[flang-commits] [flang] [llvm] [Flang] Add mock flang driver (PR #203481)

[Joseph Huber via flang-commits]

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@@ -0,0 +1,99 @@
+//===-- fakeflang.cpp - Mock Flang Driver ---------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+/// Mock driver to pass CMake's compiler introspection for
+/// CMake_Fortran_COMPILER. It's purpose is to not having to build the full
+/// flang compiler for the runtimes-configure phase in bootstrapping-runtimes
+/// builds, but only when the Fortran compiler is actually needed (e.g.
+/// flang-rt-mod, libomp-mod).
+///
+/// To detect LLVMFlang, CMake executes
+///
+///   ${CMAKE_Fortran_COMPILER} -v -c -target=... CMakeFortranCompilerId.F
+///
+/// and expects a new file to appear in the working directory. This would
+/// usually be an object file (e.g. ELF), but it doesn't matter for CMake as it
+/// parses it for the preprocessor result of CMakeFortranCompilerId.F which
+/// would appear as string literals in the binary file (CMake cannot execute the
+/// file because it might be cross-compiling). Just passing it through the
+/// preprocessor yields the same result.
+///
+/// The most relevant preprocessor definition is __flang__ which leads to
+/// CMAKE_Fortran_COMPILER_ID="LLVMFlang".
+//
+//===----------------------------------------------------------------------===//
+
+#include "flang/Version.inc"
+#include "llvm/ADT/ArrayRef.h"
+#include "llvm/ADT/STLExtras.h"
+#include "llvm/Support/FileSystem.h"
+#include "llvm/Support/InitLLVM.h"
+#include "llvm/Support/Path.h"
+#include "llvm/Support/Program.h"
+#include "llvm/Support/WithColor.h"
+#include <cstdint>
+#include <string>
+
+#define STRINGIFY(X) #X
+#define STRINGIFY_EXPANDED(X) STRINGIFY(X)
+
+static std::string getExecutablePath(const char *argv0) {
+  void *anchor = (void *)(intptr_t)getExecutablePath;
+  return llvm::sys::fs::getMainExecutable(argv0, anchor);
+}
+
+[[noreturn]] static void fail(llvm::Twine Error) {
+  llvm::WithColor::error(llvm::errs(), "fakeflang") << Error << "\n";
+  exit(EXIT_FAILURE);
+}
+
+int main(int argc, const char **argv) {
+  llvm::InitLLVM X(argc, argv);
+  std::string SelfExe = getExecutablePath(argv[0]);
+
+  if (llvm::sys::path::stem(SelfExe) == "flang") {
+    llvm::WithColor::remark(llvm::errs())
+        << "This is a mock flang compiler; Use '" STRINGIFY_EXPANDED(
+               CMAKE_MAKE_PROGRAM) " flang' to replace it with the real "
+                                   "compiler\n";
+  }
+
+  llvm::SmallString<256> ClangExe{llvm::sys::path::parent_path(SelfExe)};
+  llvm::sys::path::append(ClangExe, "clang");
+
+  llvm::ArrayRef<const char *> AllArgs(argv, static_cast<size_t>(argc));
+  bool hasDashO = AllArgs.size() > 1 &&
----------------
jhuber6 wrote:

I thought there was an option to just forward the unparsed arguments, wasn't sure how strict it had to be since the custom parsing here seems like it would hit on some other cases. But it's not a big deal.

https://github.com/llvm/llvm-project/pull/203481