Begin building libc++ and libc++abi runtimes on demand

MODULE.bazel

@@ -118,6 +118,9 @@ http_archive(
         "@carbon//bazel/llvm_project:0002_Added_Bazel_build_for_compiler_rt_fuzzer.patch",
         "@carbon//bazel/llvm_project:0003_Comment_out_unloaded_proto_library_dependencies.patch",
         "@carbon//bazel/llvm_project:0004_Introduce_basic_sources_exporting_for_libunwind.patch",
+        "@carbon//bazel/llvm_project:0005_Introduce_basic_sources_exporting_for_libcxx_and_libcxxabi.patch",
+        "@carbon//bazel/llvm_project:0006_Add_a_filegroup_containing__all__sources_to_the_libc_build_rules.patch",
+        "@carbon//bazel/llvm_project:0007_Remove_target_compatibility_restrictions_for_float128.patch",
     ],
     strip_prefix = "llvm-project-{0}".format(llvm_project_version),
     urls = ["https://github.com/llvm/llvm-project/archive/{0}.tar.gz".format(llvm_project_version)],

bazel/check_deps/check_non_test_cc_deps.py

@@ -41,11 +41,14 @@
         # Other packages in the LLVM project shouldn't be accidentally used
         # in Carbon. We can expand the above list if use cases emerge.
         if package not in (
-            "llvm",
-            "lld",
             "clang",
             "clang-tools-extra/clangd",
+            "libc",
+            "libcxx",
+            "libcxxabi",
             "libunwind",
+            "lld",
+            "llvm",
             # While this is in a `third_party` directory, its code is documented
             # as part of LLVM and for use in compiler-rt.
             "third-party/siphash",

bazel/llvm_project/0005_Introduce_basic_sources_exporting_for_libcxx_and_libcxxabi.patch

@@ -0,0 +1,96 @@
+Commit ID: e4ff7299fe7e35e70ba79f5d8e2c58658cfba678
+Change ID: mstnwoqruyypnoouksnyqssllrsozpos
+Bookmarks: bz-libcxx bz-libcxx@git bz-libcxx@origin
+Author   : Chandler Carruth <[email protected]> (2025-09-25 22:55:26)
+Committer: Chandler Carruth <[email protected]> (2025-11-22 09:23:52)
+
+    Introduce basic sources exporting for libcxx and libcxxabi
+
+    This exports the source files directly so that they can be used to build
+    a libcxx runtime library on demand.
+
+diff --git a/utils/bazel/llvm-project-overlay/libcxx/BUILD.bazel b/utils/bazel/llvm-project-overlay/libcxx/BUILD.bazel
+new file mode 100644
+index 0000000000..a81a64c649
+--- /dev/null
++++ b/utils/bazel/llvm-project-overlay/libcxx/BUILD.bazel
+@@ -0,0 +1,49 @@
++# This file is licensed under the Apache License v2.0 with LLVM Exceptions.
++# See https://llvm.org/LICENSE.txt for license information.
++# SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
++
++licenses(["notice"])
++
++package(
++    default_visibility = ["//visibility:public"],
++)
++
++exports_files(["include/__config_site.in"])
++
++exports_files(["vendor/llvm/default_assertion_handler.in"])
++
++filegroup(
++    name = "libcxx_hdrs",
++    srcs = glob(
++        [
++            # Top level includes and those in `experimental` and `ext` sometimes
++            # have no extension.
++            "include/*",
++            "include/experimental/*",
++            "include/ext/*",
++
++            # Implementation detail headers all use `.h` extensions
++            "include/**/*.h",
++        ],
++        exclude = [
++            # Omit CMake and CMake-configured files that get caught by the
++            # extension-less patterns.
++            "**/*.in",
++            "**/CMakeLists.txt",
++
++            # Omit C++03 compatibility headers as current users don't need them.
++            "include/__cxx03/**",
++        ],
++    ),
++)
++
++filegroup(
++    name = "libcxx_srcs",
++    srcs = glob(
++        [
++            "src/**/*.cpp",
++            "src/**/*.h",
++            "src/**/*.ipp",
++        ],
++    ),
++)
+diff --git a/utils/bazel/llvm-project-overlay/libcxxabi/BUILD.bazel b/utils/bazel/llvm-project-overlay/libcxxabi/BUILD.bazel
+new file mode 100644
+index 0000000000..cf491e4e46
+--- /dev/null
++++ b/utils/bazel/llvm-project-overlay/libcxxabi/BUILD.bazel
+@@ -0,0 +1,24 @@
++# This file is licensed under the Apache License v2.0 with LLVM Exceptions.
++# See https://llvm.org/LICENSE.txt for license information.
++# SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
++
++licenses(["notice"])
++
++package(
++    default_visibility = ["//visibility:public"],
++)
++
++filegroup(
++    name = "libcxxabi_hdrs",
++    srcs = glob(["include/*.h"]),
++)
++
++filegroup(
++    name = "libcxxabi_srcs",
++    srcs = glob([
++        "src/**/*.cpp",
++        "src/**/*.def",
++        "src/**/*.inc",
++        "src/**/*.h",
++    ]),
++)

bazel/llvm_project/0006_Add_a_filegroup_containing__all__sources_to_the_libc_build_rules.patch

@@ -0,0 +1,31 @@
+Commit ID: e524424ed32e945bdced8c19ca480f461d04397a
+Change ID: lslousqwyovpkqlxvyrzmmypxkxxulwn
+Bookmarks: push-lslousqwyovp push-lslousqwyovp@git push-lslousqwyovp@origin
+Author   : Chandler Carruth <[email protected]> (2025-11-20 03:05:36)
+Committer: Chandler Carruth <[email protected]> (2025-11-22 09:23:52)
+
+    Add a filegroup containing _all_ sources to the libc build rules
+
+    These rules already expose a filegroup containing the _dependencies_,
+    but that misses the source files directly in the top level library.
+
+    Without this filegroup, there isn't a way to access the source files
+    used by libcxx when building it, etc.
+
+diff --git a/utils/bazel/llvm-project-overlay/libc/libc_build_rules.bzl b/utils/bazel/llvm-project-overlay/libc/libc_build_rules.bzl
+index 0f2965369c..c871334dd9 100644
+--- a/utils/bazel/llvm-project-overlay/libc/libc_build_rules.bzl
++++ b/utils/bazel/llvm-project-overlay/libc/libc_build_rules.bzl
+@@ -247,6 +247,12 @@
+         **kwargs
+     )
+
++    _libc_srcs_filegroup(
++        name = name + "_hdrs",
++        libs = [":" + name],
++        enforce_headers_only = True,
++    )
++
+ def libc_generated_header(name, hdr, yaml_template, other_srcs = []):
+     """Generates a libc header file from YAML template.
+

bazel/llvm_project/0007_Remove_target_compatibility_restrictions_for_float128.patch

@@ -0,0 +1,36 @@
+Commit ID: 94325bae07213d2dd8263945f5362720dee7bcb9
+Change ID: kskkxwvnyqwuzkswkuxpnmyovqxoypnz
+Bookmarks: push-kskkxwvnyqwu push-kskkxwvnyqwu@git push-kskkxwvnyqwu@origin
+Author   : Chandler Carruth <[email protected]> (2025-11-24 07:26:06)
+Committer: Chandler Carruth <[email protected]> (2025-11-24 07:40:58)
+
+    Remove target compatibility restrictions for float128
+
+    The restrictions here aren't nearly as much about the OS as the compiler
+    and architecture, but the Bazel restriction was OS-based. Everything
+    seems to work well on even Arm64 macOS, and I would expect most BSDs and
+    other OSes to work well with Clang's support on x86-64.
+
+    The source code here already handles detecting when there is compiler
+    support for the type. And the users of this don't `select` or do
+    anything else to conditionally include the header, so it seems better to
+    not restrict access to the header from the build system, and instead
+    continue making the source code compatible or a no-op on relevant
+    configurations.
+
+diff --git a/utils/bazel/llvm-project-overlay/libc/BUILD.bazel b/utils/bazel/llvm-project-overlay/libc/BUILD.bazel
+index bd48222856..e3962d9b5f 100644
+--- a/utils/bazel/llvm-project-overlay/libc/BUILD.bazel
++++ b/utils/bazel/llvm-project-overlay/libc/BUILD.bazel
+@@ -171,11 +171,6 @@
+ libc_support_library(
+     name = "llvm_libc_types_float128",
+     hdrs = ["include/llvm-libc-types/float128.h"],
+-    target_compatible_with = select({
+-        "@platforms//os:linux": [],
+-        "@platforms//os:windows": [],
+-        "//conditions:default": ["@platforms//:incompatible"],
+-    }),
+     deps = [":llvm_libc_macros_float_macros"],
+ )
+

toolchain/base/runtime_sources.bzl

@@ -38,7 +38,9 @@ BUILTINS_FILEGROUPS = {
 }
 
 RUNTIMES_FILEGROUPS = {
-    "libunwind_srcs": "@llvm-project//libunwind:libunwind_srcs",
+    "libcxx": "@llvm-project//libcxx:libcxx_srcs",
+    "libcxxabi": "@llvm-project//libcxxabi:libcxxabi_srcs",
+    "libunwind": "@llvm-project//libunwind:libunwind_srcs",
 }
 
 _TEMPLATE = """
@@ -89,8 +91,16 @@ inline constexpr llvm::StringLiteral BuiltinsI386Srcs[] = {{
 {i386_srcs}
 }};
 
+constexpr inline llvm::StringLiteral LibcxxSrcs[] = {{
+{libcxx}
+}};
+
+constexpr inline llvm::StringLiteral LibcxxabiSrcs[] = {{
+{libcxxabi}
+}};
+
 constexpr inline llvm::StringLiteral LibunwindSrcs[] = {{
-{libunwind_srcs}
+{libunwind}
 }};
 
 }}  // namespace Carbon::RuntimeSources
@@ -118,14 +128,14 @@ def _get_path(file_attr, to_path_fn):
 
     return '"{0}"'.format(to_path_fn(files[0]))
 
-def _get_paths(files_attr, to_path_fn):
+def _get_paths(files_attr, to_path_fn, prefix = ""):
     files = []
     for src in files_attr:
         files.extend(src[DefaultInfo].files.to_list())
         files.extend(src[DefaultInfo].default_runfiles.files.to_list())
 
     return "\n".join([
-        '    "{0}",'.format(to_path_fn(f))
+        '    "{0}{1}",'.format(prefix, to_path_fn(f))
         for f in files
     ])
 
@@ -138,7 +148,9 @@ def _generate_runtime_sources_h_rule(ctx):
         k: _get_paths(getattr(ctx.attr, "_" + k), _builtins_path)
         for k in BUILTINS_FILEGROUPS.keys()
     } | {
-        k: _get_paths(getattr(ctx.attr, "_" + k), _runtimes_path)
+        # Other runtimes are installed under separate directories named the same
+        # as their key.
+        k: _get_paths(getattr(ctx.attr, "_" + k), _runtimes_path, k + "/")
         for k in RUNTIMES_FILEGROUPS.keys()
     })))
     return [DefaultInfo(files = depset([h_file]))]

toolchain/driver/BUILD

@@ -82,7 +82,7 @@ cc_test(
 
 cc_test(
     name = "clang_runtimes_test",
-    size = "small",
+    size = "medium",
     srcs = ["clang_runtimes_test.cpp"],
     data = ["//toolchain/install:install_data"],
     deps = [

toolchain/driver/build_runtimes_subcommand.cpp

@@ -97,9 +97,13 @@ auto BuildRuntimesSubcommand::RunInternal(DriverEnv& driver_env)
   ClangArchiveRuntimesBuilder<Runtimes::LibUnwind> lib_unwind_builder(
       &runner, driver_env.thread_pool, llvm::Triple(features.target),
       &runtimes);
+  ClangArchiveRuntimesBuilder<Runtimes::Libcxx> libcxx_builder(
+      &runner, driver_env.thread_pool, llvm::Triple(features.target),
+      &runtimes);
 
   CARBON_RETURN_IF_ERROR(std::move(resource_dir_builder).Wait());
   CARBON_RETURN_IF_ERROR(std::move(lib_unwind_builder).Wait());
+  CARBON_RETURN_IF_ERROR(std::move(libcxx_builder).Wait());
 
   return runtimes.base_path();
 }