[llvm-rc] Resolve the executable path if not present in Argv[0]

[mstorsjo]

Summary

The llvm-rc tool tries to locate a suitable Clang executable to
use for preprocessing. For this purpose, it first checks within
the same directory as the llvm-rc tool, checking with a couple
different names, followed by checking all of $PATH for another
couple names.

On Windows, the InitLLVM() function always sets up Argv[0] with the
full path to the executable, while on Unix, Argv[0] is kept as is.

Therefore, call getMainExecutable to try to resolve the directory of
the executable before looking for colocated Clang executables.

This makes 282744a9ce18120dc0a6eceb02693b36980d9498 actually have
the desired effect.

[thieta]

LGTM, I would change P to nullptr, unless there is a good reason to check the symbol there.

llvm/tools/llvm-rc/llvm-rc.cpp

128

You can just pass nullptr to getMainExecutable instead of a symbol, unless there is a good reason here (I don't think so?).

[mstorsjo]

Thanks for the review!

llvm/tools/llvm-rc/llvm-rc.cpp

128

As far as I know, the address argument can be used for resolving the main executable as a late fallback implementation - see https://github.com/llvm/llvm-project/blob/llvmorg-17.0.0-rc1/llvm/lib/Support/Unix/Path.inc#L323. If we pass nullptr there, the getMainExecutable function won't work in that particular codepath.

llvm/tools/llvm-rc/llvm-rc.cpp

std::string createTempFile(const Twine &Prefix, StringRef Suffix) {
   std::error_code EC;
   SmallString<128> FileName;
   if ((EC = sys::fs::createTemporaryFile(Prefix, Suffix, FileName)))
     fatalError("Unable to create temp file: " + EC.message());
   return static_cast<std::string>(FileName);
 }
 
 ErrorOr<std::string> findClang(const char *Argv0, StringRef Triple) {
-  StringRef Parent = llvm::sys::path::parent_path(Argv0);
+  // This just needs to be some symbol in the binary.
+  void *P = (void*) (intptr_t) findClang;
+  std::string MainExecPath = llvm::sys::fs::getMainExecutable(Argv0, P);
+  if (MainExecPath.empty())
+    MainExecPath = Argv0;
+
+  StringRef Parent = llvm::sys::path::parent_path(MainExecPath);
   ErrorOr<std::string> Path = std::error_code();
   std::string TargetClang = (Triple + "-clang").str();
   std::string VersionedClang = ("clang-" + Twine(LLVM_VERSION_MAJOR)).str();
   if (!Parent.empty()) {
     // First look for the tool with all potential names in the specific
     // directory of Argv0, if known
     for (const auto *Name :
          {TargetClang.c_str(), VersionedClang.c_str(), "clang", "clang-cl"}) {