[llvm-branch-commits] [clang] release/21.x: [Flang] Search flang_rt in clang_rt path (#151954) (PR #152458)

[via llvm-branch-commits]

https://github.com/llvmbot created https://github.com/llvm/llvm-project/pull/152458

Backport 8de481913353a1e37264687d5cc73db0de19e6cc

Requested by: @Meinersbur

>From 8a59c3705a92e904f9cdcbfe73342d6197659db0 Mon Sep 17 00:00:00 2001
From: Michael Kruse <llvm-project at meinersbur.de>
Date: Wed, 6 Aug 2025 16:58:08 +0200
Subject: [PATCH] [Flang] Search flang_rt in clang_rt path (#151954)

The clang/flang driver has two separate systems for find the location of
clang_rt (simplified):

* `getCompilerRTPath()`, e.g. `../lib/clang/22/lib/windows`,
   used when `LLVM_ENABLE_PER_TARGET_RUNTIME_DIR=0`
* `getRuntimePath()`, e.g. `../lib/clang/22/lib/x86_64-pc-windows-msvc`,
   used when `LLVM_ENABLE_PER_TARGET_RUNTIME_DIR=1`

To simplify the search path, Flang-RT normally assumes only
`getRuntimePath()`, i.e. ignoring `LLVM_ENABLE_PER_TARGET_RUNTIME_DIR`
and always using the `LLVM_ENABLE_PER_TARGET_RUNTIME_DIR=1` mechanism.
There is an exception for Apple Darwin triples where `getRuntimePath()`
returns nothing. The flang-rt/compiler-rt CMake code for library
location also ignores `LLVM_ENABLE_PER_TARGET_RUNTIME_DIR` but uses the
`LLVM_ENABLE_PER_TARGET_RUNTIME_DIR=0` path instead. Since only
`getRuntimePath()` is automatically added to the linker command line,
this patch explicitly adds `getCompilerRTPath()` to the path when
linking flang_rt.

Fixes #151031

(cherry picked from commit 8de481913353a1e37264687d5cc73db0de19e6cc)
---
 clang/lib/Driver/ToolChain.cpp | 29 +++++++++++++++++++++--------
 1 file changed, 21 insertions(+), 8 deletions(-)

diff --git a/clang/lib/Driver/ToolChain.cpp b/clang/lib/Driver/ToolChain.cpp
index 3f9b808b2722e..07a3ae925f96d 100644
--- a/clang/lib/Driver/ToolChain.cpp
+++ b/clang/lib/Driver/ToolChain.cpp
@@ -837,17 +837,30 @@ void ToolChain::addFortranRuntimeLibs(const ArgList &Args,
 
 void ToolChain::addFortranRuntimeLibraryPath(const llvm::opt::ArgList &Args,
                                              ArgStringList &CmdArgs) const {
-  // Default to the <driver-path>/../lib directory. This works fine on the
-  // platforms that we have tested so far. We will probably have to re-fine
-  // this in the future. In particular, on some platforms, we may need to use
-  // lib64 instead of lib.
+  auto AddLibSearchPathIfExists = [&](const Twine &Path) {
+    // Linker may emit warnings about non-existing directories
+    if (!llvm::sys::fs::is_directory(Path))
+      return;
+
+    if (getTriple().isKnownWindowsMSVCEnvironment())
+      CmdArgs.push_back(Args.MakeArgString("-libpath:" + Path));
+    else
+      CmdArgs.push_back(Args.MakeArgString("-L" + Path));
+  };
+
+  // Search for flang_rt.* at the same location as clang_rt.* with
+  // LLVM_ENABLE_PER_TARGET_RUNTIME_DIR=0. On most platforms, flang_rt is
+  // located at the path returned by getRuntimePath() which is already added to
+  // the library search path. This exception is for Apple-Darwin.
+  AddLibSearchPathIfExists(getCompilerRTPath());
+
+  // Fall back to the non-resource directory <driver-path>/../lib. We will
+  // probably have to refine this in the future. In particular, on some
+  // platforms, we may need to use lib64 instead of lib.
   SmallString<256> DefaultLibPath =
       llvm::sys::path::parent_path(getDriver().Dir);
   llvm::sys::path::append(DefaultLibPath, "lib");
-  if (getTriple().isKnownWindowsMSVCEnvironment())
-    CmdArgs.push_back(Args.MakeArgString("-libpath:" + DefaultLibPath));
-  else
-    CmdArgs.push_back(Args.MakeArgString("-L" + DefaultLibPath));
+  AddLibSearchPathIfExists(DefaultLibPath);
 }
 
 void ToolChain::addFlangRTLibPath(const ArgList &Args,