Add @_used and @_section attributes for global variables and top-level functions (#65901)

* Add @_used and @_section attributes for global variables and top-level functions

This adds:
- @_used attribute that flags as a global variable or a top-level function as
  "do not dead-strip" via llvm.used, roughly the equivalent of
  __attribute__((used)) in C/C++.
- @_section("...") attribute that places a global variable or a top-level
  function into a section with that name, roughly the equivalent of
  __attribute__((section("..."))) in C/C++.

Author: kubamracek

docs/ReferenceGuides/UnderscoredAttributes.md

@@ -832,6 +832,12 @@ Fully bypasses access control, allowing access to private declarations
 in the imported module. The imported module needs to be compiled with
 `-Xfrontend -enable-private-imports` for this to work.
 
+## `@_section("section_name")`
+
+Places a global variable or a top-level function into a section of the object
+file with the given name. It's the equivalent of clang's
+`__attribute__((section))`.
+
 ## `@_semantics("uniquely.recognized.id")`
 
 Allows the optimizer to make use of some key invariants in performance critical
@@ -994,6 +1000,12 @@ for more details.
 
 This `async` function uses the pre-SE-0338 semantics of unsafely inheriting the caller's executor.  This is an underscored feature because the right way of inheriting an executor is to pass in the required executor and switch to it.  Unfortunately, there are functions in the standard library which need to inherit their caller's executor but cannot change their ABI because they were not defined as `@_alwaysEmitIntoClient` in the initial release.
 
+## `@_used`
+
+Marks a global variable or a top-level function as "used externally" even if it
+does not have visible users in the compilation unit. It's the equivalent of
+clang's `__attribute__((used))`.
+
 ## `@_weakLinked`
 
 Allows a declaration to be weakly-referenced, i.e., any references emitted by

include/swift/AST/Attr.h

@@ -502,6 +502,24 @@ class SILGenNameAttr : public DeclAttribute {
   }
 };
 
+/// Defines the @_section attribute.
+class SectionAttr : public DeclAttribute {
+public:
+  SectionAttr(StringRef Name, SourceLoc AtLoc, SourceRange Range, bool Implicit)
+    : DeclAttribute(DAK_Section, AtLoc, Range, Implicit),
+      Name(Name) {}
+
+  SectionAttr(StringRef Name, bool Implicit)
+    : SectionAttr(Name, SourceLoc(), SourceRange(), Implicit) {}
+
+  /// The section name.
+  const StringRef Name;
+
+  static bool classof(const DeclAttribute *DA) {
+    return DA->getKind() == DAK_Section;
+  }
+};
+
 /// Defines the @_cdecl attribute.
 class CDeclAttr : public DeclAttribute {
 public:

include/swift/AST/DiagnosticsSIL.def

@@ -332,6 +332,8 @@ ERROR(performance_unknown_callees,none,
       "called function is not known at compile time and can have unpredictable performance", ())
 ERROR(performance_callee_unavailable,none,
       "called function is not available in this module and can have unpredictable performance", ())
+ERROR(section_attr_on_non_const_global,none,
+      "global variable must be a compile-time constant to use @_section attribute", ())
 NOTE(performance_called_from,none,
       "called from here", ())
 

include/swift/AST/DiagnosticsSema.def

@@ -1702,6 +1702,16 @@ ERROR(cdecl_empty_name,none,
 ERROR(cdecl_throws,none,
       "raising errors from @_cdecl functions is not supported", ())
 
+// @_used and @_section
+ERROR(section_linkage_markers_disabled,none,
+      "attribute requires '-enable-experimental-feature SymbolLinkageMarkers'", ())
+ERROR(used_not_at_top_level,none,
+      "@_used can only be applied to global functions and variables", ())
+ERROR(section_not_at_top_level,none,
+      "@_section can only be applied to global functions and variables", ())
+ERROR(section_empty_name,none,
+      "@_section section name cannot be empty", ())
+
 ERROR(expose_only_non_other_attr,none,
       "@_expose attribute cannot be applied to an '%0' declaration", (StringRef))
 ERROR(expose_inside_unexposed_decl,none,

include/swift/Basic/Features.def

@@ -122,6 +122,9 @@ EXPERIMENTAL_FEATURE(FlowSensitiveConcurrencyCaptures, false)
 EXPERIMENTAL_FEATURE(CodeItemMacros, true)
 EXPERIMENTAL_FEATURE(TupleConformances, false)
 
+// Whether to enable @_used and @_section attributes
+EXPERIMENTAL_FEATURE(SymbolLinkageMarkers, true)
+
 // FIXME: MoveOnlyClasses is not intended to be in production,
 // but our tests currently rely on it, and we want to run those
 // tests in non-asserts builds too.

include/swift/SIL/SILFunction.h

@@ -291,6 +291,9 @@ class SILFunction
   /// The function's remaining set of specialize attributes.
   std::vector<SILSpecializeAttr*> SpecializeAttrSet;
 
+  /// Name of a section if @_section attribute was used, otherwise empty.
+  StringRef Section;
+
   /// Has value if there's a profile for this function
   /// Contains Function Entry Count
   ProfileCounter EntryCount;
@@ -346,6 +349,9 @@ class SILFunction
   /// would indicate.
   unsigned HasCReferences : 1;
 
+  /// Whether attribute @_used was present
+  unsigned MarkedAsUsed : 1;
+
   /// Whether cross-module references to this function should always use weak
   /// linking.
   unsigned IsAlwaysWeakImported : 1;
@@ -1234,6 +1240,14 @@ class SILFunction
     return V && V->getAttrs().hasAttribute<AlwaysEmitIntoClientAttr>();
   }
 
+  /// Return whether this function has attribute @_used on it
+  bool markedAsUsed() const { return MarkedAsUsed; }
+  void setMarkedAsUsed(bool value) { MarkedAsUsed = value; }
+
+  /// Return custom section name if @_section was used, otherwise empty
+  StringRef section() const { return Section; }
+  void setSection(StringRef value) { Section = value; }
+
   /// Returns true if this function belongs to a declaration that returns
   /// an opaque result type with one or more availability conditions that are
   /// allowed to produce a different underlying type at runtime.

include/swift/SIL/SILGlobalVariable.h

@@ -190,6 +190,21 @@ class SILGlobalVariable
     StaticInitializerBlock.eraseAllInstructions(Module);
   }
 
+  /// Returns true if this global variable has `@_used` attribute.
+  bool markedAsUsed() const {
+    auto *V = getDecl();
+    return V && V->getAttrs().hasAttribute<UsedAttr>();
+  }
+
+  /// Returns a SectionAttr if this global variable has `@_section` attribute.
+  SectionAttr *getSectionAttr() const {
+    auto *V = getDecl();
+    if (!V)
+      return nullptr;
+
+    return V->getAttrs().getAttribute<SectionAttr>();
+  }
+
   /// Return whether this variable corresponds to a Clang node.
   bool hasClangNode() const;
 

lib/AST/ASTPrinter.cpp

@@ -3211,6 +3211,17 @@ static bool usesFeatureTupleConformances(Decl *decl) {
   return false;
 }
 
+static bool usesFeatureSymbolLinkageMarkers(Decl *decl) {
+  auto &attrs = decl->getAttrs();
+  return std::any_of(attrs.begin(), attrs.end(), [](auto *attr) {
+    if (isa<UsedAttr>(attr))
+      return true;
+    if (isa<SectionAttr>(attr))
+      return true;
+    return false;
+  });
+}
+
 static bool usesFeatureLayoutPrespecialization(Decl *decl) {
   auto &attrs = decl->getAttrs();
   return std::any_of(attrs.begin(), attrs.end(), [](auto *attr) {

lib/AST/Attr.cpp

@@ -1130,6 +1130,11 @@ bool DeclAttribute::printImpl(ASTPrinter &Printer, const PrintOptions &Options,
     Printer << ")";
     break;
 
+  case DAK_Section:
+    Printer.printAttrName("@_section");
+    Printer << "(\"" << cast<SectionAttr>(this)->Name << "\")";
+    break;
+
   case DAK_ObjC: {
     Printer.printAttrName("@objc");
     llvm::SmallString<32> scratch;
@@ -1602,6 +1607,8 @@ StringRef DeclAttribute::getAttrName() const {
     return "backDeployed";
   case DAK_Expose:
     return "_expose";
+  case DAK_Section:
+    return "_section";
   case DAK_Documentation:
     return "_documentation";
   case DAK_MacroRole:

lib/IRGen/GenDecl.cpp

@@ -2699,6 +2699,12 @@ Address IRGenModule::getAddrOfSILGlobalVariable(SILGlobalVariable *var,
     }
     if (!forDefinition)
       gvar->setComdat(nullptr);
+
+    // Mark as llvm.used if @_used, set section if @_section
+    if (var->markedAsUsed())
+      addUsedGlobal(gvar);
+    if (auto *sectionAttr = var->getSectionAttr())
+      gvar->setSection(sectionAttr->Name);
   }
   if (forDefinition && !gvar->hasInitializer()) {
     if (initVal) {
@@ -3468,6 +3474,12 @@ llvm::Function *IRGenModule::getAddrOfSILFunction(
   fn = createFunction(*this, link, signature, insertBefore,
                       f->getOptimizationMode(), shouldEmitStackProtector(f));
 
+  // Mark as llvm.used if @_used, set section if @_section
+  if (f->markedAsUsed())
+    addUsedGlobal(fn);
+  if (!f->section().empty())
+    fn->setSection(f->section());
+
   // If `hasCReferences` is true, then the function is either marked with
   // @_silgen_name OR @_cdecl.  If it is the latter, it must have a definition
   // associated with it.  The combination of the two allows us to identify the