[lld][WebAssembly] Add allow-multiple-definition flag (#97699)

Add `allow-multiple-definition` flag to `wasm-ld`. This follows the ELF
linker logic. In case of duplication, the first symbol met is used.

This PR resolves the #97543

Author: mzukovec

lld/test/wasm/Inputs/allow-multiple-definition.s

@@ -0,0 +1,6 @@
+  .hidden foo
+  .globl  foo
+foo:
+  .functype foo () -> (i32)
+  i32.const 1
+  end_function

lld/test/wasm/allow-multiple-definition.s

@@ -0,0 +1,38 @@
+# RUN: llvm-mc -filetype=obj -triple=wasm32-unknown-unknown %s -o %t1
+# RUN: llvm-mc -filetype=obj -triple=wasm32-unknown-unknown %p/Inputs/allow-multiple-definition.s -o %t2
+# RUN: not wasm-ld %t1 %t2 -o /dev/null
+# RUN: not wasm-ld --allow-multiple-definition --no-allow-multiple-definition %t1 %t2 -o /dev/null
+# RUN: wasm-ld --allow-multiple-definition --fatal-warnings %t1 %t2 -o %t3
+# RUN: wasm-ld --allow-multiple-definition --fatal-warnings %t2 %t1 -o %t4
+# RUN: llvm-objdump --no-print-imm-hex -d %t3 | FileCheck %s
+# RUN: llvm-objdump --no-print-imm-hex -d %t4 | FileCheck --check-prefix=REVERT %s
+
+# RUN: wasm-ld --noinhibit-exec %t2 %t1 -o /dev/null 2>&1 | FileCheck %s --check-prefix=WARN
+# WARN: warning: duplicate symbol: foo
+
+# RUN: wasm-ld -z muldefs --fatal-warnings  %t1 %t2 -o %t3
+# RUN: wasm-ld -z muldefs --fatal-warnings  %t2 %t1 -o %t4
+# RUN: llvm-objdump --no-print-imm-hex -d %t3 | FileCheck %s
+# RUN: llvm-objdump --no-print-imm-hex -d %t4 | FileCheck --check-prefix=REVERT %s
+
+# CHECK:  i32.const 0
+# REVERT:  i32.const 1
+
+# inputs contain different constants for function foo return.
+# Tests below checks that order of files in command line
+# affects on what symbol will be used.
+# If flag allow-multiple-definition is enabled the first
+# meet symbol should be used.
+
+  .hidden foo
+  .globl  foo
+foo:
+  .functype foo () -> (i32)
+  i32.const 0
+  end_function
+
+  .globl _start
+_start:
+  .functype  _start () -> (i32)
+  call foo
+  end_function

lld/wasm/Config.h

@@ -12,6 +12,7 @@
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/ADT/StringRef.h"
 #include "llvm/ADT/StringSet.h"
+#include "llvm/ADT/Twine.h"
 #include "llvm/BinaryFormat/Wasm.h"
 #include "llvm/Support/CachePruning.h"
 #include <optional>
@@ -43,6 +44,7 @@ enum class BuildIdKind { None, Fast, Sha1, Hexstring, Uuid };
 // and such fields have the same name as the corresponding options.
 // Most fields are initialized by the driver.
 struct Configuration {
+  bool allowMultipleDefinition;
   bool bsymbolic;
   bool checkFeatures;
   bool compressRelocations;
@@ -64,6 +66,7 @@ struct Configuration {
   bool importUndefined;
   std::optional<bool> is64;
   bool mergeDataSegments;
+  bool noinhibitExec;
   bool pie;
   bool printGcSections;
   bool relocatable;
@@ -148,6 +151,8 @@ struct Ctx {
 
 extern Ctx ctx;
 
+void errorOrWarn(const llvm::Twine &msg);
+
 } // namespace lld::wasm
 
 #endif

lld/wasm/Driver.cpp

@@ -47,6 +47,13 @@ namespace lld::wasm {
 Configuration *config;
 Ctx ctx;
 
+void errorOrWarn(const llvm::Twine &msg) {
+  if (config->noinhibitExec)
+    warn(msg);
+  else
+    error(msg);
+}
+
 void Ctx::reset() {
   objectFiles.clear();
   stubFiles.clear();
@@ -99,6 +106,16 @@ class LinkerDriver {
 
   std::vector<InputFile *> files;
 };
+
+static bool hasZOption(opt::InputArgList &args, StringRef key) {
+  bool ret = false;
+  for (const auto *arg : args.filtered(OPT_z))
+    if (key == arg->getValue()) {
+      ret = true;
+      arg->claim();
+    }
+  return ret;
+}
 } // anonymous namespace
 
 bool link(ArrayRef<const char *> args, llvm::raw_ostream &stdoutOS,
@@ -467,6 +484,10 @@ getBuildId(opt::InputArgList &args) {
 
 // Initializes Config members by the command line options.
 static void readConfigs(opt::InputArgList &args) {
+  config->allowMultipleDefinition =
+      hasZOption(args, "muldefs") ||
+      args.hasFlag(OPT_allow_multiple_definition,
+                   OPT_no_allow_multiple_definition, false);
   config->bsymbolic = args.hasArg(OPT_Bsymbolic);
   config->checkFeatures =
       args.hasFlag(OPT_check_features, OPT_no_check_features, true);
@@ -479,6 +500,7 @@ static void readConfigs(opt::InputArgList &args) {
   config->exportAll = args.hasArg(OPT_export_all);
   config->exportTable = args.hasArg(OPT_export_table);
   config->growableTable = args.hasArg(OPT_growable_table);
+  config->noinhibitExec = args.hasArg(OPT_noinhibit_exec);
 
   if (args.hasArg(OPT_import_memory_with_name)) {
     config->memoryImport =
@@ -1173,7 +1195,7 @@ static void splitSections() {
 
 static bool isKnownZFlag(StringRef s) {
   // For now, we only support a very limited set of -z flags
-  return s.starts_with("stack-size=");
+  return s.starts_with("stack-size=") || s.starts_with("muldefs");
 }
 
 // Report a warning for an unknown -z option.

lld/wasm/Options.td

@@ -42,6 +42,10 @@ def Bdynamic: F<"Bdynamic">, HelpText<"Link against shared libraries">;
 
 def Bstatic: F<"Bstatic">, HelpText<"Do not link against shared libraries (default)">;
 
+defm allow_multiple_definition: B<"allow-multiple-definition",
+    "Allow multiple definitions",
+    "Do not allow multiple definitions (default)">;
+
 def build_id: F<"build-id">, HelpText<"Alias for --build-id=fast">;
 
 def build_id_eq: J<"build-id=">, HelpText<"Generate build ID note">,
@@ -105,6 +109,9 @@ defm mllvm: Eq<"mllvm", "Additional arguments to forward to LLVM's option proces
 
 defm Map: Eq<"Map", "Print a link map to the specified file">;
 
+def noinhibit_exec: F<"noinhibit-exec">,
+  HelpText<"Retain the executable output file whenever it is still usable">;
+
 def o: JoinedOrSeparate<["-"], "o">, MetaVarName<"<path>">,
   HelpText<"Path to file to write output">;
 

lld/wasm/SymbolTable.cpp

@@ -319,9 +319,12 @@ static bool shouldReplace(const Symbol *existing, InputFile *newFile,
   }
 
   // Neither symbol is week. They conflict.
-  error("duplicate symbol: " + toString(*existing) + "\n>>> defined in " +
-        toString(existing->getFile()) + "\n>>> defined in " +
-        toString(newFile));
+  if (config->allowMultipleDefinition)
+    return false;
+
+  errorOrWarn("duplicate symbol: " + toString(*existing) + "\n>>> defined in " +
+              toString(existing->getFile()) + "\n>>> defined in " +
+              toString(newFile));
   return true;
 }