[WebAssembly] Handle undefined data symbols in shared libraries

[sbc100]

Summary

When linking shared libraries, we import a mutable wasm global
to represent the address of each undefined data symbol.

This is a step towards supporting dynamic linking and shared
libraries.

[ruiu]

LGTM

lld/wasm/Symbols.h

263

nit: add a blank line between function definitions. (But if it fits in 80 cols, I believe the coding standard requires it to format in a single line like this:

bool hasGlobalIndex() const { return GlolbalIndex != INVALID_INDEX; }

)

lld/test/wasm/shared.ll

 ; RUN: llc -O0 -filetype=obj %s -o %t.o
 ; RUN: wasm-ld -shared -o %t.wasm %t.o
 ; RUN: obj2yaml %t.wasm | FileCheck %s
 
 target triple = "wasm32-unknown-unknown"
 
 @data = hidden global i32 2, align 4
-@indirect_func = local_unnamed_addr global void ()* @foo, align 4
+@indirect_func = local_unnamed_addr global i32 ()* @foo, align 4
 @indirect_func_external = local_unnamed_addr global void ()* @func_external, align 4
 
-define default void @foo() {
+define default i32 @foo() {
 entry:
   ; To ensure we use __stack_pointer
   %ptr = alloca i32
   %0 = load i32, i32* @data, align 4
   %1 = load i32, i32* @data_external, align 4
-  %2 = load void ()*, void ()** @indirect_func, align 4
-  call void %2()
-  ret void
+  %2 = load i32 ()*, i32 ()** @indirect_func, align 4
+  call i32 %2()
+  ret i32 %1
 }
 
 declare void @func_external()
 
 @data_external = external global i32
 
 
 ; check for dylink section at start

 ; CHECK-NEXT:         Kind:            GLOBAL
 ; CHECK-NEXT:         GlobalType:      I32
 ; CHECK-NEXT:         GlobalMutable:   false
 ; CHECK-NEXT:       - Module:          env
 ; CHECK-NEXT:         Field:           __table_base
 ; CHECK-NEXT:         Kind:            GLOBAL
 ; CHECK-NEXT:         GlobalType:      I32
 ; CHECK-NEXT:         GlobalMutable:   false
+; CHECK-NEXT:       - Module:          env
+; CHECK-NEXT:         Field:           data_external
+; CHECK-NEXT:         Kind:            GLOBAL
+; CHECK-NEXT:         GlobalType:      I32
+; CHECK-NEXT:         GlobalMutable:   true
+; CHECK-NEXT:       - Module:          env
+; CHECK-NEXT:         Field:           func_external
+; CHECK-NEXT:         Kind:            FUNCTION
+; CHECK-NEXT:         SigIndex:        1
 
 ; check for elem segment initialized with __table_base global as offset
 
 ; CHECK:        - Type:            ELEM
 ; CHECK-NEXT:     Segments:
 ; CHECK-NEXT:       - Offset:
 ; CHECK-NEXT:           Opcode:          GLOBAL_GET
 ; CHECK-NEXT:           Index:           2

lld/wasm/Driver.cpp

static void setConfigs(opt::InputArgList &Args) {
   Config->StackFirst = Args.hasArg(OPT_stack_first);
   Config->Trace = Args.hasArg(OPT_trace);
   Config->ThinLTOCacheDir = Args.getLastArgValue(OPT_thinlto_cache_dir);
   Config->ThinLTOCachePolicy = CHECK(
       parseCachePruningPolicy(Args.getLastArgValue(OPT_thinlto_cache_policy)),
       "--thinlto-cache-policy: invalid cache policy");
   Config->ThinLTOJobs = args::getInteger(Args, OPT_thinlto_jobs, -1u);
   errorHandler().Verbose = Args.hasArg(OPT_verbose);
+  LLVM_DEBUG(errorHandler().Verbose = true);
   ThreadsEnabled = Args.hasFlag(OPT_threads, OPT_no_threads, true);
 
   Config->InitialMemory = args::getInteger(Args, OPT_initial_memory, 0);
   Config->GlobalBase = args::getInteger(Args, OPT_global_base, 1024);
   Config->MaxMemory = args::getInteger(Args, OPT_max_memory, 0);
   Config->ZStackSize =
       args::getZOptionValue(Args, OPT_z, "stack-size", WasmPageSize);
 }

lld/wasm/Symbols.h

 
 class UndefinedData : public DataSymbol {
 public:
   UndefinedData(StringRef Name, uint32_t Flags, InputFile *File = nullptr)
       : DataSymbol(Name, UndefinedDataKind, Flags, File) {}
   static bool classof(const Symbol *S) {
     return S->kind() == UndefinedDataKind;
   }
+
+  // Undefined data symbols are imported as wasm globals so also have a global
+  // index.
+  uint32_t getGlobalIndex() const {
+    assert(GlobalIndex != INVALID_INDEX);
+    return GlobalIndex;
+  }
+  void setGlobalIndex(uint32_t Index) {
+    assert(GlobalIndex == INVALID_INDEX);
+    GlobalIndex = Index;
+  }
+  bool hasGlobalIndex() const {
+    return GlobalIndex != INVALID_INDEX;
+  }
+
+protected:
+  uint32_t GlobalIndex = INVALID_INDEX;
 };
 
 class GlobalSymbol : public Symbol {
 public:
   static bool classof(const Symbol *S) {
     return S->kind() == DefinedGlobalKind || S->kind() == UndefinedGlobalKind;
   }
 

lld/wasm/Writer.cpp

for (const Symbol *Sym : ImportedSymbols) {
     } else {
       Import.Field = Sym->getName();
       Import.Module = DefaultModule;
     }
 
     if (auto *FunctionSym = dyn_cast<FunctionSymbol>(Sym)) {
       Import.Kind = WASM_EXTERNAL_FUNCTION;
       Import.SigIndex = lookupType(*FunctionSym->Signature);
+    } else if (auto *DataSym = dyn_cast<UndefinedData>(Sym)) {
+      Import.Kind = WASM_EXTERNAL_GLOBAL;
+      Import.Global = {WASM_TYPE_I32, true};
     } else if (auto *GlobalSym = dyn_cast<GlobalSymbol>(Sym)) {
       Import.Kind = WASM_EXTERNAL_GLOBAL;
       Import.Global = *GlobalSym->getGlobalType();
     } else {
       auto *EventSym = cast<EventSymbol>(Sym);
       Import.Kind = WASM_EXTERNAL_EVENT;
       Import.Event.Attribute = EventSym->getEventType()->Attribute;
       Import.Event.SigIndex = lookupType(*EventSym->Signature);
for (OutputSection *S : OutputSections) {
     FileSize += S->getSize();
   }
 }
 
 void Writer::calculateImports() {
   for (Symbol *Sym : Symtab->getSymbols()) {
     if (!Sym->isUndefined())
       continue;
-    if (isa<DataSymbol>(Sym))
-      continue;
     if (Sym->isWeak() && !Config->Relocatable)
       continue;
     if (!Sym->isLive())
       continue;
     if (!Sym->IsUsedInRegularObj)
       continue;
+    // In relocatable output we don't generate imports for data symbols.
+    // These live only in the symbol table.
+    if (Config->Relocatable && isa<DataSymbol>(Sym))
+      continue;
 
     LLVM_DEBUG(dbgs() << "import: " << Sym->getName() << "\n");
     ImportedSymbols.emplace_back(Sym);
     if (auto *F = dyn_cast<FunctionSymbol>(Sym))
       F->setFunctionIndex(NumImportedFunctions++);
     else if (auto *G = dyn_cast<GlobalSymbol>(Sym))
       G->setGlobalIndex(NumImportedGlobals++);
+    else if (auto *D = dyn_cast<UndefinedData>(Sym))
+      D->setGlobalIndex(NumImportedGlobals++);
     else
       cast<EventSymbol>(Sym)->setEventIndex(NumImportedEvents++);
   }
 }
 
 void Writer::calculateExports() {
   if (Config->Relocatable)
     return;