Merge pull request #62159 from edymtt/use-libcxx-from-darwin-sdks-llvm-product

Use libcxx from Darwin SDKs when building LLVM and Swift (LLVM product)

Those are present since Xcode 12.5, so we don't need to copy them
anymore from the toolchain

In this scenario, clean up any existing symlink in incremental builds to
avoid masking or causing errors in the future.

Took the chance to extract this logic to a different function in an attempt to improve readability

To ease review, the PR has two commits -- the first to extract the logic as is (showing that I did not alter the existing logic) and the second one to change its behaviour for Darwin (and address a minor issue when printing about the creation of symlinks)

Addresses rdar://102387542

Author: edymtt

utils/swift_build_support/swift_build_support/products/llvm.py

@@ -346,19 +346,42 @@ def build(self, host_target):
         self.cmake_options.extend(host_config.cmake_options)
         self.cmake_options.extend(llvm_cmake_options)
 
+        self._handle_cxx_headers(host_target, platform)
+
+        self.build_with_cmake(build_targets, self.args.llvm_build_variant, [])
+
+        # copy over the compiler-rt builtins for iOS/tvOS/watchOS to ensure
+        # that Swift's stdlib can use compiler-rt builtins when targeting
+        # iOS/tvOS/watchOS.
+        if self.args.build_llvm and system() == 'Darwin':
+            self.copy_embedded_compiler_rt_builtins_from_darwin_host_toolchain(
+                self.build_dir)
+
+    def _handle_cxx_headers(self, host_target, platform):
         # When we are building LLVM create symlinks to the c++ headers. We need
         # to do this before building LLVM since compiler-rt depends on being
         # built with the just built clang compiler. These are normally put into
         # place during the cmake step of LLVM's build when libcxx is in
         # tree... but we are not building llvm with libcxx in tree when we build
         # swift. So we need to do configure's work here.
-        host_cxx_headers_dir = None
         if system() == 'Darwin':
-            host_cxx_dir = os.path.dirname(self.toolchain.cxx)
-            host_cxx_headers_dir = os.path.join(host_cxx_dir, os.pardir, os.pardir,
-                                                'usr', 'include', 'c++')
+            # We don't need this for Darwin since libcxx is present in SDKs present
+            # in Xcode 12.5 and onward (build-script requires Xcode 13.0 at a minimum),
+            # and clang knows how to find it there
+            # However, we should take care of removing the symlink
+            # laid down by a previous invocation, so to avoid failures
+            # finding c++ headers should the target folder become invalid
+            cxx_include_symlink = os.path.join(self.build_dir, 'include', 'c++')
+            if os.path.islink(cxx_include_symlink):
+                print('removing the symlink to system headers in the local '
+                      f'clang build directory {cxx_include_symlink} .',
+                      flush=True)
+                shell.remove(cxx_include_symlink)
+
+            return
 
-        elif system() == 'Haiku':
+        host_cxx_headers_dir = None
+        if system() == 'Haiku':
             host_cxx_headers_dir = '/boot/system/develop/headers/c++'
 
         # This means we're building natively on Android in the Termux
@@ -384,18 +407,9 @@ def build(self, host_target):
             os.makedirs(built_cxx_include_dir)
         print('symlinking the system headers ({}) into the local '
               'clang build directory ({}).'.format(
-                  host_cxx_headers_dir, built_cxx_include_dir))
+                  host_cxx_headers_dir, built_cxx_include_dir), flush=True)
         shell.call(['ln', '-s', '-f', host_cxx_headers_dir, built_cxx_include_dir])
 
-        self.build_with_cmake(build_targets, self.args.llvm_build_variant, [])
-
-        # copy over the compiler-rt builtins for iOS/tvOS/watchOS to ensure
-        # that Swift's stdlib can use compiler-rt builtins when targeting
-        # iOS/tvOS/watchOS.
-        if self.args.build_llvm and system() == 'Darwin':
-            self.copy_embedded_compiler_rt_builtins_from_darwin_host_toolchain(
-                self.build_dir)
-
     def should_test(self, host_target):
         """should_test() -> Bool
 

utils/swift_build_support/swift_build_support/shell.py

@@ -202,6 +202,15 @@ def symlink(source, dest, dry_run=None, echo=True):
     os.symlink(source, dest)
 
 
+def remove(path, dry_run=None, echo=True):
+    dry_run = _coerce_dry_run(dry_run)
+    if dry_run or echo:
+        _echo_command(dry_run, ['rm', path])
+    if dry_run:
+        return
+    os.remove(path)
+
+
 # Initialized later
 lock = None