llvm: use --gcc_install_dir in config files (#43795)

var/spack/repos/builtin/packages/llvm/package.py

@@ -936,7 +936,9 @@ def cmake_args(self):
 
         cmake_args.append(from_variant("LIBOMP_TSAN_SUPPORT", "libomp_tsan"))
 
-        if self.compiler.name == "gcc":
+        # From clang 16 onwards we use a more precise --gcc-install-dir flag in post-install
+        # generated config files.
+        if self.spec.satisfies("@:15 %gcc"):
             cmake_args.append(define("GCC_INSTALL_PREFIX", self.compiler.prefix))
 
         if self.spec.satisfies("~code_signing platform=darwin"):
@@ -976,12 +978,24 @@ def cmake_args(self):
                         runtimes_order.index(x) if x in runtimes_order else len(runtimes_order)
                     )
                 )
+
+            # CMake args passed just to runtimes
+            runtime_cmake_args = [define("CMAKE_INSTALL_RPATH_USE_LINK_PATH", True)]
+
+            # When building runtimes, just-built clang has to know where GCC is.
+            gcc_install_dir_flag = get_gcc_install_dir_flag(spec, self.compiler)
+            if gcc_install_dir_flag:
+                runtime_cmake_args.extend(
+                    [
+                        define("CMAKE_C_FLAGS", gcc_install_dir_flag),
+                        define("CMAKE_CXX_FLAGS", gcc_install_dir_flag),
+                    ]
+                )
+
             cmake_args.extend(
                 [
                     define("LLVM_ENABLE_RUNTIMES", runtimes),
-                    define(
-                        "RUNTIMES_CMAKE_ARGS", [define("CMAKE_INSTALL_RPATH_USE_LINK_PATH", True)]
-                    ),
+                    define("RUNTIMES_CMAKE_ARGS", runtime_cmake_args),
                 ]
             )
 
@@ -1031,6 +1045,19 @@ def post_install(self):
         with working_dir(self.build_directory):
             install_tree("bin", join_path(self.prefix, "libexec", "llvm"))
 
+        cfg_files = []
+        if spec.satisfies("+clang"):
+            cfg_files.extend(("clang.cfg", "clang++.cfg"))
+        if spec.satisfies("@19: +flang"):
+            # The config file is `flang.cfg` even though the executable is `flang-new`.
+            # `--gcc-install-dir` / `--gcc-toolchain` support was only added in LLVM 19.
+            cfg_files.append("flang.cfg")
+        gcc_install_dir_flag = get_gcc_install_dir_flag(spec, self.compiler)
+        if gcc_install_dir_flag:
+            for cfg in cfg_files:
+                with open(os.path.join(self.prefix.bin, cfg), "w") as f:
+                    print(gcc_install_dir_flag, file=f)
+
     def llvm_config(self, *args, **kwargs):
         lc = Executable(self.prefix.bin.join("llvm-config"))
         if not kwargs.get("output"):
@@ -1042,6 +1069,18 @@ def llvm_config(self, *args, **kwargs):
             return ret
 
 
+def get_gcc_install_dir_flag(spec: Spec, compiler) -> Optional[str]:
+    """Get the --gcc-install-dir=... flag, so that clang does not do a system scan for GCC."""
+    if not spec.satisfies("@16: %gcc"):
+        return None
+    gcc = Executable(compiler.cc)
+    libgcc_path = gcc("-print-file-name=libgcc.a", output=str, fail_on_error=False).strip()
+    if not os.path.isabs(libgcc_path):
+        return None
+    libgcc_dir = os.path.dirname(libgcc_path)
+    return f"--gcc-install-dir={libgcc_dir}" if os.path.exists(libgcc_dir) else None
+
+
 def get_llvm_targets_to_build(spec):
     targets = spec.variants["targets"].value