[clang] Ensure --print-runtime-dir path exists

[aganea]

Before this PR, clang --print-runtime-dir on Windows used to report a non-existent directory if LLVM_ENABLE_PER_TARGET_RUNTIME_DIR=OFF.

We now check if any of the known runtime directories exist before printing any of them on stdout. If none exists, we print (runtime dir is not present).

[llvmbot]

@llvm/pr-subscribers-clang-driver

@llvm/pr-subscribers-clang

Author: Alexandre Ganea (aganea)

Changes

Before this PR, clang --print-runtime-dir used to report a non-existent directory if LLVM_ENABLE_PER_TARGET_RUNTIME_DIR=OFF.

We now check if any of the known runtime directories exist before printing on stdout. If it doesn't, we print (runtime dir is not present).

---

Full diff: https://github.com/llvm/llvm-project/pull/102834.diff

1 Files Affected:

• (modified) clang/lib/Driver/Driver.cpp (+8-4)

diff --git a/clang/lib/Driver/Driver.cpp b/clang/lib/Driver/Driver.cpp
index f4e909b79389bc..4c8cd36dd118ee 100644
--- a/clang/lib/Driver/Driver.cpp
+++ b/clang/lib/Driver/Driver.cpp
@@ -2230,10 +2230,14 @@ bool Driver::HandleImmediateArgs(Compilation &C) {
   }
 
   if (C.getArgs().hasArg(options::OPT_print_runtime_dir)) {
-    if (std::optional<std::string> RuntimePath = TC.getRuntimePath())
-      llvm::outs() << *RuntimePath << '\n';
-    else
-      llvm::outs() << TC.getCompilerRTPath() << '\n';
+    for (auto RuntimePath :
+         {TC.getRuntimePath(), std::make_optional(TC.getCompilerRTPath())}) {
+      if (RuntimePath && getVFS().exists(*RuntimePath)) {
+        llvm::outs() << *RuntimePath << '\n';
+        return false;
+      }
+    }
+    llvm::outs() << "(runtime dir is not present)" << '\n';
     return false;
   }
 

[MaskRay]

--print-runtime-dir can be defined in multiple ways.
The current definition is to print a directory suitable for LLVM_ENABLE_PER_TARGET_RUNTIME_DIR=on configurations.
This interppretation makes sense to me, as the "runtime" in "print-runtime-dir" matches LLVM_ENABLE_PER_TARGET_RUNTIME_DIR.

With this interpretation, --print-runtime-dir should print the directory regardless of whether the LLVM_ENABLE_PER_TARGET_RUNTIME_DIR=on directory is present or not.
If the directory is absent, --print-runtime-dir prints a directory that the user should ensure presence.

Perhaps the TC.getCompilerRTPath() fallback leads to confusion. Ideally it probably should not be present.

[aganea]

To give the whole picture, the story is that while building the LLVM package on Windows, I was seeing this:

[51/52] Running sanitizer_common testsllvm-lit.py: C:\src\git\llvm-project\llvm\utils\lit\lit\llvm\config.py:57: note: using lit tools: C:\Program Files\Git\usr\bin\
llvm-lit.py: C:\src\git\llvm-project\compiler-rt\test\lit.common.cfg.py:60: warning: Path reported by clang does not exist: "C:\src\git\llvm-project\llvm_package_20.0.0\build64\lib\clang\20\lib\x86_64-pc-windows-msvc". This path was found by running ['C:/src/git/llvm-project/llvm_package_20.0.0/build64/./bin/clang.exe', '--target=x86_64-pc-windows-msvc', '-Wthread-safety', '-Wthread-safety-reference', '-Wthread-safety-beta', '-print-runtime-dir'].
llvm-lit.py: C:\src\git\llvm-project\compiler-rt\test\lit.common.cfg.py:60: warning: Path reported by clang does not exist: "C:\src\git\llvm-project\llvm_package_20.0.0\build64\lib\clang\20\lib\x86_64-pc-windows-msvc". This path was found by running ['C:/src/git/llvm-project/llvm_package_20.0.0/build64/./bin/clang.exe', '--target=x86_64-pc-windows-msvc', '-Wthread-safety', '-Wthread-safety-reference', '-Wthread-safety-beta', '-print-runtime-dir'].

This is, we don't set LLVM_ENABLE_PER_TARGET_RUNTIME_DIR=ON when building compiler-rt, so we end up taking this cmake codepath: https://github.com/llvm/llvm-project/blob/main/compiler-rt/cmake/base-config-ix.cmake#L106 which ends up with the compiler runtime libs in {build_folder}\lib\clang\{version}\lib\windows. However ToolChain::getRuntimePath() checks for the triple folder, as in {build_folder}\lib\clang\{version}\lib\x86_64-pc-windows-msvc which don't exist. clang -print-runtime-dir will just display that, without checking the folder's existence. However if we do clang-cl a.cpp -fsanitize=address then we check the library paths (which runtime is part of at this point) for existence: https://github.com/llvm/llvm-project/blob/main/clang/lib/Driver/ToolChains/MSVC.cpp#L153

I think we want to keep the dynamic aspect on supporting both LLVM_ENABLE_PER_TARGET_RUNTIME_DIR=ON/OFF at runtime in Clang like @mstorsjo was suggesting in https://discourse.llvm.org/t/runtime-directory-fallback/76860/2, I feel we should check the existence of the path before printing it, thus this PR.

[aganea]

Ping.

This still occurs as of today on main, clang-cl -print-runtime-dir doesn't print the right folder path when targetting Windows.
On disk we have: C:\Program Files\LLVM\lib\clang\22\lib\windows,
whereas -print-runtime-path prints the full triple as on the other platforms: C:\Program Files\LLVM\lib\clang\22\lib\x86_64-pc-windows-msvc.

What is it that we want to do here? Add a dynamic behavior as in this PR, that checks if the path is there, and adjust accordingly? Rename the "windows" component to instead use the target triple, as the other plateforms (this seems to be some work). Or conversly, accept that on Windows the path is "windows" and not the target triple? Anything else?

[rnk]

I found this at the bottom of my inbox after a long cleanout.

I'm struggling to remember it at this point in the evening, but we've done things like this before as part of the runtime path changes, and I don't see any reason to object to this one.

Part of me wants to test this behavior since we have the indirection layer, we don't have to copy the clang binary and create an entire runtime tree in the test directory, but I wouldn't block this over that concern.

In conclusion, I think we should merge the patch.