[WebAssembly] Add warnings for -shared and -pie

[sunfish]

Summary

The meaning of -shared and -pie are expected to be changed in the
future when Module Linking-style libraries are implemented. Begin
issuing warnings to give people a heads-up that they will be changing.

For compatibility with Emscripten, add a --emscripten-pic flag which
disables these warnings.

[sbc100]

lld/wasm/Driver.cpp

481

Since this is just a warning and linker will actual produce a binary.. how about something like like : "the shared library is not yet stable"? Or some other wording to that effect.

This wording sounds like it would be hard error and not output anything.

lld/wasm/Options.td

205

I'm a little loath to explicitly call out emscripten. We could use some term like "experimental" or "v1" or something similar?

Once argument for this the emscripten ABI is actually quite different to the output of lld. We use a binaryen transforms which transforms the lld output, for example it internalizes/removes all the GOT imports that lld generates.

[sunfish]

• Rename -emscripten-pic to -experimental-pic
• Change the warning text to say "not stable" rather than "not yet implemented"

[sunfish]

lld/wasm/Driver.cpp

481

Changed to say "not yet stable".

lld/wasm/Options.td

205

Changed to experimental.

lld/wasm/Config.h

 // and such fields have the same name as the corresponding options.
 // Most fields are initialized by the driver.
 struct Configuration {
   bool allowUndefined;
   bool checkFeatures;
   bool compressRelocations;
   bool demangle;
   bool disableVerify;
+  bool experimentalPic;
   bool emitRelocs;
   bool exportAll;
   bool exportDynamic;
   bool exportTable;
   bool growableTable;
   bool gcSections;
   bool importMemory;
   bool sharedMemory;

lld/wasm/Driver.cpp

 static void readConfigs(opt::InputArgList &args) {
   config->allowUndefined = args.hasArg(OPT_allow_undefined);
   config->checkFeatures =
       args.hasFlag(OPT_check_features, OPT_no_check_features, true);
   config->compressRelocations = args.hasArg(OPT_compress_relocations);
   config->demangle = args.hasFlag(OPT_demangle, OPT_no_demangle, true);
   config->disableVerify = args.hasArg(OPT_disable_verify);
   config->emitRelocs = args.hasArg(OPT_emit_relocs);
+  config->experimentalPic = args.hasArg(OPT_experimental_pic);
   config->entry = getEntry(args);
   config->exportAll = args.hasArg(OPT_export_all);
   config->exportTable = args.hasArg(OPT_export_table);
   config->growableTable = args.hasArg(OPT_growable_table);
   errorHandler().fatalWarnings =
       args.hasFlag(OPT_fatal_warnings, OPT_no_fatal_warnings, false);
   config->importMemory = args.hasArg(OPT_import_memory);
   config->sharedMemory = args.hasArg(OPT_shared_memory);
if (config->compressRelocations)
       error("-r -and --compress-relocations may not be used together");
     if (args.hasArg(OPT_undefined))
       error("-r -and --undefined may not be used together");
     if (config->pie)
       error("-r and -pie may not be used together");
     if (config->sharedMemory)
       error("-r and --shared-memory may not be used together");
   }
+
+  // To begin to prepare for Module Linking-style shared libraries, start
+  // warning about uses of `-shared` and related flags outside of Experimental
+  // mode, to give anyone using them a heads-up that they will be changing.
+  //
+  // Also, warn about flags which request explicit exports.
+  if (!config->experimentalPic) {
+    // -shared will change meaning when Module Linking is implemented.
+    if (config->shared) {
+      warn("creating shared libraries, with -shared, is not yet stable");
+    }
+
+    // -pie will change meaning when Module Linking is implemented.
+    if (config->pie) {
+      warn("creating PIEs, with -pie, is not yet stable");
+    }
+  }
 }
 
 // Force Sym to be entered in the output. Used for -u or equivalent.
 static Symbol *handleUndefined(StringRef name) {
   Symbol *sym = symtab->find(name);
   if (!sym)
     return nullptr;
 

lld/wasm/Options.td

def lto_partitions: J<"lto-partitions=">,
   HelpText<"Number of LTO codegen partitions">;
 def disable_verify: F<"disable-verify">;
 def save_temps: F<"save-temps">;
 def thinlto_cache_dir: J<"thinlto-cache-dir=">,
   HelpText<"Path to ThinLTO cached object file directory">;
 defm thinlto_cache_policy: Eq<"thinlto-cache-policy", "Pruning policy for the ThinLTO cache">;
 def thinlto_jobs: J<"thinlto-jobs=">,
   HelpText<"Number of ThinLTO jobs. Default to --threads=">;
+
+// Experimental PIC mode.
+def experimental_pic: F<"experimental-pic">,
+  HelpText<"Enable Experimental PIC">;