wasm-ld: Implement function pointer alignment

lld/test/wasm/function-pointer-alignmnet.s

@@ -0,0 +1,61 @@
+# RUN: llvm-mc -filetype=obj -triple=wasm32-unknown-unknown %s -o %t.o
+
+# RUN: wasm-ld -o %t.wasm %t.o
+# RUN: obj2yaml %t.wasm | FileCheck %s  -check-prefix=CHECK-DEFAULT
+
+# RUN: wasm-ld --function-pointer-alignment=2 -o %t.wasm %t.o
+# RUN: obj2yaml %t.wasm | FileCheck %s  -check-prefix=CHECK-2
+
+# RUN: wasm-ld --function-pointer-alignment=3 -o %t.wasm %t.o
+# RUN: obj2yaml %t.wasm | FileCheck %s  -check-prefix=CHECK-3
+
+.functype func1 () -> ()
+.functype func2 () -> ()
+.functype func3 () -> ()
+
+.globl _start
+_start:
+  .functype _start () -> ()
+  i32.const func1
+  drop
+  i32.const func2
+  drop
+  i32.const func3
+  drop
+  end_function
+
+.globl func1
+func1:
+  .functype func1 () -> ()
+  end_function
+
+.globl func2
+func2:
+  .functype func2 () -> ()
+  end_function
+
+.globl func3
+func3:
+  .functype func3 () -> ()
+  end_function
+
+# CHECK-DEFAULT:        - Type:            ELEM
+# CHECK-DEFAULT-NEXT:     Segments:
+# CHECK-DEFAULT-NEXT:       - Offset:
+# CHECK-DEFAULT-NEXT:           Opcode:          I32_CONST
+# CHECK-DEFAULT-NEXT:           Value:           1
+# CHECK-DEFAULT-NEXT:         Functions:       [ 1, 2, 3 ]
+
+# CHECK-2:        - Type:            ELEM
+# CHECK-2-NEXT:     Segments:
+# CHECK-2-NEXT:       - Offset:
+# CHECK-2-NEXT:           Opcode:          I32_CONST
+# CHECK-2-NEXT:           Value:           1
+# CHECK-2-NEXT:         Functions:       [ 1, 2, 0, 3, 0 ]
+
+# CHECK-3:        - Type:            ELEM
+# CHECK-3-NEXT:     Segments:
+# CHECK-3-NEXT:       - Offset:
+# CHECK-3-NEXT:           Opcode:          I32_CONST
+# CHECK-3-NEXT:           Value:           1
+# CHECK-3-NEXT:         Functions:       [ 1, 0, 2, 0, 0, 3, 0, 0 ]

lld/wasm/Config.h

@@ -98,6 +98,7 @@ struct Config {
   // for shared libraries (since they always added to a dynamic offset at
   // runtime).
   uint64_t tableBase;
+  uint64_t functionPointerAlignment;
   uint64_t zStackSize;
   uint64_t pageSize;
   unsigned ltoPartitions;

lld/wasm/Driver.cpp

@@ -632,6 +632,8 @@ 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

@@ -256,6 +256,9 @@ 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

@@ -581,6 +581,13 @@ 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() {
@@ -617,9 +624,9 @@ void ElemSection::writeBody() {
   writeUleb128(os, indirectFunctions.size(), "elem count");
   uint32_t tableIndex = ctx.arg.tableBase;
   for (const FunctionSymbol *sym : indirectFunctions) {
-    assert(sym->getTableIndex() == tableIndex);
+    assert(!sym || sym->getTableIndex() == tableIndex);
     (void) tableIndex;
-    writeUleb128(os, sym->getFunctionIndex(), "function index");
+    writeUleb128(os, sym ? sym->getFunctionIndex() : 0, "function index");
     ++tableIndex;
   }
 }