SILOptimizer: Skip unreachable cases optimization when elements are available.

Cache a bit on `EnumDecl` indicating whether there are any elements that are
unavailable during lowering and then use that bit to decided whether to attempt
optimization for `switch_enum`/`switch_enum_addr` instructions.

Author: tshortli

include/swift/AST/Decl.h

@@ -650,13 +650,16 @@ class alignas(1 << DeclAlignInBits) Decl : public ASTAllocated<Decl> {
                         /// address diversified ptrauth qualified field.
                         IsNonTrivialPtrAuth : 1);
 
-  SWIFT_INLINE_BITFIELD(EnumDecl, NominalTypeDecl, 2+1,
+  SWIFT_INLINE_BITFIELD(EnumDecl, NominalTypeDecl, 2+1+1,
     /// True if the enum has cases and at least one case has associated values.
     HasAssociatedValues : 2,
     /// True if the enum has at least one case that has some availability
     /// attribute.  A single bit because it's lazily computed along with the
     /// HasAssociatedValues bit.
-    HasAnyUnavailableValues : 1
+    HasAnyUnavailableValues : 1,
+    /// True if \c isAvailableDuringLowering() is false for any cases. Lazily
+    /// computed along with HasAssociatedValues.
+    HasAnyUnavailableDuringLoweringValues : 1
   );
 
   SWIFT_INLINE_BITFIELD(ModuleDecl, TypeDecl, 1+1+1+1+1+1+1+1+1+1+1+1+1+1+1+1,
@@ -4294,6 +4297,11 @@ class EnumDecl final : public NominalTypeDecl {
   /// Note that this is false for enums with absolutely no cases.
   bool hasPotentiallyUnavailableCaseValue() const;
 
+  /// True if \c isAvailableDuringLowering() is false for any cases.
+  ///
+  /// Note that this is false for enums with absolutely no cases.
+  bool hasCasesUnavailableDuringLowering() const;
+
   /// True if the enum has cases.
   bool hasCases() const {
     return !getAllElements().empty();

lib/AST/Decl.cpp

@@ -5945,12 +5945,23 @@ EnumCaseDecl *EnumCaseDecl::create(SourceLoc CaseLoc,
 
 bool EnumDecl::hasPotentiallyUnavailableCaseValue() const {
   switch (static_cast<AssociatedValueCheck>(Bits.EnumDecl.HasAssociatedValues)) {
-    case AssociatedValueCheck::Unchecked:
-      // Compute below
-      this->hasOnlyCasesWithoutAssociatedValues();
-      LLVM_FALLTHROUGH;
-    default:
-      return static_cast<bool>(Bits.EnumDecl.HasAnyUnavailableValues);
+  case AssociatedValueCheck::Unchecked:
+    this->hasOnlyCasesWithoutAssociatedValues();
+    LLVM_FALLTHROUGH;
+  default:
+    return static_cast<bool>(Bits.EnumDecl.HasAnyUnavailableValues);
+  }
+}
+
+bool EnumDecl::hasCasesUnavailableDuringLowering() const {
+  switch (
+      static_cast<AssociatedValueCheck>(Bits.EnumDecl.HasAssociatedValues)) {
+  case AssociatedValueCheck::Unchecked:
+    this->hasOnlyCasesWithoutAssociatedValues();
+    LLVM_FALLTHROUGH;
+  default:
+    return static_cast<bool>(
+        Bits.EnumDecl.HasAnyUnavailableDuringLoweringValues);
   }
 }
 
@@ -5970,6 +5981,7 @@ bool EnumDecl::hasOnlyCasesWithoutAssociatedValues() const {
   }
 
   bool hasAnyUnavailableValues = false;
+  bool hasAnyUnavailableDuringLoweringValues = false;
   bool hasAssociatedValues = false;
 
   for (auto elt : getAllElements()) {
@@ -5980,13 +5992,18 @@ bool EnumDecl::hasOnlyCasesWithoutAssociatedValues() const {
       }
     }
 
+    if (!elt->isAvailableDuringLowering())
+      hasAnyUnavailableDuringLoweringValues = true;
+
     if (elt->hasAssociatedValues())
       hasAssociatedValues = true;
   }
 
   EnumDecl *enumDecl = const_cast<EnumDecl *>(this);
 
   enumDecl->Bits.EnumDecl.HasAnyUnavailableValues = hasAnyUnavailableValues;
+  enumDecl->Bits.EnumDecl.HasAnyUnavailableDuringLoweringValues =
+      hasAnyUnavailableDuringLoweringValues;
   enumDecl->Bits.EnumDecl.HasAssociatedValues = static_cast<unsigned>(
       hasAssociatedValues ? AssociatedValueCheck::HasAssociatedValues
                           : AssociatedValueCheck::NoAssociatedValues);

lib/SILOptimizer/Mandatory/DiagnoseUnreachable.cpp

@@ -890,8 +890,12 @@ static bool eliminateSwitchDispatchOnUnavailableElements(
   if (!SWI)
     return false;
 
-  // FIXME: Return early if EnumDecl doesn't have any elements that are
-  // unavailable.
+  EnumDecl *ED = SWI->getOperand(0)->getType().getEnumOrBoundGenericEnum();
+  assert(ED && "operand is not an enum");
+
+  // No need to check the instruction if all elements are available.
+  if (!ED->hasCasesUnavailableDuringLowering())
+    return false;
 
   ASTContext &ctx = BB.getModule().getASTContext();
   SILLocation Loc = SWI->getLoc();