[WebAssembly] Add --export-default/--no-export-default options

[sbc100]

Summary

These option control weather or not symbols marked as visibility
default are exported in the output binary.

By default this is true, but emscripten prefers to control the
exported symbol list explicitly at link time and ignore the
symbol attributes.

[ruiu]

Maybe we should handle it as an error if a user passes both --export-all and --export-default?

[sbc100]

Hmm, I see what you mean. I thing is that --export-default is the default, so we would need to detect weather it was set from the command or not which would complicate the code a little. Also, all is a superset of default I'm not sure its necessary.

[sbc100]

OK to land?

[ruiu]

LGTM

wasm/Options.td

34 ↗

(On Diff #165147)

Add "(default)" at the end of this message as this is the default choice.

lld/trunk/test/wasm/visibility-hidden.ll

 ; RUN: llc -filetype=obj -o %t.o %s
 ; RUN: llc -filetype=obj %S/Inputs/hidden.ll -o %t2.o
 ; RUN: rm -f %t2.a
 ; RUN: llvm-ar rcs %t2.a %t2.o
-; RUN: wasm-ld %t.o %t2.a -o %t.wasm
-; RUN: obj2yaml %t.wasm | FileCheck %s
 
 ; Test that hidden symbols are not exported, whether pulled in from an archive
 ; or directly.
+; RUN: wasm-ld %t.o %t2.a -o %t.wasm
+; RUN: obj2yaml %t.wasm | FileCheck %s
+
+; Test that symbols with default visitiblity are not exported when
+; --no-export-default is passed.
+; RUN: wasm-ld --no-export-default %t.o %t2.a -o %t.nodef.wasm
+; RUN: obj2yaml %t.nodef.wasm | FileCheck %s -check-prefix=NO-DEFAULT
+
 
 target triple = "wasm32-unknown-unknown"
 
 define hidden i32 @objectHidden() {
 entry:
     ret i32 0
 }
 

 ; CHECK-NEXT:         Index:           2
 ; CHECK-NEXT:       - Name:            _start
 ; CHECK-NEXT:         Kind:            FUNCTION
 ; CHECK-NEXT:         Index:           3
 ; CHECK-NEXT:       - Name:            archiveDefault
 ; CHECK-NEXT:         Kind:            FUNCTION
 ; CHECK-NEXT:         Index:           5
 ; CHECK-NEXT:   - Type:
+
+
+; NO-DEFAULT:        - Type:            EXPORT
+; NO-DEFAULT-NEXT:     Exports:
+; NO-DEFAULT-NEXT:       - Name:            memory
+; NO-DEFAULT-NEXT:         Kind:            MEMORY
+; NO-DEFAULT-NEXT:         Index:           0
+; NO-DEFAULT-NEXT:   - Type:

lld/trunk/wasm/Config.h

 namespace wasm {
 
 struct Configuration {
   bool AllowUndefined;
   bool CompressRelocTargets;
   bool Demangle;
   bool DisableVerify;
   bool ExportAll;
+  bool ExportDefault;
   bool ExportTable;
   bool GcSections;
   bool ImportMemory;
   bool ImportTable;
   bool MergeDataSegments;
   bool PrintGcSections;
   bool Relocatable;
   bool SaveTemps;

lld/trunk/wasm/Driver.cpp

void LinkerDriver::link(ArrayRef<const char *> ArgsArr) {
 
   errorHandler().ErrorLimit = args::getInteger(Args, OPT_error_limit, 20);
 
   Config->AllowUndefined = Args.hasArg(OPT_allow_undefined);
   Config->Demangle = Args.hasFlag(OPT_demangle, OPT_no_demangle, true);
   Config->DisableVerify = Args.hasArg(OPT_disable_verify);
   Config->Entry = getEntry(Args, Args.hasArg(OPT_relocatable) ? "" : "_start");
   Config->ExportAll = Args.hasArg(OPT_export_all);
+  Config->ExportDefault = Args.hasFlag(OPT_export_default,
+      OPT_no_export_default, true);
   Config->ExportTable = Args.hasArg(OPT_export_table);
   errorHandler().FatalWarnings =
       Args.hasFlag(OPT_fatal_warnings, OPT_no_fatal_warnings, false);
   Config->ImportMemory = Args.hasArg(OPT_import_memory);
   Config->ImportTable = Args.hasArg(OPT_import_table);
   Config->LTOO = args::getInteger(Args, OPT_lto_O, 2);
   Config->LTOPartitions = args::getInteger(Args, OPT_lto_partitions, 1);
   Config->Optimize = args::getInteger(Args, OPT_O, 0);

lld/trunk/wasm/Options.td

 
 def compress_relocations: F<"compress-relocations">,
   HelpText<"Compress the relocation targets in the code section.">;
 
 defm demangle: B<"demangle",
     "Demangle symbol names",
     "Do not demangle symbol names">;
 
+defm export_default: B<"export-default",
+    "Export symbols marked as 'default' visibility (default)",
+    "Do not export symbols marked as 'default' visibility">;
+
 def entry: S<"entry">, MetaVarName<"<entry>">,
   HelpText<"Name of entry point symbol">;
 
 def error_limit: J<"error-limit=">,
   HelpText<"Maximum number of errors to emit before stopping (0 = no limit)">;
 
 def fatal_warnings: F<"fatal-warnings">,
   HelpText<"Treat warnings as errors">;

lld/trunk/wasm/Symbols.cpp

 
 bool Symbol::isExported() const {
   if (!isDefined() || isLocal())
     return false;
 
   if (ForceExport || Config->ExportAll)
     return true;
 
+  if (!Config->ExportDefault)
+    return false;
+
   return !isHidden();
 }
 
 uint32_t FunctionSymbol::getFunctionIndex() const {
   if (auto *F = dyn_cast<DefinedFunction>(this))
     return F->Function->getFunctionIndex();
   assert(FunctionIndex != INVALID_INDEX);
   return FunctionIndex;