[WebAssembly] Setting export_name implies no_dead_strip

[sbc100]

Summary

This change updates the clang front end to add symbols to llvm.used
when they have explicit export_name attribute.

In also causes the .export_name assempler directive to imply the
MCSA_NoDeadStrip symbol attribute.

[sbc100]

rebase

[sunfish]

llvm/lib/Target/WebAssembly/AsmParser/WebAssemblyAsmParser.cpp

725

It feels like this is a little inconsistent. If export_name doesn't automatically imply used in LLVM IR or .o files, why does it do so in .s files?

[sbc100]

llvm/lib/Target/WebAssembly/AsmParser/WebAssemblyAsmParser.cpp

725

OK, reverted that part.

[sbc100]

• revert part

clang/lib/Sema/SemaDeclAttr.cpp

if (FD->isThisDeclarationADefinition()) {
     return;
   }
 
   StringRef Str;
   SourceLocation ArgLoc;
   if (!S.checkStringLiteralArgumentAttr(AL, 0, Str, &ArgLoc))
     return;
 
-  FD->addAttr(::new (S.Context)
-                  WebAssemblyExportNameAttr(S.Context, AL, Str));
+  D->addAttr(::new (S.Context) WebAssemblyExportNameAttr(S.Context, AL, Str));
+  D->addAttr(UsedAttr::CreateImplicit(S.Context));
 }
 
 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;
   }

clang/test/CodeGen/wasm-export-name.c

 // RUN: %clang_cc1 -triple wasm32-unknown-unknown-wasm -emit-llvm -o - %s | FileCheck %s
 
 int __attribute__((export_name("bar"))) foo(void);
 
 int foo(void) {
   return 43;
 }
 
+// CHECK: @llvm.used = appending global [1 x i8*] [i8* bitcast (i32 ()* @foo to i8*)]
+
 // CHECK: define i32 @foo() [[A:#[0-9]+]]
 
 // CHECK: attributes [[A]] = {{{.*}} "wasm-export-name"="bar" {{.*}}}

llvm/lib/Target/WebAssembly/AsmParser/WebAssemblyAsmParser.cpp

if (DirectiveID.getString() == ".export_name") {
       if (SymName.empty())
         return true;
       if (expect(AsmToken::Comma, ","))
         return true;
       auto ExportName = expectIdent();
       auto WasmSym = cast<MCSymbolWasm>(Ctx.getOrCreateSymbol(SymName));
       WasmSym->setExportName(ExportName);
       TOut.emitExportName(WasmSym, ExportName);
+      Out.EmitSymbolAttribute(WasmSym, MCSA_NoDeadStrip);
     }
 
     if (DirectiveID.getString() == ".import_module") {
       auto SymName = expectIdent();
       if (SymName.empty())
         return true;
       if (expect(AsmToken::Comma, ","))
         return true;

llvm/test/MC/WebAssembly/export-name.s

 # CHECK-OBJ-NEXT:         Index:           0
 
 # CHECK-OBJ:          Name:            linking
 # CHECK-OBJ-NEXT:     Version:         2
 # CHECK-OBJ-NEXT:     SymbolTable:
 # CHECK-OBJ-NEXT:       - Index:           0
 # CHECK-OBJ-NEXT:         Kind:            FUNCTION
 # CHECK-OBJ-NEXT:         Name:            foo
-# CHECK-OBJ-NEXT:         Flags:           [ EXPORTED ]
+# CHECK-OBJ-NEXT:         Flags:           [ EXPORTED, NO_STRIP ]
 # CHECK-OBJ-NEXT:         Function:        0