[clang] [compiler-rt] [clang][Darwin] Prefer the toolchain-provided libc++.dylib if there is one (PR #170930)
Louis Dionne via llvm-commits
llvm-commits at lists.llvm.org
Fri Dec 5 13:54:08 PST 2025
https://github.com/ldionne created https://github.com/llvm/llvm-project/pull/170930
When libc++ is bootstrapped with clang, the resulting clang uses the just-built libc++ headers from <install>/bin/../include/c++/v1. However, before this patch, clang would still use the system-provided libc++.dylib (usually in the Apple SDK) because it would fail to add the corresponding linker flag to find the just-built libc++. After this patch, Clang will instead link against the toolchain provided `libc++.dylib` in `<install>/lib` if it exists, which results in programs being compiled and linked against corresponding libc++ headers and dylib.
This is a re-application of #170303 which was temporarily reverted due to build breakages. This also contains minor fixes to the tests to make the more portable across platforms.
Fixes #77653
rdar://107060541
>From fd318711d70258c3a46f8d34f46e711d70604a3b Mon Sep 17 00:00:00 2001
From: Louis Dionne <ldionne.2 at gmail.com>
Date: Fri, 5 Dec 2025 16:46:16 -0500
Subject: [PATCH] [clang][Darwin] Prefer the toolchain-provided libc++.dylib if
there is one
When libc++ is bootstrapped with clang, the resulting clang uses the
just-built libc++ headers from <install>/bin/../include/c++/v1. However,
before this patch, clang would still use the system-provided libc++.dylib
(usually in the Apple SDK) because it would fail to add the corresponding
linker flag to find the just-built libc++. After this patch, Clang will
instead link against the toolchain provided `libc++.dylib` in
`<install>/lib` if it exists, which results in programs being compiled
and linked against corresponding libc++ headers and dylib.
This is a re-application of #170303 which was temporarily reverted
due to build breakages. This also contains minor fixes to the tests
to make the more portable across platforms.
Fixes #77653
rdar://107060541
---
clang/lib/Driver/ToolChains/Darwin.cpp | 44 ++++++++-
.../usr/lib/libc++.dylib | 0
.../usr/lib/libc++experimental.a | 0
.../usr/bin/.keep | 0
.../usr/include/c++/v1/.keep | 0
.../usr/lib/libc++.a | 0
.../usr/lib/libc++experimental.a | 0
.../Driver/darwin-header-search-libcxx.cpp | 4 +-
clang/test/Driver/darwin-link-libcxx.cpp | 95 +++++++++++++++++++
.../test/Driver/experimental-library-flag.cpp | 8 +-
compiler-rt/cmake/config-ix.cmake | 15 +++
11 files changed, 158 insertions(+), 8 deletions(-)
create mode 100644 clang/test/Driver/Inputs/basic_darwin_toolchain/usr/lib/libc++.dylib
create mode 100644 clang/test/Driver/Inputs/basic_darwin_toolchain/usr/lib/libc++experimental.a
create mode 100644 clang/test/Driver/Inputs/basic_darwin_toolchain_static/usr/bin/.keep
create mode 100644 clang/test/Driver/Inputs/basic_darwin_toolchain_static/usr/include/c++/v1/.keep
create mode 100644 clang/test/Driver/Inputs/basic_darwin_toolchain_static/usr/lib/libc++.a
create mode 100644 clang/test/Driver/Inputs/basic_darwin_toolchain_static/usr/lib/libc++experimental.a
create mode 100644 clang/test/Driver/darwin-link-libcxx.cpp
diff --git a/clang/lib/Driver/ToolChains/Darwin.cpp b/clang/lib/Driver/ToolChains/Darwin.cpp
index fc3cd9030f71d..30c53389dc22f 100644
--- a/clang/lib/Driver/ToolChains/Darwin.cpp
+++ b/clang/lib/Driver/ToolChains/Darwin.cpp
@@ -2846,11 +2846,49 @@ void AppleMachO::AddCXXStdlibLibArgs(const ArgList &Args,
CXXStdlibType Type = GetCXXStdlibType(Args);
switch (Type) {
- case ToolChain::CST_Libcxx:
- CmdArgs.push_back("-lc++");
+ case ToolChain::CST_Libcxx: {
+ // On Darwin, we prioritize a libc++ located in the toolchain to a libc++
+ // located in the sysroot. Unlike the driver for most other platforms, on
+ // Darwin we do that by explicitly passing the library path to the linker
+ // to avoid having to add the toolchain's `lib/` directory to the linker
+ // search path, which would make other libraries findable as well.
+ //
+ // Prefering the toolchain library over the sysroot library matches the
+ // behavior we have for headers, where we prefer headers in the toolchain
+ // over headers in the sysroot if there are any. Note that it's important
+ // for the header search path behavior to match the link-time search path
+ // behavior to ensure that we link the program against a library that
+ // matches the headers that were used to compile it.
+ //
+ // Otherwise, we end up compiling against some set of headers and then
+ // linking against a different library (which, confusingly, shares the same
+ // name) which may have been configured with different options, be at a
+ // different version, etc.
+ SmallString<128> InstallLib = llvm::sys::path::parent_path(getDriver().Dir);
+ llvm::sys::path::append(InstallLib, "lib"); // <install>/lib
+ auto Link = [&](StringRef Library) {
+ SmallString<128> Shared(InstallLib);
+ llvm::sys::path::append(Shared,
+ SmallString<4>("lib") + Library + ".dylib");
+ SmallString<128> Static(InstallLib);
+ llvm::sys::path::append(Static, SmallString<4>("lib") + Library + ".a");
+ SmallString<32> Relative("-l");
+ Relative += Library;
+
+ if (getVFS().exists(Shared)) {
+ CmdArgs.push_back(Args.MakeArgString(Shared));
+ } else if (getVFS().exists(Static)) {
+ CmdArgs.push_back(Args.MakeArgString(Static));
+ } else {
+ CmdArgs.push_back(Args.MakeArgString(Relative));
+ }
+ };
+
+ Link("c++");
if (Args.hasArg(options::OPT_fexperimental_library))
- CmdArgs.push_back("-lc++experimental");
+ Link("c++experimental");
break;
+ }
case ToolChain::CST_Libstdcxx:
// Unfortunately, -lstdc++ doesn't always exist in the standard search path;
diff --git a/clang/test/Driver/Inputs/basic_darwin_toolchain/usr/lib/libc++.dylib b/clang/test/Driver/Inputs/basic_darwin_toolchain/usr/lib/libc++.dylib
new file mode 100644
index 0000000000000..e69de29bb2d1d
diff --git a/clang/test/Driver/Inputs/basic_darwin_toolchain/usr/lib/libc++experimental.a b/clang/test/Driver/Inputs/basic_darwin_toolchain/usr/lib/libc++experimental.a
new file mode 100644
index 0000000000000..e69de29bb2d1d
diff --git a/clang/test/Driver/Inputs/basic_darwin_toolchain_static/usr/bin/.keep b/clang/test/Driver/Inputs/basic_darwin_toolchain_static/usr/bin/.keep
new file mode 100644
index 0000000000000..e69de29bb2d1d
diff --git a/clang/test/Driver/Inputs/basic_darwin_toolchain_static/usr/include/c++/v1/.keep b/clang/test/Driver/Inputs/basic_darwin_toolchain_static/usr/include/c++/v1/.keep
new file mode 100644
index 0000000000000..e69de29bb2d1d
diff --git a/clang/test/Driver/Inputs/basic_darwin_toolchain_static/usr/lib/libc++.a b/clang/test/Driver/Inputs/basic_darwin_toolchain_static/usr/lib/libc++.a
new file mode 100644
index 0000000000000..e69de29bb2d1d
diff --git a/clang/test/Driver/Inputs/basic_darwin_toolchain_static/usr/lib/libc++experimental.a b/clang/test/Driver/Inputs/basic_darwin_toolchain_static/usr/lib/libc++experimental.a
new file mode 100644
index 0000000000000..e69de29bb2d1d
diff --git a/clang/test/Driver/darwin-header-search-libcxx.cpp b/clang/test/Driver/darwin-header-search-libcxx.cpp
index cc8ec9ceb89b3..e8985a4e22b2f 100644
--- a/clang/test/Driver/darwin-header-search-libcxx.cpp
+++ b/clang/test/Driver/darwin-header-search-libcxx.cpp
@@ -92,7 +92,7 @@
// CHECK-LIBCXX-SYSROOT_AND_TOOLCHAIN-1: "-internal-isystem" "[[TOOLCHAIN]]/usr/bin/../include/c++/v1"
// CHECK-LIBCXX-SYSROOT_AND_TOOLCHAIN-1-NOT: "-internal-isystem" "[[SYSROOT]]/usr/include/c++/v1"
-// Make sure that using -nostdinc, -nostdinc++ or -nostdlib will drop both the toolchain
+// Make sure that using -nostdinc, -nostdinc++ or -nostdlibinc will drop both the toolchain
// C++ include path and the sysroot one.
//
// RUN: %clang -### %s -fsyntax-only 2>&1 \
@@ -116,7 +116,7 @@
// RUN: -isysroot %S/Inputs/basic_darwin_sdk_usr_cxx_v1 \
// RUN: -stdlib=platform \
// RUN: -nostdinc++ \
-// RUN: | FileCheck -DSYSROOT=%S/Inputs/basic_darwin_sdk_usr \
+// RUN: | FileCheck -DSYSROOT=%S/Inputs/basic_darwin_sdk_usr_cxx_v1 \
// RUN: -DTOOLCHAIN=%S/Inputs/basic_darwin_toolchain \
// RUN: --check-prefix=CHECK-LIBCXX-NOSTDINCXX %s
// CHECK-LIBCXX-NOSTDINCXX: "-cc1"
diff --git a/clang/test/Driver/darwin-link-libcxx.cpp b/clang/test/Driver/darwin-link-libcxx.cpp
new file mode 100644
index 0000000000000..fee48f43fb14b
--- /dev/null
+++ b/clang/test/Driver/darwin-link-libcxx.cpp
@@ -0,0 +1,95 @@
+// UNSUPPORTED: system-windows
+
+// Tests to check that we link against the toolchain-provided libc++ built library when it is provided.
+// This is required to prefer the toolchain's libc++ over the system's libc++, which matches the behavior
+// we have for header search paths.
+//
+// Note that we explicitly specify the linker path to use to make this test portable across platforms.
+
+// When libc++.dylib is NOT in the toolchain, we should use -lc++ and fall back to the libc++
+// in the sysroot.
+//
+// (1) Without -fexperimental-library.
+// RUN: %clangxx -### %s 2>&1 \
+// RUN: --target=x86_64-apple-darwin \
+// RUN: --ld-path=%S/Inputs/lld/ld.lld \
+// RUN: -ccc-install-dir %S/Inputs/basic_darwin_toolchain_no_libcxx/usr/bin \
+// RUN: | FileCheck -DTOOLCHAIN=%S/Inputs/basic_darwin_toolchain_no_libcxx \
+// RUN: -DINPUTS=%S/Inputs \
+// RUN: --check-prefix=CHECK-1 %s
+// CHECK-1: "[[INPUTS]]/lld/ld.lld"
+// CHECK-1: "-lc++"
+// CHECK-1-NOT: "[[TOOLCHAIN]]/usr/lib"
+//
+// (2) With -fexperimental-library.
+// RUN: %clangxx -### %s 2>&1 \
+// RUN: --target=x86_64-apple-darwin \
+// RUN: --ld-path=%S/Inputs/lld/ld.lld \
+// RUN: -ccc-install-dir %S/Inputs/basic_darwin_toolchain_no_libcxx/usr/bin \
+// RUN: -fexperimental-library \
+// RUN: | FileCheck -DTOOLCHAIN=%S/Inputs/basic_darwin_toolchain_no_libcxx \
+// RUN: -DINPUTS=%S/Inputs \
+// RUN: --check-prefix=CHECK-2 %s
+// CHECK-2: "[[INPUTS]]/lld/ld.lld"
+// CHECK-2: "-lc++" "-lc++experimental"
+// CHECK-2-NOT: "[[TOOLCHAIN]]/usr/lib"
+
+// When we have libc++.dylib in the toolchain, it should be used over the one in the sysroot.
+// There are a few cases worth testing.
+//
+// (1) Without -fexperimental-library.
+// RUN: %clangxx -### %s 2>&1 \
+// RUN: --target=x86_64-apple-darwin \
+// RUN: --ld-path=%S/Inputs/lld/ld.lld \
+// RUN: -ccc-install-dir %S/Inputs/basic_darwin_toolchain/usr/bin \
+// RUN: | FileCheck -DTOOLCHAIN=%S/Inputs/basic_darwin_toolchain \
+// RUN: -DINPUTS=%S/Inputs \
+// RUN: --check-prefix=CHECK-3 %s
+// CHECK-3: "[[INPUTS]]/lld/ld.lld"
+// CHECK-3: "[[TOOLCHAIN]]/usr/lib/libc++.dylib"
+// CHECK-3-NOT: "-lc++"
+//
+// (2) With -fexperimental-library.
+// RUN: %clangxx -### %s 2>&1 \
+// RUN: --target=x86_64-apple-darwin \
+// RUN: --ld-path=%S/Inputs/lld/ld.lld \
+// RUN: -ccc-install-dir %S/Inputs/basic_darwin_toolchain/usr/bin \
+// RUN: -fexperimental-library \
+// RUN: | FileCheck -DTOOLCHAIN=%S/Inputs/basic_darwin_toolchain \
+// RUN: -DINPUTS=%S/Inputs \
+// RUN: --check-prefix=CHECK-4 %s
+// CHECK-4: "[[INPUTS]]/lld/ld.lld"
+// CHECK-4: "[[TOOLCHAIN]]/usr/lib/libc++.dylib"
+// CHECK-4: "[[TOOLCHAIN]]/usr/lib/libc++experimental.a"
+// CHECK-4-NOT: "-lc++"
+// CHECK-4-NOT: "-lc++experimental"
+
+// When we have libc++.a in the toolchain instead of libc++.dylib, it should be
+// used over the one in the sysroot.
+//
+// (1) Without -fexperimental-library.
+// RUN: %clangxx -### %s 2>&1 \
+// RUN: --target=x86_64-apple-darwin \
+// RUN: --ld-path=%S/Inputs/lld/ld.lld \
+// RUN: -ccc-install-dir %S/Inputs/basic_darwin_toolchain_static/usr/bin \
+// RUN: | FileCheck -DTOOLCHAIN=%S/Inputs/basic_darwin_toolchain_static \
+// RUN: -DINPUTS=%S/Inputs \
+// RUN: --check-prefix=CHECK-5 %s
+// CHECK-5: "[[INPUTS]]/lld/ld.lld"
+// CHECK-5: "[[TOOLCHAIN]]/usr/lib/libc++.a"
+// CHECK-5-NOT: "-lc++"
+//
+// (2) With -fexperimental-library.
+// RUN: %clangxx -### %s 2>&1 \
+// RUN: --target=x86_64-apple-darwin \
+// RUN: --ld-path=%S/Inputs/lld/ld.lld \
+// RUN: -ccc-install-dir %S/Inputs/basic_darwin_toolchain_static/usr/bin \
+// RUN: -fexperimental-library \
+// RUN: | FileCheck -DTOOLCHAIN=%S/Inputs/basic_darwin_toolchain_static \
+// RUN: -DINPUTS=%S/Inputs \
+// RUN: --check-prefix=CHECK-6 %s
+// CHECK-6: "[[INPUTS]]/lld/ld.lld"
+// CHECK-6: "[[TOOLCHAIN]]/usr/lib/libc++.a"
+// CHECK-6: "[[TOOLCHAIN]]/usr/lib/libc++experimental.a"
+// CHECK-6-NOT: "-lc++"
+// CHECK-6-NOT: "-lc++experimental"
diff --git a/clang/test/Driver/experimental-library-flag.cpp b/clang/test/Driver/experimental-library-flag.cpp
index 62b007516897e..0546a09d5518b 100644
--- a/clang/test/Driver/experimental-library-flag.cpp
+++ b/clang/test/Driver/experimental-library-flag.cpp
@@ -18,6 +18,8 @@
// CHECK: -fexperimental-library
// Depending on the stdlib in use, we should (or not) pass -lc++experimental.
-// CHECK-LIBCXX: -lc++experimental
-// CHECK-LIBSTDCXX-NOT: -lc++experimental
-// CHECK-NOSTDLIB-NOT: -lc++experimental
+// Note that we don't check for `-lc++experimental` specifically, since some targets
+// like Darwin pass the path to the library explicitly instead of using `-lx`.
+// CHECK-LIBCXX: c++experimental
+// CHECK-LIBSTDCXX-NOT: c++experimental
+// CHECK-NOSTDLIB-NOT: c++experimental
diff --git a/compiler-rt/cmake/config-ix.cmake b/compiler-rt/cmake/config-ix.cmake
index 084a7060e8d13..30a78b69515cc 100644
--- a/compiler-rt/cmake/config-ix.cmake
+++ b/compiler-rt/cmake/config-ix.cmake
@@ -507,8 +507,23 @@ if(APPLE)
set(CMAKE_OSX_DEPLOYMENT_TARGET "")
set(DARWIN_COMMON_CFLAGS -stdlib=libc++)
+
+ # This is tricky: We want to link against libc++, however the libc++ for the
+ # architecture we're currently building may not have been built yet, since
+ # compiler-rt on Darwin builds for all targets at once while libc++ builds for
+ # a single target. Hence, we pass -nostdlib++ to disable the default mechanism
+ # for finding libc++, and we pass -lc++ which will end up finding libc++ in the
+ # SDK currently in use. That libc++ is the wrong libc++ to use if we're using
+ # headers from a just-built libc++, but at least it contains all the architectures
+ # we should be interested in.
+ #
+ # Fixing this properly would require removing the impedence mismatch between
+ # the compiler-rt build on Darwin (which wants to build all architectures at
+ # once) and the libc++ build, which produces a single architecture per CMake
+ # invocation.
set(DARWIN_COMMON_LINK_FLAGS
-stdlib=libc++
+ -nostdlib++
-lc++
-lc++abi)
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