[lld][WebAssebmly] Preserve custom import attributes with LTO

Undefined symbols in WebAssembly can come with custom `import-module`
and `import-field` attributes.  However when reading symbols from
bitcode object files during LTO those curtom attributes are not
available.

Once we compile the LTO object and read in the symbol table from the
object file we have access to these custom attributes.  In this case,
when undefined symbols are added and a symbol already exists in the
SymbolTable we can't simple return it, we may need to update the
symbol's attributes.

Fixes: PR43211

Differential Revision: https://reviews.llvm.org/D68959

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@375081 91177308-0d34-0410-b5e6-96231b3b80d8

test/wasm/Inputs/import-attributes.ll

@@ -0,0 +1,10 @@
+target triple = "wasm32-unknown-unknown"
+
+define void @call_foo() {
+  call void @foo();
+  ret void
+}
+
+declare void @foo() #0
+
+attributes #0 = { "wasm-import-module"="baz" }

test/wasm/import-attribute-mismatch.ll

@@ -0,0 +1,18 @@
+; RUN: llc -filetype=obj %s -o %t1.o
+; RUN: llc -filetype=obj %S/Inputs/import-attributes.ll -o %t2.o
+; RUN: not wasm-ld --export call_foo --allow-undefined -o %t.wasm %t1.o %t2.o 2>&1 | FileCheck %s
+
+target triple = "wasm32-unknown-unknown-wasm"
+
+define void @_start() {
+  call void @foo();
+  ret void
+}
+
+declare void @foo() #0
+
+attributes #0 = { "wasm-import-module"="bar" }
+
+; CHECK: wasm-ld: error: import module mismatch for symbol: foo
+; CHECK: >>> defined as bar in {{.*}}1.o
+; CHECK: >>> defined as baz in {{.*}}2.o

test/wasm/lto/import-attributes.ll

@@ -0,0 +1,22 @@
+; RUN: llvm-as %s -o %t.o
+; RUN: wasm-ld --allow-undefined -o %t.wasm %t.o
+; RUN: obj2yaml %t.wasm | FileCheck %s
+
+target triple = "wasm32-unknown-unknown-wasm"
+target datalayout = "e-m:e-p:32:32-i64:64-n32:64-S128"
+
+define void @_start() {
+  call void @foo();
+  ret void
+}
+
+declare void @foo() #0
+
+attributes #0 = { "wasm-import-module"="bar" "wasm-import-name"="customfoo" }
+
+; CHECK:        - Type:            IMPORT
+; CHECK-NEXT:     Imports:
+; CHECK-NEXT:       - Module:          bar
+; CHECK-NEXT:         Field:           customfoo
+; CHECK-NEXT:         Kind:            FUNCTION
+; CHECK-NEXT:         SigIndex:        0

wasm/InputFiles.cpp

@@ -527,8 +527,8 @@ static Symbol *createBitcodeSymbol(const std::vector<bool> &keptComdats,
   if (objSym.isUndefined() || excludedByComdat) {
     flags |= WASM_SYMBOL_UNDEFINED;
     if (objSym.isExecutable())
-      return symtab->addUndefinedFunction(name, name, defaultModule, flags, &f,
-                                          nullptr, true);
+      return symtab->addUndefinedFunction(name, "", "", flags, &f, nullptr,
+                                          true);
     return symtab->addUndefinedData(name, flags, &f);
   }
 

wasm/LTO.cpp

@@ -77,9 +77,8 @@ BitcodeCompiler::~BitcodeCompiler() = default;
 
 static void undefine(Symbol *s) {
   if (auto f = dyn_cast<DefinedFunction>(s))
-    replaceSymbol<UndefinedFunction>(f, f->getName(), f->getName(),
-                                     defaultModule, 0,
-                                     f->getFile(), f->signature);
+    replaceSymbol<UndefinedFunction>(f, f->getName(), "", "", 0, f->getFile(),
+                                     f->signature);
   else if (isa<DefinedData>(s))
     replaceSymbol<UndefinedData>(s, s->getName(), 0, s->getFile());
   else

wasm/SymbolTable.cpp

@@ -393,6 +393,36 @@ Symbol *SymbolTable::addDefinedEvent(StringRef name, uint32_t flags,
   return s;
 }
 
+// This function get called when an undefined symbol is added, and there is
+// already an existing one in the symbols table.  In this case we check that
+// custom 'import-module' and 'import-field' symbol attributes agree.
+// With LTO these attributes are not avialable when the bitcode is read and only
+// become available when the LTO object is read.  In this case we silently
+// replace the empty attributes with the valid ones.
+template <typename T>
+static void setImportAttributes(T *existing, StringRef importName,
+                                StringRef importModule, InputFile *file) {
+  if (!importName.empty()) {
+    if (existing->importName.empty())
+      existing->importName = importName;
+    if (existing->importName != importName)
+      error("import name mismatch for symbol: " + toString(*existing) +
+            "\n>>> defined as " + existing->importName + " in " +
+            toString(existing->getFile()) + "\n>>> defined as " + importName +
+            " in " + toString(file));
+  }
+
+  if (!importModule.empty()) {
+    if (existing->importModule.empty())
+      existing->importModule = importModule;
+    if (existing->importModule != importModule)
+      error("import module mismatch for symbol: " + toString(*existing) +
+            "\n>>> defined as " + existing->importModule + " in " +
+            toString(existing->getFile()) + "\n>>> defined as " + importModule +
+            " in " + toString(file));
+  }
+}
+
 Symbol *SymbolTable::addUndefinedFunction(StringRef name, StringRef importName,
                                           StringRef importModule,
                                           uint32_t flags, InputFile *file,
@@ -424,10 +454,10 @@ Symbol *SymbolTable::addUndefinedFunction(StringRef name, StringRef importName,
       reportTypeError(s, file, WASM_SYMBOL_TYPE_FUNCTION);
       return s;
     }
+    auto *existingUndefined = dyn_cast<UndefinedFunction>(existingFunction);
     if (!existingFunction->signature && sig)
       existingFunction->signature = sig;
     if (isCalledDirectly && !signatureMatches(existingFunction, sig)) {
-      auto* existingUndefined = dyn_cast<UndefinedFunction>(existingFunction);
       // If the existing undefined functions is not called direcltly then let
       // this one take precedence.  Otherwise the existing function is either
       // direclty called or defined, in which case we need a function variant.
@@ -436,6 +466,8 @@ Symbol *SymbolTable::addUndefinedFunction(StringRef name, StringRef importName,
       else if (getFunctionVariant(s, sig, file, &s))
         replaceSym();
     }
+    if (existingUndefined)
+      setImportAttributes(existingUndefined, importName, importModule, file);
   }
 
   return s;