[clang][Driver][Darwin] Optionally use xcselect to find macOS SDK

clang/CMakeLists.txt

@@ -214,6 +214,39 @@ if(GCC_INSTALL_PREFIX AND NOT USE_DEPRECATED_GCC_INSTALL_PREFIX)
     "See https://github.com/llvm/llvm-project/pull/77537 for detail.")
 endif()
 
+if(APPLE)
+  check_include_file(xcselect.h CLANG_HAVE_XCSELECT_H)
+  if(CLANG_HAVE_XCSELECT_H)
+    include(CheckSymbolExists)
+    list(APPEND CMAKE_REQUIRED_LIBRARIES xcselect)
+    check_symbol_exists(xcselect_host_sdk_path xcselect.h CLANG_HAVE_XCSELECT_HOST_SDK_PATH)
+    list(REMOVE_ITEM CMAKE_REQUIRED_LIBRARIES xcselect)
+  endif()
+endif()
+
+cmake_dependent_option(CLANG_USE_XCSELECT "Use libxcselect to find the macOS SDK." OFF
+  "APPLE;CLANG_HAVE_XCSELECT_HOST_SDK_PATH" OFF)
+
+if(DEFAULT_SYSROOT AND CLANG_USE_XCSELECT)
+  message(FATAL_ERROR "Setting DEFAULT_SYSROOT is incompatible with CLANG_USE_XCSELECT.")
+endif()
+
+if(CLANG_USE_XCSELECT)
+  set(XCSELECT_VALID_POLICIES LATEST MATCHING_ONLY MATCHING_PREFERRED)
+  set(CLANG_XCSELECT_HOST_SDK_POLICY "LATEST" CACHE STRING
+    "Policy to use for xcselect. One of: ${XCSELECT_VALID_POLICIES}")
+  set_property(CACHE CLANG_XCSELECT_HOST_SDK_POLICY PROPERTY STRINGS ${XCSELECT_VALID_POLICIES})
+  string(TOUPPER ${CLANG_XCSELECT_HOST_SDK_POLICY} CLANG_XCSELECT_HOST_SDK_POLICY)
+  list(JOIN XCSELECT_VALID_POLICIES "|" XCSELECT_POLICY_REGEX)
+  if(NOT CLANG_XCSELECT_HOST_SDK_POLICY MATCHES "^XCSELECT_HOST_SDK_POLICY_(${XCSELECT_POLICY_REGEX})$")
+    if(NOT CLANG_XCSELECT_HOST_SDK_POLICY IN_LIST XCSELECT_VALID_POLICIES)
+      message(FATAL_ERROR
+        "CLANG_XCSELECT_HOST_SDK_POLICY (${CLANG_XCSELECT_HOST_SDK_POLICY}) must be one of: ${XCSELECT_VALID_POLICIES}")
+    endif()
+    set(CLANG_XCSELECT_HOST_SDK_POLICY "XCSELECT_HOST_SDK_POLICY_${CLANG_XCSELECT_HOST_SDK_POLICY}")
+  endif()
+endif()
+
 set(ENABLE_LINKER_BUILD_ID OFF CACHE BOOL "pass --build-id to ld")
 
 set(ENABLE_X86_RELAX_RELOCATIONS ON CACHE BOOL

clang/include/clang/Config/config.h.cmake

@@ -86,4 +86,10 @@
 /* Whether CIR is built into Clang */
 #cmakedefine01 CLANG_ENABLE_CIR
 
+/* Whether to use xcselect to find the macOS SDK */
+#cmakedefine CLANG_USE_XCSELECT
+
+/* Policy to use for xcselect */
+#cmakedefine CLANG_XCSELECT_HOST_SDK_POLICY ${CLANG_XCSELECT_HOST_SDK_POLICY}
+
 #endif

clang/lib/Driver/CMakeLists.txt

@@ -14,6 +14,10 @@ if(WIN32)
   set(system_libs version)
 endif()
 
+if(CLANG_USE_XCSELECT)
+  set(system_libs xcselect)
+endif()
+
 add_clang_library(clangDriver
   Action.cpp
   Compilation.cpp

clang/lib/Driver/ToolChains/Darwin.cpp

@@ -29,6 +29,10 @@
 #include "llvm/TargetParser/Triple.h"
 #include <cstdlib> // ::getenv
 
+#ifdef CLANG_USE_XCSELECT
+#include <xcselect.h> // ::xcselect_host_sdk_path
+#endif
+
 using namespace clang::driver;
 using namespace clang::driver::tools;
 using namespace clang::driver::toolchains;
@@ -2257,17 +2261,26 @@ void Darwin::AddDeploymentTarget(DerivedArgList &Args) const {
     // Warn if the path does not exist.
     if (!getVFS().exists(A->getValue()))
       getDriver().Diag(clang::diag::warn_missing_sysroot) << A->getValue();
-  } else {
-    if (char *env = ::getenv("SDKROOT")) {
-      // We only use this value as the default if it is an absolute path,
-      // exists, and it is not the root path.
-      if (llvm::sys::path::is_absolute(env) && getVFS().exists(env) &&
-          StringRef(env) != "/") {
-        Args.append(Args.MakeSeparateArg(
-            nullptr, Opts.getOption(options::OPT_isysroot), env));
-      }
+  } else if (const char *env = ::getenv("SDKROOT"); env && *env) {
+    // We only use this value as the default if it is an absolute path,
+    // exists, and it is not the root path.
+    if (llvm::sys::path::is_absolute(env) && getVFS().exists(env) &&
+        StringRef(env) != "/") {
+      Args.append(Args.MakeSeparateArg(
+          nullptr, Opts.getOption(options::OPT_isysroot), env));
     }
   }
+#ifdef CLANG_USE_XCSELECT
+  else if (const char *env = ::getenv("CLANG_NO_XCSELECT");
+           getTriple().isMacOSX() && (!env || !*env)) {
+    if (char *p;
+        !::xcselect_host_sdk_path(CLANG_XCSELECT_HOST_SDK_POLICY, &p)) {
+      Args.append(Args.MakeSeparateArg(
+          nullptr, Opts.getOption(options::OPT_isysroot), p));
+      ::free(p);
+    }
+  }
+#endif
 
   // Read the SDKSettings.json file for more information, like the SDK version
   // that we can pass down to the compiler.

clang/test/lit.cfg.py

@@ -366,3 +366,7 @@ def calculate_arch_features(arch_string):
 # possibly be present in system and user configuration files, so disable
 # default configs for the test runs.
 config.environment["CLANG_NO_DEFAULT_CONFIG"] = "1"
+
+# Configuring clang with CLANG_USE_XCSELECT=ON breaks some tests, so disable
+# its behaviour while running tests.
+config.environment["CLANG_NO_XCSELECT"] = "1"