[llvm-rc] Continue to use Argv[0] to resolve executable path

[akhuang]

Summary

In internal google builds, MainExecPath doesn't go to the directory with clang.
Fall back to using Argv0 if MainExecPath doesn't find any clangs.

[mstorsjo]

Sorry for the breakage, and thanks for making a patch!

This issue actually came up already a couple days ago, in https://github.com/llvm/llvm-project/issues/64927, and I considered doing something pretty much like this there as well - but the submitter managed to work around the issue quite neatly.

I’ve got mainly one suggestion; could you flip the nesting of the two loops? I’d like it to prefer <triple>-clang in parent(argv[0]) over clang in parent(MainExecPath).

As a side note, on windows, argv[0] is overwritten with the equivalent of MainExecPath by InitLLVM, but not on Unix.

Also, it’s nice to hear that llvm-rc actually is in use within Google - I’ve had a feeling that not many actually use this tool.

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

139

I see I had forgotten to update this comment here, as it is slightly wrong now - can you reword it s as part of this patch?

[akhuang]

Flip for loops and edit the comment

[mstorsjo]

LGTM, thanks for fixing! And please backport this to 17.x as well.

[akhuang]

Thanks! made a backport at https://github.com/llvm/llvm-project/issues/65050

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

 
 ErrorOr<std::string> findClang(const char *Argv0, StringRef Triple) {
   // 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"}) {
+    for (const StringRef Parent :
+         {llvm::sys::path::parent_path(MainExecPath),
+          llvm::sys::path::parent_path(Argv0)}) {
+      // Look for various versions of "clang" first in the MainExecPath parent
+      // directory and then in the argv[0] parent directory.
+      // On Windows (but not Unix) argv[0] is overwritten with the eqiuvalent
+      // of MainExecPath by InitLLVM.
       Path = sys::findProgramByName(Name, Parent);
       if (Path)
         return Path;
     }
   }
+
   // If no parent directory known, or not found there, look everywhere in PATH
   for (const auto *Name : {"clang", "clang-cl"}) {
     Path = sys::findProgramByName(Name);
     if (Path)
       return Path;
   }
   return Path;
 }