[lld] Reject --read-workers when lld is built without thread support (#163925)

Also expand the #ifdef to remove unused code in this configuration. As
suggested in
https://github.com/llvm/llvm-project/pull/147134#issuecomment-3328612158.

I have also:
* Expanded the error message to explain why it's not allowed.
* Added a test for the error.
* Marked the original test as unsupported when threads are disabled.

Fixes issues we have had on Armv8 with threading disabled where this
test would crash every so often.

This change will hopefully be superseded by #157917, but that has been
in review a long time and I want to make the bot stable again.

I could just disable the test, but I'd like lld to function properly in
general in the meantime too.

Co-authored-by: John Holdsworth <github@johnholdsworth.com>

lld/MachO/Driver.cpp

@@ -291,6 +291,7 @@ struct DeferredFile {
 };
 using DeferredFiles = std::vector<DeferredFile>;
 
+#if LLVM_ENABLE_THREADS
 class SerialBackgroundQueue {
   std::deque<std::function<void()>> queue;
   std::thread *running;
@@ -359,7 +360,6 @@ void multiThreadedPageInBackground(DeferredFiles &deferred) {
       (void)t;
     }
   };
-#if LLVM_ENABLE_THREADS
   { // Create scope for waiting for the taskGroup
     std::atomic_size_t index = 0;
     llvm::parallel::TaskGroup taskGroup;
@@ -373,7 +373,6 @@ void multiThreadedPageInBackground(DeferredFiles &deferred) {
         }
       });
   }
-#endif
 #ifndef NDEBUG
   auto dt = high_resolution_clock::now() - t0;
   if (Process::GetEnv("LLD_MULTI_THREAD_PAGE"))
@@ -390,6 +389,7 @@ static void multiThreadedPageIn(const DeferredFiles &deferred) {
     multiThreadedPageInBackground(files);
   });
 }
+#endif
 
 static InputFile *processFile(std::optional<MemoryBufferRef> buffer,
                               DeferredFiles *archiveContents, StringRef path,
@@ -1430,6 +1430,7 @@ static void createFiles(const InputArgList &args) {
     }
   }
 
+#if LLVM_ENABLE_THREADS
   if (config->readWorkers) {
     multiThreadedPageIn(deferredFiles);
 
@@ -1447,6 +1448,7 @@ static void createFiles(const InputArgList &args) {
     for (auto *archive : archives)
       archive->addLazySymbols();
   }
+#endif
 }
 
 static void gatherInputSections() {
@@ -1834,6 +1836,7 @@ bool link(ArrayRef<const char *> argsArr, llvm::raw_ostream &stdoutOS,
   }
 
   if (auto *arg = args.getLastArg(OPT_read_workers)) {
+#if LLVM_ENABLE_THREADS
     StringRef v(arg->getValue());
     unsigned workers = 0;
     if (!llvm::to_integer(v, workers, 0))
@@ -1841,6 +1844,10 @@ bool link(ArrayRef<const char *> argsArr, llvm::raw_ostream &stdoutOS,
             ": expected a non-negative integer, but got '" + arg->getValue() +
             "'");
     config->readWorkers = workers;
+#else
+    error(arg->getSpelling() +
+          ": option unavailable because lld was not built with thread support");
+#endif
   }
   if (auto *arg = args.getLastArg(OPT_threads_eq)) {
     StringRef v(arg->getValue());

lld/test/CMakeLists.txt

@@ -1,5 +1,6 @@
 llvm_canonicalize_cmake_booleans(
   ENABLE_BACKTRACES
+  LLVM_ENABLE_THREADS
   LLVM_ENABLE_ZLIB
   LLVM_ENABLE_ZSTD
   LLVM_ENABLE_LIBXML2

lld/test/MachO/read-workers-no-thread-support.s

@@ -0,0 +1,10 @@
+# REQUIRES: x86 && !thread_support
+# RUN: llvm-mc -filetype=obj -triple=x86_64-apple-darwin %s -o %t.o
+
+# RUN: not %lld --read-workers=1 %t.o -o /dev/null
+
+# CHECK: error: --read-workers=: option unavailable because lld was built without thread support
+
+.globl _main
+_main:
+  ret

lld/test/MachO/read-workers.s

@@ -1,4 +1,4 @@
-# REQUIRES: x86
+# REQUIRES: x86 && thread_support
 # RUN: llvm-mc -filetype=obj -triple=x86_64-apple-darwin %s -o %t.o
 
 ## A non-negative integer is allowed.

lld/test/lit.cfg.py

@@ -182,3 +182,6 @@ if tar_executable:
 # ELF tests expect the default target for ld.lld to be ELF.
 if config.ld_lld_default_mingw:
     config.excludes.append("ELF")
+
+if config.enable_threads:
+    config.available_features.add("thread_support")

lld/test/lit.site.cfg.py.in

@@ -26,6 +26,7 @@ config.ld_lld_default_mingw = @LLD_DEFAULT_LD_LLD_IS_MINGW@
 config.build_examples = @LLVM_BUILD_EXAMPLES@
 config.has_plugins = @LLVM_ENABLE_PLUGINS@
 config.linked_bye_extension = @LLVM_BYE_LINK_INTO_TOOLS@
+config.enable_threads = @LLVM_ENABLE_THREADS@
 
 import lit.llvm
 lit.llvm.initialize(lit_config, config)