wasm-ld: Implement function pointer alignment

lld/wasm/Config.h

@@ -93,6 +93,7 @@ struct Config {
   // for shared libraries (since they always added to a dynamic offset at
   // runtime).
   uint64_t tableBase;
+  uint64_t functionPointerAlignment;
   uint64_t zStackSize;
   unsigned ltoPartitions;
   unsigned ltoo;

lld/wasm/Driver.cpp

@@ -636,6 +636,8 @@ static void readConfigs(opt::InputArgList &args) {
   LLVM_DEBUG(errorHandler().verbose = true);
 
   ctx.arg.tableBase = args::getInteger(args, OPT_table_base, 0);
+  ctx.arg.functionPointerAlignment =
+      args::getInteger(args, OPT_function_pointer_alignment, 0);
   ctx.arg.globalBase = args::getInteger(args, OPT_global_base, 0);
   ctx.arg.initialHeap = args::getInteger(args, OPT_initial_heap, 0);
   ctx.arg.initialMemory = args::getInteger(args, OPT_initial_memory, 0);

lld/wasm/Options.td

@@ -251,6 +251,9 @@ def stack_first: FF<"stack-first">,
 def table_base: JJ<"table-base=">,
   HelpText<"Table offset at which to place address taken functions (Defaults to 1)">;
 
+def function_pointer_alignment: JJ<"function-pointer-alignment=">,
+  HelpText<"Align function pointers at a given value (Defaults to 1)">;
+
 defm whole_archive: B<"whole-archive",
     "Force load of all members in a static library",
     "Do not force load of all members in a static library (default)">;

lld/wasm/SyntheticSections.cpp

@@ -568,6 +568,13 @@ void ElemSection::addEntry(FunctionSymbol *sym) {
     return;
   sym->setTableIndex(ctx.arg.tableBase + indirectFunctions.size());
   indirectFunctions.emplace_back(sym);
+
+  // Pad with null function pointers if alignment is requesting.
+  if (ctx.arg.functionPointerAlignment > 1) {
+    while ((ctx.arg.tableBase + indirectFunctions.size()) % ctx.arg.functionPointerAlignment) {
+      indirectFunctions.push_back(nullptr);
+    }
+  }
 }
 
 void ElemSection::writeBody() {
@@ -604,9 +611,11 @@ void ElemSection::writeBody() {
   writeUleb128(os, indirectFunctions.size(), "elem count");
   uint32_t tableIndex = ctx.arg.tableBase;
   for (const FunctionSymbol *sym : indirectFunctions) {
-    assert(sym->getTableIndex() == tableIndex);
+    if (sym != nullptr) {
+      assert(sym->getTableIndex() == tableIndex);
+    }
     (void) tableIndex;
-    writeUleb128(os, sym->getFunctionIndex(), "function index");
+    writeUleb128(os, sym ? sym->getFunctionIndex() : 0, "function index");
     ++tableIndex;
   }
 }