Refactor cast representation in AST and SIL, and implement 'is'.

Improve our representations of casts in the AST and SIL so that 'as!' and 'is' (and eventually 'as?') can share almost all of the same type-checking, SILGen, and IRGen code.

In the AST, we now represent 'as!' and 'is' as UnconditionalCheckedCastExpr and IsaExpr, respectively, with the semantic variations of cast (downcast, super-to-archetype, archetype-to-concrete, etc.) discriminated by an enum field. This keeps the user-visible syntactic and type behavior differences of the two forms cleanly separated for AST consumers.

At the SIL level, we transpose the representation so that the different cast semantics get their own instructions and the conditional/unconditional cast behavior is indicated by an enum, making it easy for IRGen to discriminate the different code paths for the different semantics. We also add an 'IsNonnull' instruction to cover the conditional-cast-result-to-boolean conversion common to all the forms of 'is'.

The upshot of all this is that 'x is T' now works for all the new archetype and existential cast forms supported by 'as!'.

Swift SVN r5737

include/swift/SIL/SILInstruction.h

@@ -701,20 +701,11 @@ public:
   UpcastInst(SILLocation Loc, SILValue Operand, SILType Ty)
     : UnaryInstructionBase(Loc, Operand, Ty) {}
 };
-
-/// DowncastInst - Perform an unchecked conversion of a class instance to a
-/// subclass type.
-class DowncastInst
-  : public UnaryInstructionBase<ValueKind::DowncastInst, ConversionInst>
-{
-public:
-  DowncastInst(SILLocation Loc, SILValue Operand, SILType Ty)
-    : UnaryInstructionBase(Loc, Operand, Ty) {}
-};
   
 /// AddressToPointerInst - Convert a SIL address to a Builtin.RawPointer value.
 class AddressToPointerInst
-  : public UnaryInstructionBase<ValueKind::AddressToPointerInst, ConversionInst>
+  : public UnaryInstructionBase<ValueKind::AddressToPointerInst,
+                                ConversionInst>
 {
 public:
   AddressToPointerInst(SILLocation Loc, SILValue Operand, SILType Ty)
@@ -811,29 +802,54 @@ public:
     : UnaryInstructionBase(Loc, Operand, Ty) {}
 };
 
+/// Test that an address or reference type is not null.
+class IsNonnullInst : public UnaryInstructionBase<ValueKind::IsNonnullInst> {
+public:
+  IsNonnullInst(SILLocation Loc, SILValue Operand, SILType BoolTy)
+    : UnaryInstructionBase(Loc, Operand, BoolTy) {}
+};
+
+/// Discriminates checked cast modes.
+enum class CheckedCastMode : unsigned char {
+  // Abort if the cast fails.
+  Unconditional,
+  // Return null if the cast fails.
+  Conditional,
+};
+  
+/// CheckedConversionInst - Abstract base class for checked conversions.
+class CheckedConversionInst : public ConversionInst {
+  CheckedCastMode Mode;
+public:
+  CheckedConversionInst(ValueKind Kind, SILLocation Loc, SILType Ty,
+                        CheckedCastMode Mode)
+    : ConversionInst(Kind, Loc, Ty), Mode(Mode) {}
+  
+  CheckedCastMode getMode() const { return Mode; }
+};
+  
+/// DowncastInst - Perform an unchecked conversion of a class instance to a
+/// subclass type.
+class DowncastInst
+  : public UnaryInstructionBase<ValueKind::DowncastInst, CheckedConversionInst>
+{
+public:
+  DowncastInst(SILLocation Loc, SILValue Operand, SILType Ty,
+               CheckedCastMode Mode)
+    : UnaryInstructionBase(Loc, Operand, Ty, Mode) {}
+};
+
 /// SuperToArchetypeRefInst - Given a value of a class type, initializes a
 /// class archetype with a base class constraint to contain a reference to
 /// the value.
 class SuperToArchetypeRefInst
   : public UnaryInstructionBase<ValueKind::SuperToArchetypeRefInst,
-                                ConversionInst>
+                                CheckedConversionInst>
 {
 public:
-  SuperToArchetypeRefInst(SILLocation Loc, SILValue Operand, SILType Ty)
-    : UnaryInstructionBase(Loc, Operand, Ty) {}
-};
-
-/// IsaInst - Perform a runtime check of a class instance's type.
-class IsaInst : public UnaryInstructionBase<ValueKind::IsaInst> {
-  SILType TestType;
-public:
-  IsaInst(SILLocation Loc,
-          SILValue Operand,
-          SILType TestTy,
-          SILType BoolTy)
-    : UnaryInstructionBase(Loc, Operand, BoolTy), TestType(TestTy) {}
-  
-  SILType getTestType() const { return TestType; }
+  SuperToArchetypeRefInst(SILLocation Loc, SILValue Operand, SILType Ty,
+                          CheckedCastMode Mode)
+    : UnaryInstructionBase(Loc, Operand, Ty, Mode) {}
 };
 
 /// Given the address of an opaque archetype value, dynamically checks the concrete
@@ -841,11 +857,12 @@ public:
 /// successful or crashes if not.
 class DowncastArchetypeAddrInst
   : public UnaryInstructionBase<ValueKind::DowncastArchetypeAddrInst,
-                                ConversionInst>
+                                CheckedConversionInst>
 {
 public:
-  DowncastArchetypeAddrInst(SILLocation Loc, SILValue Operand, SILType Ty)
-    : UnaryInstructionBase(Loc, Operand, Ty) {}
+  DowncastArchetypeAddrInst(SILLocation Loc, SILValue Operand, SILType Ty,
+                            CheckedCastMode Mode)
+    : UnaryInstructionBase(Loc, Operand, Ty, Mode) {}
 };
   
 /// Given a value of class archetype type, dynamically checks the concrete
@@ -853,11 +870,12 @@ public:
 /// successful or crashes if not.
 class DowncastArchetypeRefInst
   : public UnaryInstructionBase<ValueKind::DowncastArchetypeRefInst,
-                                ConversionInst>
+                                CheckedConversionInst>
 {
 public:
-  DowncastArchetypeRefInst(SILLocation Loc, SILValue Operand, SILType Ty)
-    : UnaryInstructionBase(Loc, Operand, Ty) {}
+  DowncastArchetypeRefInst(SILLocation Loc, SILValue Operand, SILType Ty,
+                           CheckedCastMode Mode)
+    : UnaryInstructionBase(Loc, Operand, Ty, Mode) {}
 };
   
 /// Given the address of an opaque existential container, dynamically checks the
@@ -865,11 +883,13 @@ public:
 /// the contained value if successful or crashes if not.
 class ProjectDowncastExistentialAddrInst
   : public UnaryInstructionBase<ValueKind::ProjectDowncastExistentialAddrInst,
-                                ConversionInst>
+                                CheckedConversionInst>
 {
 public:
-  ProjectDowncastExistentialAddrInst(SILLocation Loc, SILValue Operand, SILType Ty)
-    : UnaryInstructionBase(Loc, Operand, Ty) {}
+  ProjectDowncastExistentialAddrInst(SILLocation Loc, SILValue Operand,
+                                     SILType Ty,
+                                     CheckedCastMode Mode)
+    : UnaryInstructionBase(Loc, Operand, Ty, Mode) {}
 };
   
 /// Given a value of class archetype type, dynamically checks the concrete
@@ -877,11 +897,12 @@ public:
 /// successful or crashes if not.
 class DowncastExistentialRefInst
   : public UnaryInstructionBase<ValueKind::DowncastExistentialRefInst,
-                                ConversionInst>
+                                CheckedConversionInst>
 {
 public:
-  DowncastExistentialRefInst(SILLocation Loc, SILValue Operand, SILType Ty)
-    : UnaryInstructionBase(Loc, Operand, Ty) {}
+  DowncastExistentialRefInst(SILLocation Loc, SILValue Operand, SILType Ty,
+                             CheckedCastMode Mode)
+    : UnaryInstructionBase(Loc, Operand, Ty, Mode) {}
 };
   
 /// StructInst - Represents a constructed tuple.