[Clang][Driver] Improve config file handling on Darwin

On Darwin, the default target triple contains the version of the kernel:

    ❯ clang --print-target-triple
    arm64-apple-darwin24.0.0
    ❯ uname -r
    24.0.0

This makes writing config files for the target triple rather cumbersome,
because they require including the full version of the kernel in the
filename when one generally cares only about the major version if at
all.

Let's improve this by also checking for major-versioned and unversioned
`.cfg` files if we weren't able to find a `.cfg` file for the full
triple when targetting Darwin.

Author: carlocab

clang/lib/Driver/Driver.cpp

@@ -95,6 +95,7 @@
 #include "llvm/TargetParser/Host.h"
 #include "llvm/TargetParser/RISCVISAInfo.h"
 #include <cstdlib> // ::getenv
+#include <cstring>
 #include <map>
 #include <memory>
 #include <optional>
@@ -1222,6 +1223,24 @@ bool Driver::loadDefaultConfigFiles(llvm::cl::ExpansionContext &ExpCtx) {
   if (ExpCtx.findConfigFile(CfgFileName, CfgFilePath))
     return readConfigFile(CfgFilePath, ExpCtx);
 
+  // On Darwin, try a major-versioned triple then an unversioned triple.
+  auto pos = Triple.find("-apple-darwin");
+  auto kernelVersionStart = pos + std::strlen("-apple-darwin");
+  if (pos != Triple.npos && Triple.length() > kernelVersionStart) {
+    // First, find the major-versioned triple (e.g. arm64-apple-darwin24).
+    auto T = Triple.substr(0, Triple.find(".", kernelVersionStart));
+    CfgFileName = T + ".cfg";
+    if (ExpCtx.findConfigFile(CfgFileName, CfgFilePath))
+      return readConfigFile(CfgFilePath, ExpCtx);
+
+    // If that is not available, try an unversioned triple
+    // (e.g. arm64-apple-darwin).
+    T = Triple.substr(0, kernelVersionStart);
+    CfgFileName = T + ".cfg";
+    if (ExpCtx.findConfigFile(CfgFileName, CfgFilePath))
+      return readConfigFile(CfgFilePath, ExpCtx);
+  }
+
   // If we were unable to find a config file deduced from executable name,
   // that is not an error.
   return false;

clang/test/Driver/config-file-darwin.c

@@ -0,0 +1,24 @@
+// REQUIRES: shell
+
+// RUN: unset CLANG_NO_DEFAULT_CONFIG
+// RUN: rm -rf %t && mkdir %t
+
+//--- Major-versioned config files are used when targetting *-apple-darwin*
+//
+// RUN: mkdir -p %t/testbin
+// RUN: ln -s %clang %t/testbin/x86_64-apple-darwin24.0.0-clang
+// RUN: echo "-Werror" > %t/testbin/x86_64-apple-darwin24.cfg
+// RUN: %t/testbin/x86_64-apple-darwin24.0.0-clang --config-system-dir= --config-user-dir= -c -no-canonical-prefixes -### %s 2>&1 | FileCheck %s -check-prefix CHECK-MAJOR-VERSIONED
+//
+// CHECK-MAJOR-VERSIONED: Configuration file: {{.*}}/testbin/x86_64-apple-darwin24.cfg
+// CHECK-MAJOR-VERSIONED: -Werror
+
+//--- Unversioned config files are used when targetting *-apple-darwin*
+//
+// RUN: mkdir -p %t/testbin
+// RUN: ln -s %clang %t/testbin/arm64-apple-darwin23.1.2-clang
+// RUN: echo "-Werror" > %t/testbin/arm64-apple-darwin.cfg
+// RUN: %t/testbin/arm64-apple-darwin23.1.2-clang --config-system-dir= --config-user-dir= -c -no-canonical-prefixes -### %s 2>&1 | FileCheck %s -check-prefix CHECK-UNVERSIONED
+//
+// CHECK-UNVERSIONED: Configuration file: {{.*}}/testbin/arm64-apple-darwin.cfg
+// CHECK-UNVERSIONED: -Werror