[lld][WebAssembly]: Defer __wasm_apply_data_relocs decision to writer phase

At the time of createSyntheticSymbols, it isn't obvious if we
need these relocations.

Note: this has a side effect to stop emitting empty
__wasm_apply_data_relocs. It isn't a problem because the export
is optional in the spec (DynamicLinking.md) and all implementations
I'm aware of implement it that way. (emscripten, toywasm, wasm-tools)

Author: yamt

lld/wasm/Driver.cpp

@@ -917,18 +917,6 @@ static void createSyntheticSymbols() {
             is64 ? i64ArgSignature : i32ArgSignature,
             "__wasm_init_tls"));
   }
-
-  if (ctx.isPic ||
-      config->unresolvedSymbols == UnresolvedPolicy::ImportDynamic ||
-      !config->isStatic) {
-    // For PIC code, or when dynamically importing addresses, we create
-    // synthetic functions that apply relocations.  These get called from
-    // __wasm_call_ctors before the user-level constructors.
-    WasmSym::applyDataRelocs = symtab->addSyntheticFunction(
-        "__wasm_apply_data_relocs",
-        WASM_SYMBOL_VISIBILITY_DEFAULT | WASM_SYMBOL_EXPORTED,
-        make<SyntheticFunction>(nullSignature, "__wasm_apply_data_relocs"));
-  }
 }
 
 static void createOptionalSymbols() {

lld/wasm/Symbols.cpp

@@ -80,7 +80,6 @@ namespace wasm {
 DefinedFunction *WasmSym::callCtors;
 DefinedFunction *WasmSym::callDtors;
 DefinedFunction *WasmSym::initMemory;
-DefinedFunction *WasmSym::applyDataRelocs;
 DefinedFunction *WasmSym::applyGlobalRelocs;
 DefinedFunction *WasmSym::applyTLSRelocs;
 DefinedFunction *WasmSym::applyGlobalTLSRelocs;

lld/wasm/Symbols.h

@@ -591,18 +591,14 @@ struct WasmSym {
   // Function that calls the libc/etc. cleanup function.
   static DefinedFunction *callDtors;
 
-  // __wasm_apply_data_relocs
-  // Function that applies relocations to data segment post-instantiation.
-  static DefinedFunction *applyDataRelocs;
-
   // __wasm_apply_global_relocs
   // Function that applies relocations to wasm globals post-instantiation.
   // Unlike __wasm_apply_data_relocs this needs to run on every thread.
   static DefinedFunction *applyGlobalRelocs;
 
   // __wasm_apply_tls_relocs
-  // Like applyDataRelocs but for TLS section.  These must be delayed until
-  // __wasm_init_tls.
+  // Like __wasm_apply_data_relocs but for TLS section.  These must be
+  // delayed until __wasm_init_tls.
   static DefinedFunction *applyTLSRelocs;
 
   // __wasm_apply_global_tls_relocs

lld/wasm/Writer.cpp

@@ -1145,6 +1145,8 @@ void Writer::createSyntheticInitFunctions() {
 
   static WasmSignature nullSignature = {{}, {}};
 
+  createApplyDataRelocationsFunction();
+
   // Passive segments are used to avoid memory being reinitialized on each
   // thread's instantiation. These passive segments are initialized and
   // dropped in __wasm_init_memory, which is registered as the start function
@@ -1467,15 +1469,29 @@ void Writer::createApplyDataRelocationsFunction() {
   {
     raw_string_ostream os(bodyContent);
     writeUleb128(os, 0, "num locals");
+    uint64_t off = os.tell();
     for (const OutputSegment *seg : segments)
       if (!config->sharedMemory || !seg->isTLS())
         for (const InputChunk *inSeg : seg->inputSegments)
           inSeg->generateRelocationCode(os);
 
+    if (off == os.tell()) {
+      LLVM_DEBUG(dbgs() << "skipping empty __wasm_apply_data_relocs\n");
+      return;
+    }
     writeU8(os, WASM_OPCODE_END, "END");
   }
 
-  createFunction(WasmSym::applyDataRelocs, bodyContent);
+  // __wasm_apply_data_relocs
+  // Function that applies relocations to data segment post-instantiation.
+  static WasmSignature nullSignature = {{}, {}};
+  auto def = symtab->addSyntheticFunction(
+      "__wasm_apply_data_relocs",
+      WASM_SYMBOL_VISIBILITY_DEFAULT | WASM_SYMBOL_EXPORTED,
+      make<SyntheticFunction>(nullSignature, "__wasm_apply_data_relocs"));
+  def->markLive();
+
+  createFunction(def, bodyContent);
 }
 
 void Writer::createApplyTLSRelocationsFunction() {
@@ -1771,8 +1787,6 @@ void Writer::run() {
 
   if (!config->relocatable) {
     // Create linker synthesized functions
-    if (WasmSym::applyDataRelocs)
-      createApplyDataRelocationsFunction();
     if (WasmSym::applyGlobalRelocs)
       createApplyGlobalRelocationsFunction();
     if (WasmSym::applyTLSRelocs)