[wasm] add @_extern(wasm) attribute support

This attribute instructs the compiler that this function declaration
should be "import"ed from host environment. It's equivalent of Clang's
`__attribute__((import_module("module"), import_name("field")))`

Author: kateinoigakukun

docs/ReferenceGuides/UnderscoredAttributes.md

@@ -437,6 +437,18 @@ the export name.
 
 It's the equivalent of clang's `__attribute__((export_name))`.
 
+## `@_extern(<language>)`
+
+Indicates that a particular declaration should be imported
+from the external environment.
+
+### `@_extern(wasm, module: <"moduleName">, name: <"fieldName">)`
+
+Indicates that a particular declaration should be imported
+through WebAssembly's import interface.
+
+It's the equivalent of clang's `__attribute__((import_module("module"), import_name("field")))`.
+
 ## `@_fixed_layout`
 
 Same as `@frozen` but also works for classes.

include/swift/AST/Attr.def

@@ -418,6 +418,9 @@ DECL_ATTR(_section, Section,
 DECL_ATTR(_rawLayout, RawLayout,
   OnStruct | UserInaccessible | ABIBreakingToAdd | ABIBreakingToRemove | APIStableToAdd | APIStableToRemove,
   146)
+DECL_ATTR(_extern, Extern,
+  OnFunc | ABIStableToAdd | ABIBreakingToRemove | APIStableToAdd | APIStableToRemove,
+  147)
 CONTEXTUAL_SIMPLE_DECL_ATTR(final, Final,
   OnClass | OnFunc | OnAccessor | OnVar | OnSubscript | DeclModifier | ABIBreakingToAdd | ABIBreakingToRemove | APIStableToAdd | APIStableToRemove,
   2)

include/swift/AST/Attr.h

@@ -2335,6 +2335,28 @@ class ExposeAttr : public DeclAttribute {
   }
 };
 
+/// Define the `@_extern` attribute, used to import external declarations in
+/// the specified way to interoperate with Swift.
+class ExternAttr : public DeclAttribute {
+public:
+  ExternAttr(StringRef ModuleName, StringRef Name, SourceLoc AtLoc, SourceRange Range, bool Implicit)
+    : DeclAttribute(DAK_Extern, AtLoc, Range, Implicit),
+      ModuleName(ModuleName), Name(Name) {}
+
+  ExternAttr(StringRef ModuleName, StringRef Name, bool Implicit)
+    : ExternAttr(ModuleName, Name, SourceLoc(), SourceRange(), Implicit) {}
+
+  /// The module name to import the named declaration in it
+  const StringRef ModuleName;
+
+  /// The declaration name to import
+  const StringRef Name;
+
+  static bool classof(const DeclAttribute *DA) {
+    return DA->getKind() == DAK_Extern;
+  }
+};
+
 /// The `@_documentation(...)` attribute, used to override a symbol's visibility
 /// in symbol graphs, and/or adding arbitrary metadata to it.
 class DocumentationAttr: public DeclAttribute {

include/swift/AST/DiagnosticsCommon.def

@@ -84,6 +84,9 @@ ERROR(require_const_initializer_for_const,none,
 ERROR(func_decl_without_brace,PointsToFirstBadToken,
       "expected '{' in body of function declaration", ())
 
+ERROR(func_decl_no_body_expected,PointsToFirstBadToken,
+      "unexpected body of function declaration", ())
+
 NOTE(convert_let_to_var,none,
      "change 'let' to 'var' to make it mutable", ())
 

include/swift/AST/DiagnosticsParse.def

@@ -1867,6 +1867,8 @@ ERROR(attr_rawlayout_expected_integer_count,none,
 ERROR(attr_rawlayout_expected_params,none,
       "expected %1 argument after %0 argument in @_rawLayout attribute", (StringRef, StringRef))
 
+ERROR(attr_extern_expected_label,none,
+      "expected %0 argument to @_extern attribute", (StringRef))
 //------------------------------------------------------------------------------
 // MARK: Generics parsing diagnostics
 //------------------------------------------------------------------------------

include/swift/AST/DiagnosticsSema.def

@@ -1866,6 +1866,10 @@ ERROR(section_not_at_top_level,none,
 ERROR(section_empty_name,none,
       "@_section section name cannot be empty", ())
 
+// @_extern
+ERROR(extern_not_at_top_level_func,none,
+      "@_extern attribute can only be applied to global functions", ())
+
 ERROR(expose_wasm_not_at_top_level_func,none,
       "@_expose attribute with 'wasm' can only be applied to global functions", ())
 

include/swift/Parse/Parser.h

@@ -1095,6 +1095,10 @@ class Parser {
   ParserResult<DifferentiableAttr> parseDifferentiableAttribute(SourceLoc AtLoc,
                                                                 SourceLoc Loc);
 
+  /// Parse the @_extern attribute.
+  bool parseExternAttribute(DeclAttributes &Attributes, bool &DiscardAttribute,
+                            StringRef AttrName, SourceLoc AtLoc, SourceLoc Loc);
+
   /// Parse the arguments inside the @differentiable attribute.
   bool parseDifferentiableAttributeArguments(
       DifferentiabilityKind &diffKind,

include/swift/SIL/SILFunction.h

@@ -303,6 +303,9 @@ class SILFunction
   /// empty.
   StringRef WasmExportName;
 
+  /// Name of a Wasm import module and field if @_extern(wasm) attribute
+  llvm::Optional<std::pair<StringRef, StringRef>> WasmImportModuleAndField;
+
   /// Has value if there's a profile for this function
   /// Contains Function Entry Count
   ProfileCounter EntryCount;
@@ -1278,6 +1281,24 @@ class SILFunction
   StringRef wasmExportName() const { return WasmExportName; }
   void setWasmExportName(StringRef value) { WasmExportName = value; }
 
+  /// Return Wasm import module name if @_extern(wasm) was used otherwise empty
+  StringRef wasmImportModuleName() const {
+    if (WasmImportModuleAndField)
+      return WasmImportModuleAndField->first;
+    return StringRef();
+  }
+
+  /// Return Wasm import field name if @_extern(wasm) was used otherwise empty
+  StringRef wasmImportFieldName() const {
+    if (WasmImportModuleAndField)
+      return WasmImportModuleAndField->second;
+    return StringRef();
+  }
+
+  void setWasmImportModuleAndField(StringRef module, StringRef field) {
+    WasmImportModuleAndField = std::make_pair(module, field);
+  }
+
   /// 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.

lib/AST/Attr.cpp

@@ -1138,6 +1138,15 @@ bool DeclAttribute::printImpl(ASTPrinter &Printer, const PrintOptions &Options,
     Printer << ")";
     break;
   }
+
+  case DAK_Extern: {
+    auto *Attr = cast<ExternAttr>(this);
+    Printer.printAttrName("@_extern");
+    // For now, it accepts only "wasm" as its kind.
+    Printer << "(wasm, module: \"" << Attr->ModuleName << "\", name: \"" << Attr->Name << "\")";
+    break;
+  }
+
   case DAK_Section:
     Printer.printAttrName("@_section");
     Printer << "(\"" << cast<SectionAttr>(this)->Name << "\")";
@@ -1708,6 +1717,8 @@ StringRef DeclAttribute::getAttrName() const {
     }
   case DAK_RawLayout:
     return "_rawLayout";
+  case DAK_Extern:
+    return "_extern";
   }
   llvm_unreachable("bad DeclAttrKind");
 }

lib/IRGen/GenDecl.cpp

@@ -3577,11 +3577,18 @@ llvm::Function *IRGenModule::getAddrOfSILFunction(
     addUsedGlobal(fn);
   if (!f->section().empty())
     fn->setSection(f->section());
+
+  llvm::AttrBuilder attrBuilder(getLLVMContext());
   if (!f->wasmExportName().empty()) {
-    llvm::AttrBuilder attrBuilder(getLLVMContext());
     attrBuilder.addAttribute("wasm-export-name", f->wasmExportName());
-    fn->addFnAttrs(attrBuilder);
   }
+  if (!f->wasmImportFieldName().empty()) {
+    attrBuilder.addAttribute("wasm-import-name", f->wasmImportFieldName());
+  }
+  if (!f->wasmImportModuleName().empty()) {
+    attrBuilder.addAttribute("wasm-import-module", f->wasmImportModuleName());
+  }
+  fn->addFnAttrs(attrBuilder);
 
   // 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