[lld][WebAssembly] Honor --allow-undefined for data symbols too

[sbc100]

Summary

This was originally the way this worked before before
https://reviews.llvm.org/D60882.

In retrospect it seems inconsistent that --allow-undefined doesn't
work for all symbols. See:
https://groups.google.com/g/emscripten-discuss/c/HSRgQiIq1gI/m/Kt9oFWHiAwAJ

I'm also planning a followup change which implement the full
--unresolved-symbols=.. flags supported by ELF linkers (both ld and
ld.lld) since it seems more standard.

[sbc100]

Followup: https://reviews.llvm.org/D79248

[dschuff]

So the plan is to support unresolved-symbols and then eventually deprecate and remove this?

[sbc100]

In D79247#2021187, @dschuff wrote:

So the plan is to support unresolved-symbols and then eventually deprecate and remove this?

Eventually yes that would great. However I think it will take several llvm version before we are ready to do that. Today the --allow-undefined flag has many users including emscripten, and they rely on the current behviour of this flag which is to turn undefined function symbols into function imports. We maybe or may not want to keep the behaviour somehow (either via this flag, or one with a better name such as --import-undefined-functions), even though we now have clang/llvm/symbol attributes to mark functions for import.

lld/test/wasm/undefined-data.ll

 ; RUN: llc -filetype=obj %s -o %t.o
 ; RUN: not wasm-ld -o %t.wasm %t.o 2>&1 | FileCheck %s -check-prefix=UNDEF
-; RUN: not wasm-ld --allow-undefined -o %t.wasm %t.o 2>&1 | FileCheck %s -check-prefix=UNDEF
+; RUN: wasm-ld --allow-undefined -o %t.wasm %t.o
 ; RUN: not wasm-ld --shared -o %t.wasm %t.o 2>&1 | FileCheck %s -check-prefix=SHARED
 
 target triple = "wasm32-unknown-unknown"
 
 @data_external = external global i32
 
 define i32 @_start() {
 entry:
     %0 = load i32, i32* @data_external, align 4
     ret i32 %0
 }
 
 ; UNDEF: error: {{.*}}undefined-data.ll.tmp.o: undefined symbol: data_external
 ; SHARED: error: {{.*}}undefined-data.ll.tmp.o: relocation R_WASM_MEMORY_ADDR_LEB cannot be used against symbol data_external; recompile with -fPIC

lld/wasm/Relocations.cpp

 
 namespace lld {
 namespace wasm {
 static bool requiresGOTAccess(const Symbol *sym) {
   return config->isPic && !sym->isHidden() && !sym->isLocal();
 }
 
 static bool allowUndefined(const Symbol* sym) {
-  // Historically --allow-undefined doesn't work for data symbols since we don't
-  // have any way to represent these as imports in the final binary.  The idea
-  // behind allowing undefined symbols is to allow importing these symbols from
-  // the embedder and we can't do this for data symbols (at least not without
-  // compiling with -fPIC)
-  if (isa<DataSymbol>(sym))
-    return false;
   // Undefined functions with explicit import name are allowed to be undefined
   // at link time.
   if (auto *F = dyn_cast<UndefinedFunction>(sym))
     if (F->importName)
       return true;
   return (config->allowUndefined ||
           config->allowUndefinedSymbols.count(sym->getName()) != 0);
 }