[Embedded] Introduce a new linkage model for Embedded Swift

This linkage model gives symbols generated in Embedded Swift
LLVM's linkonce_odr linkage (which allows the linker to merge
duplicates) unless they specifically need to be emitted as strong
symbols in the object. That fits better with creating and using static
libraries with Embedded Swift, and essentially maps to the idea of
"header-only" libraries in C and C++.

Introduce the attribute @alwaysEmitIntoObjectFile that forces code for
the symbol to be generated into the generated object file as a
non-mergeable symbol. Anything it depends on will also be emitted into
the object file, most of which will be via mergeable symbols based on
the rule above. Again, as a C/C++ analogy, this is like putting the
code into a ".c" or ".cpp" file, which will then include any
header-only code it depends on as well.

@alwaysEmitIntoObjectFile also suppresses SIL serialization for that
symbol, completely hiding the definition of the function from clients
of the module. This provides a rudimentary form of implementation
hiding that is common in "Desktop" Swift but was not available in
Embedded Swift.

All of this is behind an experimental feature flag,
EmbeddedLinkageModel, but is intended to subsume both the existing
linkage model for Embedded Swift (which emits everying as a
non-mergeable symbol, so it doesn't work for static libraries)

Author: DougGregor

include/swift/AST/Decl.h

@@ -1067,6 +1067,14 @@ class alignas(1 << DeclAlignInBits) Decl : public ASTAllocated<Decl>, public Swi
   /// behaviors for it and, if it's an extension, its members.
   bool isObjCImplementation() const;
 
+  /// True if this declaration *must* be emitted to an object file, overriding
+  /// other defaults that might delay emission, including Embedded mode and
+  /// Cross-Module Optimization.
+  ///
+  /// As a user, this property can be indicated using
+  /// '@alwaysEmitIntoObjectFile'.
+  bool isEmittedToObjectFile() const;
+
   using AuxiliaryDeclCallback = llvm::function_ref<void(Decl *)>;
 
   /// Iterate over the auxiliary declarations for this declaration,

include/swift/AST/DeclAttr.def

@@ -201,7 +201,11 @@ SIMPLE_DECL_ATTR(requires_stored_property_inits, RequiresStoredPropertyInits,
   ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove | UnreachableInABIAttr,
   27)
 
-// Unused '28'
+SIMPLE_DECL_ATTR(alwaysEmitIntoObjectFile, AlwaysEmitIntoObjectFile,
+  OnAbstractFunction| OnVar | OnSubscript,
+  UserInaccessible | ABIBreakingToAdd | ABIBreakingToRemove | APIStableToAdd | APIStableToRemove | ForbiddenInABIAttr,
+  28)
+
 // Unused '29'
 
 SIMPLE_DECL_ATTR(nonobjc, NonObjC,

include/swift/AST/DiagnosticsSema.def

@@ -1000,6 +1000,12 @@ ERROR(cannot_import_non_embedded_module,none,
       (Identifier))
 ERROR(volatile_is_experimental,none,
       "importing _Volatile module requires '-enable-experimental-feature Volatile'", ())
+ERROR(in_object_file_experimental,none,
+      "'@alwaysEmitIntoObjectFile' attribute requires '-enable-experimental-feature EmbeddedLinkageModel'", ())
+ERROR(in_object_file_generic,none,
+      "'@alwaysEmitIntoObjectFile' cannot be used on a generic function", ())
+ERROR(in_object_file_inlinable,none,
+      "'@alwaysEmitIntoObjectFile' cannot be used with inlinable attribute", ())
 
 ERROR(need_cxx_interop_to_import_module,none,
       "module %0 was built with C++ interoperability enabled, but "

include/swift/Basic/Features.def

@@ -420,6 +420,9 @@ EXPERIMENTAL_FEATURE(RawLayout, true)
 /// Enables the "embedded" swift mode (no runtime).
 EXPERIMENTAL_FEATURE(Embedded, true)
 
+/// Embedded linkage model.
+EXPERIMENTAL_FEATURE(EmbeddedLinkageModel, true)
+
 /// Enables importing the Volatile module
 EXPERIMENTAL_FEATURE(Volatile, true)
 

include/swift/IRGen/Linking.h

@@ -1850,6 +1850,14 @@ class LinkEntity {
   bool isTypeKind() const { return isTypeKind(getKind()); }
 
   bool isAlwaysSharedLinkage() const;
+
+  /// Whether the link entity's definitions must be considered non-unique.
+  ///
+  /// This applies only in the Embedded Swift linkage model, and is used for
+  /// any symbols that have not been explicitly requested to have unique
+  /// definitions (e.g., with @alwaysEmitIntoObjectFile).
+  bool hasNonUniqueDefinition() const;
+
 #undef LINKENTITY_GET_FIELD
 #undef LINKENTITY_SET_FIELD
 

include/swift/SIL/SILDeclRef.h

@@ -401,6 +401,14 @@ struct SILDeclRef {
   /// True if the function has the @backDeployed attribute.
   bool isBackDeployed() const;
 
+  /// True if this function must be emitted to an object file, overriding any
+  /// defaults.
+  bool isEmittedToObjectFile() const;
+
+  /// True if this function should have a non-unique definition based on the
+  /// embedded linkage model.
+  bool hasNonUniqueDefinition() const;
+
   /// Return the expected linkage for a definition of this declaration.
   SILLinkage getDefinitionLinkage() const;
 

lib/AST/ASTDumper.cpp

@@ -5055,6 +5055,7 @@ class PrintAttribute : public AttributeVisitor<PrintAttribute, void, Label>,
   TRIVIAL_ATTR_PRINTER(WeakLinked, weak_linked)
   TRIVIAL_ATTR_PRINTER(Nonexhaustive, nonexhaustive)
   TRIVIAL_ATTR_PRINTER(Concurrent, concurrent)
+  TRIVIAL_ATTR_PRINTER(AlwaysEmitIntoObjectFile, alwaysEmitIntoObjectFile)
 
 #undef TRIVIAL_ATTR_PRINTER
 

lib/AST/Decl.cpp

@@ -2266,6 +2266,13 @@ bool Decl::isObjCImplementation() const {
   return getAttrs().hasAttribute<ObjCImplementationAttr>(/*AllowInvalid=*/true);
 }
 
+bool Decl::isEmittedToObjectFile() const {
+ if (getSemanticAttrs().hasAttribute<AlwaysEmitIntoObjectFileAttr>())
+   return true;
+
+  return false;
+}
+
 PatternBindingDecl::PatternBindingDecl(SourceLoc StaticLoc,
                                        StaticSpellingKind StaticSpelling,
                                        SourceLoc VarLoc,

lib/AST/FeatureSet.cpp

@@ -118,6 +118,7 @@ UNINTERESTING_FEATURE(DoExpressions)
 UNINTERESTING_FEATURE(ImplicitLastExprResults)
 UNINTERESTING_FEATURE(RawLayout)
 UNINTERESTING_FEATURE(Embedded)
+UNINTERESTING_FEATURE(EmbeddedLinkageModel)
 UNINTERESTING_FEATURE(Volatile)
 UNINTERESTING_FEATURE(SuppressedAssociatedTypes)
 UNINTERESTING_FEATURE(StructLetDestructuring)

lib/ASTGen/Sources/ASTGen/DeclAttrs.swift

@@ -261,6 +261,7 @@ extension ASTGenVisitor {
         .ImplicitSelfCapture,
         .InheritsConvenienceInitializers,
         .Inlinable,
+        .AlwaysEmitIntoObjectFile,
         .Isolated,
         .LexicalLifetimes,
         .LLDBDebuggerFunction,