[Windows] Emit dependency to flang_rt.builtins.lib in every flang-compiled object file.

The dependency is already transitive due to flang-rt depending on it, so
it shouldn't be necessary. However, CMake may depend on it, and the
transsitivity may be ignored in DLLs.

To avoid breaking things, keep the original behaviour but remove the
redundant condition and update the wrong comment.

IMHO this is a bug in LLVM: If clang/clang-cl emits calls not in user-code,
the driver should als care about linking the correct library. Even better,
do not generate such calls and adhere to the msvc-defined ABI.

Author: Meinersbur

clang/lib/Driver/ToolChains/Flang.cpp

@@ -291,6 +291,14 @@ static void processVSRuntimeLibrary(const ToolChain &TC, const ArgList &Args,
   assert(TC.getTriple().isKnownWindowsMSVCEnvironment() &&
          "can only add VS runtime library on Windows!");
 
+  // Flang/Clang (including clang-cl) -compiled programs targeting the MSVC ABI
+  // should only depend on msv(u)crt. LLVM still emits libgcc/compiler-rt
+  // functions in some cases like 128-bit integer math (__udivti3, __modti3,
+  // __fixsfti, __floattidf, ...) that msvc does not support. We are injecting a
+  // dependency to Compiler-RT's builtin library where these are implemented.
+  CmdArgs.push_back(Args.MakeArgString(
+      "--dependent-lib=" + TC.getCompilerRTBasename(Args, "builtins")));
+
   unsigned RTOptionID = options::OPT__SLASH_MT;
   if (auto *rtl = Args.getLastArg(options::OPT_fms_runtime_lib_EQ)) {
     RTOptionID = llvm::StringSwitch<unsigned>(rtl->getValue())

flang-rt/cmake/modules/AddFlangRT.cmake

@@ -95,9 +95,9 @@ function (add_flangrt_library name)
 
   # Flang/Clang (including clang-cl) -compiled programs targeting the MSVC ABI
   # should only depend on msv(u)crt. LLVM still emits libgcc/compiler-rt
-  # functions for 128-bit integer math (__udivti3, __modti3, __fixsfti,
-  # __floattidf, ...) that msvc does not support. We are injecting a dependency
-  # to Compiler-RT where these are implemented.
+  # functions in some cases like 128-bit integer math (__udivti3, __modti3,
+  # __fixsfti, __floattidf, ...) that msvc does not support. We are injecting a
+  # dependency to Compiler-RT's builtin library where these are implemented.
   if (MSVC AND (CMAKE_CXX_COMPILER_ID MATCHES ".*Clang") AND FLANG_RT_LIBCALL)
     target_compile_options(${name} PRIVATE "$<$<COMPILE_LANGUAGE:CXX,C>:-Xclang>$<$<COMPILE_LANGUAGE:Fortran>:-Xflang>" "--dependent-lib=${FLANG_RT_LIBCALL}")
   endif ()