[wasm] Add function-pointer-alignment function

Author: ethanaobrien

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

@@ -638,6 +638,7 @@ static void readConfigs(opt::InputArgList &args) {
   LLVM_DEBUG(errorHandler().verbose = true);
 
   config->tableBase = args::getInteger(args, OPT_table_base, 0);
+  config->functionPointerAlignment = args::getInteger(args, OPT_function_pointer_alignment, 0);
   config->globalBase = args::getInteger(args, OPT_global_base, 0);
   config->initialHeap = args::getInteger(args, OPT_initial_heap, 0);
   config->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

@@ -567,20 +567,11 @@ void ElemSection::addEntry(FunctionSymbol *sym) {
   if (sym->hasTableIndex() || sym->isStub)
     return;
 
-  uint32_t padding = 0;
-  uint64_t alignment = 1;
-
-  if (indirectFunctions.size() == 0 && padding > 0) {
-    for (uint32_t i=0; i<padding; i++) {
-      indirectFunctions.push_back(nullptr);
-    }
-  }
-
   sym->setTableIndex(config->tableBase + indirectFunctions.size());
   indirectFunctions.emplace_back(sym);
 
-  if (alignment > 1) {
-    for (uint32_t i=0; i<alignment-1; i++) {
+  if (config->functionPointerAlignment > 1) {
+    for (uint32_t i=0; i<config->functionPointerAlignment-1; i++) {
       indirectFunctions.push_back(nullptr);
     }
   }