/C++. As the language evolve, there are many corners where
+the semantics around initialization are not well-defined, and causing confusion or leading to surprising behaviors.
+
+This proposal attempts to provide a design on where we want the language to be in terms of how initialization is handled in all different places.
+
+Related Work
+------------
+
+C++ has many different ways and syntax to initialize an object: through explicit constructor calls, initialization list, or implicitly in a member/variable declaration.
+A variable in C++ can also be in an uninitialized state after its declaration. HLSL inherits most of these behvior from C++ by allowing variables to be uninitialized.
+
+On the other hand, languages like C# and Swift has a set of well defined rules to ensure every variable is initialized after its declaration.
+C++ allows using the initilization list syntax to initialize an object. The semantics of initialization lists depends on whether or not explicit constructors
+are defined on the type.
+
+Proposed Approach
+-----------------
+
+In this section, we document all concepts and rules related to initialization, constructors and initialization lists.
+
+### Default Initializable type
+
+A type is considered "default-initializable" if it provides a constructor that can take 0 arguments, so that it can be constructed with `T()`.
+
+### Variable Initialization
+
+Generally, a variable is considered uninitialized at its declaration site without an explicit value expression.
+For example,
+```csharp
+struct MyType { int x ; }
+
+void foo()
+{
+  MyType t; // t is uninitialized.
+  var t1 : MyType; // same in modern syntax, t1 is uninitialized.
+}
+```
+
+However, the Slang language has been allowing implicit initialization of variables whose types are default initializable types.
+For example,
+```csharp
+struct MyType1 {
+  int x;
+  __init() { x = 0; }
+}
+void foo() {
+  MyType t1; // `t1` is initialized with a call to `__init`.
+}
+```
+
+We would like to move away from this legacy behavior towards a consistent semantics of never implicitly initializing a variable.
+To maintain backward compatibility, we will keep the legacy behavior, but remove the implicit initialization when the variable is defined
+in modern syntax:
+```csharp
+void foo() {
+  var t1: MyType; // `t1` will no longer be initialized.
+}
+```
+We will also remove the default initilaization semantics for traditional syntax in modern Slang modules that comes with an explicit `module` declaration.
+
+Trying to use a variable without initializing it first is an error.
+For backward compatibility, we will introduce a compiler option to turn this error into a warning, but we may deprecate this option in the future.
+
+### Generic Type Parameter
+
+A generic type parameter is not considered default-initializable by-default. As a result, the following code should leave `t` in an uninitialized state:
+```csharp
+void foo<T>()
+{
+    T t; // `t` is uninitialized at declaration.
+}
+```
+
+### Synthesis of constructors for member initialization
+
+If a type already defines any explicit constructors, do not synthesize any constructors for initializer list call. An intializer list expression
+for the type must exactly match one of the explicitly defined constructors.
+
+If the type doesn't provide any explicit constructors, the compiler need to synthesize the constructors for the calls that that the intializer
+lists translate into, so that an initializer list expression can be used to initialize a variable of the type.
+
+For each type, we will synthesize one constructor at the same visibility of the type itself:
+
+The signature for the synthesized initializer for type `V struct T` is:
+```csharp
+V T.__init(member0: typeof(member0) = default(member0), member1 : typeof(member1) = default(member1), ...)
+```
+where `V` is a visibility modifier, `(member0, member1, ... memberN)` is the set of members that has visibility `V`, and `default(member0)`
+is the value defined by the initialization expression in `member0` if it exist, or the default value of `member0`'s type.
+If `member0`'s type is not default initializable and the the member doesn't provide an initial value, then the parameter will not have a default value.
+
+The synthesized constructor will be marked as `[Synthesized]` by the compiler, so the call site can inject additional compatibility logic when calling a synthesized constructor.
+
+The body of the constructor will initialize each member with the value comming from the corresponding constructor argument if such argument exists,
+otherwise the member will be initialized to its default value either defined by the init expr of the member, or the default value of the type if the 
+type is default-initializable. If the member type is not default-initializable and a default value isn't provided on the member, then such the constructor
+synthesis will fail and the constructor will not be added to the type. Failure to synthesis a constructor is not an error, and an error will appear
+if the user is trying to initialize a value of the type in question assuming such a constructor exist.
+
+Note that if every member of a struct contains a default expression, the synthesized `__init` method can be called with 0 arguments, however, this will not cause a variable declaration to be implicitly initialized. Implicit initialization is a backward compatibility feature that only work for user-defined `__init()` methods.
+
+### Single argument constructor call
+
+Call to a constructor with a single argument is always treated as a syntactic sugar of type cast:
+```csharp
+int x = int(1.0f); // is treated as (int) 1.0f;
+MyType y = MyType(arg); // is treated as (MyType)arg;
+MyType x = MyType(y); // equivalent to `x = y`.
+```
+
+The compiler will attempt to resolve all type casts using type coercion rules, if that failed, will fall back to resolve it as a constructor call.
+
+### Initialization List
+
+Slang allows initialization of a variable by assigning it with an initialization list. 
+Generally, Slang will always try to resolve initialization list coercion as if it is an explicit constructor invocation.
+For example, given:
+```csharp
+S obj = {1,2};
+```
+Slang will try to convert the code into:
+```csharp
+S obj = S(1,2);
+```
+
+Following the same logic, an empty initializer list will translate into a default-initialization:
+```csharp
+S obj = {};
+// equivalent to:
+S obj = S();
+```
+
+Note that initializer list of a single argument does not translate into a type cast, unlike the constructor call syntax. Initializing with a single element in the intializer list always translates directly into a constructor call. For example:
+```csharp
+void test()
+{
+  MyType t = {1};
+  // translates to direct constructor call:
+  // MyType t = MyType.__init(1);
+  // which is NOT the same as:
+  // MyType t = MyType(t)
+  // or:
+  // MyType t = (MyType)t;
+}
+```
+
+If the above code passes type check, then it will be used as the way to initialize `obj`.
+
+If the above code does not pass type check, and if there is only one constructor for`MyType` that is synthesized as described in the previous section (and therefore marked as `[Synthesized]`, Slang continues to check if `S` meets the standard of a "legacy C-style struct` type.
+A type is a "legacy C-Style struct" if all of the following conditions are met:
+- It is a user-defined struct type or a basic scalar, vector or matrix type, e.g. `int`, `float4x4`.
+- It does not contain any explicit constructors defined by the user.
+- All its members have the same visibility as the type itself.
+- All its members are legacy C-Style structs or arrays of legacy C-style structs.
+Note that C-Style structs are allowed to have member default values.
+In such case, we perform a legacy "read data" style consumption of the initializer list to synthesize the arguments to call the constructor, so that the following behavior is valid:
+
+```csharp
+struct Inner { int x; int y; };
+struct Outer { Inner i; Inner j; }
+
+// Initializes `o` into `{ Inner{1,2}, Inner{3,0} }`, by synthesizing the
+// arguments to call `Outer.__init(Inner(1,2), Inner(3, 0))`.
+Outer o = {1, 2, 3};
+```
+
+If the type is not a legacy C-Style struct, Slang should produce an error.
+
+### Legacy HLSL syntax to cast from 0
+
+HLSL allows a legacy syntax to cast from literal `0` to a struct type, for example:
+```hlsl
+MyStruct s { int x; }
+void test()
+{
+  MyStruct s = (MyStruct)0;
+}
+```
+
+Slang treats this as equivalent to a empty-initialization:
+```csharp
+MyStruct s = (MyStruct)0;
+// is equivalent to
+MyStruct s = {};
+```
+
+Examples
+-------------------
+```csharp
+
+// Assume everything below is public unless explicitly declared.
+
+struct Empty
+{
+  // compiler synthesizes:
+  // __init();
+}
+void test()
+{
+  Empty s0 = {}; // Works, `s` is considered initialized via ctor call.
+  Empty s1; // `s1` is considered uninitialized.
+}
+
+struct CLike
+{
+  int x; int y;
+  // compiler synthesizes:
+  // __init(int x, int y);
+}
+void test1()
+{
+  CLike c0; // `c0` is uninitialized.
+
+  // case 1: initialized with synthesized ctor call using legacy logic to form arguments,
+  // and `c1` is now `{0,0}`.
+  // (we will refer to this scenario as "initialized with legacy logic" for
+  // the rest of the examples):
+  CLike c1 = {}; 
+
+  // case 2: initialized with legacy initializaer list logic, `c1` is now `{1,0}`:
+  CLike c2 = {1}; 
+
+  // case 3: initilaized with ctor call `CLike(1,2)`, `c3` is now `{1,2}`:
+  CLike c3 = {1, 2};
+}
+
+struct ExplicitCtor
+{
+   int x;
+   int y;
+   __init(int x) {...}
+  // compiler does not synthesize any ctors.
+}
+void test2()
+{
+  ExplicitCtor e0; // `e0` is uninitialized.
+  ExplicitCtor e1 = {1}; // calls `__init`.
+  ExplicitCtor e2 = {1, 2}; // error, no ctor matches initializer list.
+}
+
+struct DefaultMember {
+  int x = 0;
+  int y = 1;
+  // compiler synthesizes:
+  // __init(int x = 0, int y = 1);
+}
+void test3()
+{
+  DefaultMember m; // `m` is uninitialized.
+  DefaultMember m1 = {}; // calls `__init()`, initialized to `{0,1}`.
+  DefaultMember m2 = {1}; // calls `__init(1)`, initialized to `{1,1}`.
+  DefaultMember m3 = {1,2}; // calls `__init(1,2)`, initialized to `{1,2}`.
+}
+
+struct PartialInit {
+  // warning: not all members are initialized.
+  // members should either be all-uninitialized or all-initialized with
+  // default expr.
+  int x;
+  int y = 1;
+  // compiler synthesizes:
+  // __init(int x, int y = 1);
+}
+void test4()
+{
+  PartialInit i; // `i` is not initialized.
+  PartialInit i1 = {2}; // calls `__init`, result is `{2,1}`.
+  PartialInit i2 = {2, 3}; // calls `__init`, result is {2, 3}
+}
+
+struct PartialInit2 {
+  int x = 1;
+  int y; // warning: not all members are initialized.
+  // compiler synthesizes:
+  // __init(int x, int y);
+}
+void test5()
+{
+  PartialInit2 j; // `j` is not initialized.
+  PartialInit2 j1 = {2}; // error, no ctor match.
+  PartialInit2 j2 = {2, 3}; // calls `__init`, result is {2, 3}
+}
+
+public struct Visibility1
+{
+  internal int x;
+  public int y = 0;
+  // the compiler does not synthesize any ctor.
+  // the compiler will try to synthesize:
+  //     public __init(int y);
+  // but then it will find that `x` cannot be initialized.
+  // so this synthesis will fail and no ctor will be added
+  // to the type.
+}
+void test6()
+{
+  Visibility1 t = {0, 0}; // error, no matching ctor
+  Visibility1 t1 = {}; // error, no matching ctor
+  Visibility1 t2 = {1}; // error, no matching ctor
+}
+
+public struct Visibility2
+{
+  // Visibility2 type contains members of different visibility,
+  // which disqualifies it from being considered as C-style struct.
+  // Therefore we will not attempt the legacy fallback logic for
+  // initializer-list syntax.
+  internal int x = 1;
+  public int y = 0;
+  // compiler synthesizes:
+  // public __init(int y = 0);
+}
+void test7()
+{
+  Visibility2 t = {0, 0}; // error, no matching ctor.
+  Visibility2 t1 = {}; // OK, initialized to {1,0} via ctor call.
+  Visibility2 t2 = {1}; // OK, initialized to {1,1} via ctor call.
+}
+
+internal struct Visibility3
+{
+  // Visibility3 type is considered as C-style struct.
+  // Because all members have the same visibility as the type.
+  // Therefore we will attempt the legacy fallback logic for
+  // initializer-list syntax.
+  // Note that c-style structs can still have init exprs on members.
+  internal int x;
+  internal int y = 2;
+  // compiler synthesizes:
+  // internal __init(int x, int y = 2);
+}
+internal void test8()
+{
+  Visibility3 t = {0, 0}; // OK, initialized to {0,0} via ctor call.
+  Visibility3 t1 = {1}; // OK, initialized to {1,2} via ctor call.
+  Visibility3 t2 = {}; // OK, initialized to {0, 2} via legacy logic.
+}
+
+internal struct Visibility4
+{
+  // Visibility4 type is considered as C-style struct.
+  // And we still synthesize a ctor for member initialization.
+  // Because Visiblity4 has no public members, the synthesized
+  // ctor will take 0 arguments.
+  internal int x = 1;
+  internal int y = 2;
+  // compiler synthesizes:
+  // internal __init(int x = 1, int y = 2);
+}
+internal void test9()
+{
+  Visibility4 t = {0, 0}; // OK, initialized to {0,0} via ctor call.
+  Visibility4 t1 = {3}; // OK, initialized to {3,2} via ctor call.
+  Visibility4 t2 = {}; // OK, initialized to {1,2} via ctor call.
+}
+```
+
+### Zero Initialization
+
+The Slang compiler supported an option to force zero-initialization of all local variables.
+This is currently implemented by adding `IDefaultInitializable` conformance to all user
+defined types. With the direction we are heading, we should remove this option in the future.
+For now we can continue to provide this functionality but through an IR rewrite pass instead
+of changing the frontend semantics. 
+
+When users specifies `-zero-initialize`, we should still use the same front-end logic for
+all the checking. After lowering to IR, we should insert a `store` after all `IRVar : T` to
+initialize them to `defaultConstruct(T)`.
+
+
+Q&A
+-----------
+
+### Should global static and groupshared variables be default initialized?
+
+Similar to local variables, all declarations are not default initialized at its declaration site.
+In particular, it is difficult to efficiently initialized global variables safely and correctly in a general way on platforms such as Vulkan,
+so implicit initialization for these variables can come with serious performance consequences.
+
+### Should `out` parameters be default initialized?
+
+Following the same philosphy of not initializing any declarations, `out` parameters are also not default-initialized.
+
+Alternatives Considered
+-----------------------
+
+One important decision point is whether or not Slang should allow variables to be left in uninitialized state after its declaration as it is allowed in C++. In contrast, C# forces everything to be default initialized at its declaration site, which come at the cost of incurring the burden to developers to come up with a way to define the default value for each type.
+Our opinion is we want to allow things as uninitialized, and to have the compiler validation checks to inform
+the developer something is wrong if they try to use a variable in uninitialized state. We believe it is desirable to tell the developer what's wrong instead of using a heavyweight mechanism to ensure everything is initialized at declaration sites, which can have non-trivial performance consequences for GPU programs, especially when the variable is declared in groupshared memory.
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