AST: Adopt split caching for RenamedDeclRequest.

In the common case where there is no renamed decl for the attribute, just set a
few bits in inline storage for `AvailableAttr`.

Author: tshortli

include/swift/AST/Attr.h

@@ -147,13 +147,17 @@ class DeclAttribute : public AttributeBase {
       Value : 32
     );
 
-    SWIFT_INLINE_BITFIELD(AvailableAttr, DeclAttribute, 8+8+1+1,
+    SWIFT_INLINE_BITFIELD(AvailableAttr, DeclAttribute, 8+8+1+1+1+1,
       /// A `PlatformKind` value.
       Platform : 8,
 
       /// A `PlatformAgnosticAvailabilityKind` value.
       PlatformAgnostic : 8,
 
+      /// State storage for `RenamedDeclRequest`.
+      HasComputedRenamedDecl : 1,
+      HasRenamedDecl : 1,
+
       /// Whether this attribute was spelled `@_spi_available`.
       IsSPI : 1,
 
@@ -862,6 +866,22 @@ class AvailableAttr : public DeclAttribute {
   static bool classof(const DeclAttribute *DA) {
     return DA->getKind() == DeclAttrKind::Available;
   }
+
+  bool hasCachedRenamedDecl() const {
+    return Bits.AvailableAttr.HasRenamedDecl;
+  }
+
+private:
+  friend class RenamedDeclRequest;
+
+  bool hasComputedRenamedDecl() const {
+    return Bits.AvailableAttr.HasComputedRenamedDecl;
+  }
+
+  void setComputedRenamedDecl(bool hasRenamedDecl) {
+    Bits.AvailableAttr.HasComputedRenamedDecl = true;
+    Bits.AvailableAttr.HasRenamedDecl = hasRenamedDecl;
+  }
 };
 
 /// Indicates that the given declaration is visible to Objective-C.

include/swift/AST/TypeCheckRequests.h

@@ -4085,8 +4085,9 @@ class ConditionalRequirementsRequest
 
 class RenamedDeclRequest
     : public SimpleRequest<RenamedDeclRequest,
-                           ValueDecl *(const ValueDecl *, const AvailableAttr *),
-                           RequestFlags::Cached> {
+                           ValueDecl *(const ValueDecl *,
+                                       const AvailableAttr *),
+                           RequestFlags::Cached | RequestFlags::SplitCached> {
 public:
   using SimpleRequest::SimpleRequest;
 
@@ -4098,6 +4099,8 @@ class RenamedDeclRequest
 
 public:
   bool isCached() const { return true; }
+  std::optional<ValueDecl *> getCachedResult() const;
+  void cacheResult(ValueDecl *value) const;
 };
 
 using AvailableAttrDeclPair = std::pair<const AvailableAttr *, const Decl *>;

lib/AST/Attr.cpp

@@ -2236,6 +2236,8 @@ AvailableAttr::AvailableAttr(
       INIT_VER_TUPLE(Obsoleted), ObsoletedRange(ObsoletedRange) {
   Bits.AvailableAttr.Platform = static_cast<uint8_t>(Platform);
   Bits.AvailableAttr.PlatformAgnostic = static_cast<uint8_t>(PlatformAgnostic);
+  Bits.AvailableAttr.HasComputedRenamedDecl = false;
+  Bits.AvailableAttr.HasRenamedDecl = false;
   Bits.AvailableAttr.IsSPI = IsSPI;
 
   if (IsForEmbedded) {

lib/AST/TypeCheckRequests.cpp

@@ -1622,6 +1622,34 @@ void ResolveRawLayoutTypeRequest::cacheResult(Type value) const {
   }
 }
 
+//----------------------------------------------------------------------------//
+// RenamedDeclRequest computation.
+//----------------------------------------------------------------------------//
+
+std::optional<ValueDecl *> RenamedDeclRequest::getCachedResult() const {
+  auto decl = std::get<0>(getStorage());
+  auto attr = std::get<1>(getStorage());
+
+  if (attr->hasComputedRenamedDecl()) {
+    if (attr->hasCachedRenamedDecl())
+      return decl->getASTContext().evaluator.getCachedNonEmptyOutput(*this);
+
+    return nullptr;
+  }
+
+  return std::nullopt;
+}
+
+void RenamedDeclRequest::cacheResult(ValueDecl *value) const {
+  auto decl = std::get<0>(getStorage());
+  auto attr = const_cast<AvailableAttr *>(std::get<1>(getStorage()));
+
+  attr->setComputedRenamedDecl(value != nullptr);
+  if (value)
+    decl->getASTContext().evaluator.cacheNonEmptyOutput(*this,
+                                                        std::move(value));
+}
+
 //----------------------------------------------------------------------------//
 // TypeCheckSourceFileRequest computation.
 //----------------------------------------------------------------------------//