ci: build LLVM with thread support on Windows

.github/workflows/windows.yml

@@ -66,7 +66,7 @@ jobs:
         uses: actions/cache/restore@v3
         id: cache-llvm-build
         with:
-          key: llvm-build-15-windows-v5
+          key: llvm-build-15-windows-v6
           path: llvm-build
       - name: Build LLVM
         if: steps.cache-llvm-build.outputs.cache-hit != 'true'

Makefile

@@ -120,10 +120,8 @@ ifeq ($(OS),Windows_NT)
     START_GROUP = -Wl,--start-group
     END_GROUP = -Wl,--end-group
 
-    # LLVM compiled using MinGW on Windows appears to have problems with threads.
-    # Without this flag, linking results in errors like these:
-    #     libLLVMSupport.a(Threading.cpp.obj):Threading.cpp:(.text+0x55): undefined reference to `std::thread::hardware_concurrency()'
-    LLVM_OPTION += -DLLVM_ENABLE_THREADS=OFF -DLLVM_ENABLE_PIC=OFF
+    # PIC needs to be disabled for libclang to work.
+    LLVM_OPTION += -DLLVM_ENABLE_PIC=OFF
 
     CGO_CPPFLAGS += -DCINDEX_NO_EXPORTS
     CGO_LDFLAGS += -static -static-libgcc -static-libstdc++

builder/lld.cpp

@@ -3,25 +3,39 @@
 // This file provides C wrappers for liblld.
 
 #include <lld/Common/Driver.h>
+#include <llvm/Support/Parallel.h>
 
 extern "C" {
 
+static void configure() {
+#if _WIN64
+	// This is a hack to work around a hang in the LLD linker on Windows, with
+	// -DLLVM_ENABLE_THREADS=ON. It has a similar effect as the -threads=1
+	// linker flag, but with support for the COFF linker.
+	llvm::parallel::strategy = llvm::hardware_concurrency(1);
+#endif
+}
+
 bool tinygo_link_elf(int argc, char **argv) {
+	configure();
 	std::vector<const char*> args(argv, argv + argc);
 	return lld::elf::link(args, llvm::outs(), llvm::errs(), false, false);
 }
 
 bool tinygo_link_macho(int argc, char **argv) {
+	configure();
 	std::vector<const char*> args(argv, argv + argc);
 	return lld::macho::link(args, llvm::outs(), llvm::errs(), false, false);
 }
 
 bool tinygo_link_mingw(int argc, char **argv) {
+	configure();
 	std::vector<const char*> args(argv, argv + argc);
 	return lld::mingw::link(args, llvm::outs(), llvm::errs(), false, false);
 }
 
 bool tinygo_link_wasm(int argc, char **argv) {
+	configure();
 	std::vector<const char*> args(argv, argv + argc);
 	return lld::wasm::link(args, llvm::outs(), llvm::errs(), false, false);
 }