[WebAssembly] Add an import_module function attribute

[sunfish]

Summary

This adds a C/C++ attribute which corresponds to the LLVM IR wasm-import-module attribute. It allows code to specify an explicit import module.

include/clang/Basic/Attr.td

 def TargetRISCV : TargetArch<["riscv32", "riscv64"]>;
 def TargetX86 : TargetArch<["x86"]>;
 def TargetAnyX86 : TargetArch<["x86", "x86_64"]>;
+def TargetWebAssembly : TargetArch<["wasm32", "wasm64"]>;
 def TargetWindows : TargetArch<["x86", "x86_64", "arm", "thumb", "aarch64"]> {
   let OSes = ["Win32"];
 }

   let Subjects = SubjectList<[Function], ErrorDiag, "kernel functions">;
 }
 
+def WebAssemblyImportModule : InheritableAttr,
+                              TargetSpecificAttr<TargetWebAssembly> {
+  let Spellings = [Clang<"import_module">];
+  let Args = [StringArgument<"ImportModuleName">];
+  let Documentation = [WebAssemblyImportModuleDocs];
+  let Subjects = SubjectList<[Function], ErrorDiag>;
+}
+
 def NoSplitStack : InheritableAttr {
   let Spellings = [GCC<"no_split_stack">];
   let Subjects = SubjectList<[Function], ErrorDiag>;

include/clang/Basic/AttrDocs.td

 For more information see
 `gcc documentation <https://gcc.gnu.org/onlinedocs/gcc-7.2.0/gcc/Microsoft-Windows-Variable-Attributes.html>`_
 or `msvc documentation <https://docs.microsoft.com/pl-pl/cpp/cpp/selectany>`_.
-}];
+}]; }
+
+def WebAssemblyImportModuleDocs : Documentation {
+  let Category = DocCatFunction;
+  let Content = [{
+Clang supports the ``__attribute__((import_module(<module_name>)))`` 
+attribute for the WebAssembly target. This attribute may be attached to a
+function declaration, where it modifies how the symbol is to be imported
+within the WebAssembly linking environment.
+
+WebAssembly imports use a two-level namespace scheme, consisting of a module
+name, which typically identifies a module from which to import, and a field
+name, which typically identifies a field from that module to import. By
+default, module names for C/C++ symbols are assigned automatically by the
+linker. This attribute can be used to override the default behavior, and
+reuqest a specific module name be used instead.
+  }];
 }
 
 def ArtificialDocs : Documentation {

lib/CodeGen/TargetInfo.cpp

 
   void setTargetAttributes(const Decl *D, llvm::GlobalValue *GV,
                            CodeGen::CodeGenModule &CGM) const override {
+    TargetCodeGenInfo::setTargetAttributes(D, GV, CGM);
+    if (const auto *FD = dyn_cast_or_null<FunctionDecl>(D)) {
+      if (const auto *Attr = FD->getAttr<WebAssemblyImportModuleAttr>()) {
+        llvm::Function *Fn = cast<llvm::Function>(GV);
+        llvm::AttrBuilder B;
+        B.addAttribute("wasm-import-module", Attr->getImportModuleName());
+        Fn->addAttributes(llvm::AttributeList::FunctionIndex, B);
+      }
+    }
+
     if (auto *FD = dyn_cast_or_null<FunctionDecl>(D)) {
       llvm::Function *Fn = cast<llvm::Function>(GV);
       if (!FD->doesThisDeclarationHaveABody() && !FD->hasPrototype())

lib/Sema/SemaDeclAttr.cpp

   handleSimpleAttribute<AVRSignalAttr>(S, D, AL);
 }
 
+static void handleWebAssemblyImportModuleAttr(Sema &S, Decl *D, const ParsedAttr &AL) {
+  if (!isFunctionOrMethod(D)) {
+    S.Diag(D->getLocation(), diag::warn_attribute_wrong_decl_type)
+        << "'import_module'" << ExpectedFunction;
+    return;
+  }
 
+  auto *FD = cast<FunctionDecl>(D);
+  if (FD->isThisDeclarationADefinition()) {
+    S.Diag(D->getLocation(), diag::err_alias_is_definition) << FD << 0;
+    return;
+  }
+
+  StringRef Str;
+  SourceLocation ArgLoc;
+  if (!S.checkStringLiteralArgumentAttr(AL, 0, Str, &ArgLoc))
+    return;
+
+  FD->addAttr(::new (S.Context) WebAssemblyImportModuleAttr(
+      AL.getRange(), S.Context, Str,
+      AL.getAttributeSpellingListIndex()));
+}
+
 static void handleRISCVInterruptAttr(Sema &S, Decl *D,
                                      const ParsedAttr &AL) {
   // Warn about repeated attributes.

   case ParsedAttr::AT_AVRSignal:
     handleAVRSignalAttr(S, D, AL);
     break;
+  case ParsedAttr::AT_WebAssemblyImportModule:
+    handleWebAssemblyImportModuleAttr(S, D, AL);
+    break;
   case ParsedAttr::AT_IBAction:
     handleSimpleAttribute<IBActionAttr>(S, D, AL);
     break;

test/CodeGen/wasm-import-module.c

+// RUN: %clang_cc1 -triple wasm32-unknown-unknown-wasm -emit-llvm -o - %s | FileCheck %s
+
+void __attribute__((import_module("bar"))) foo(void);
+
+void call(void) {
+  foo();
+}
+
+// CHECK: declare void @foo() [[A:#[0-9]+]]
+
+// CHECK: attributes [[A]] = {{{.*}} "wasm-import-module"="bar" {{.*}}}