[Fuchsia][cmake] Allow using FatLTO when building runtimes

[ilovepi]

We'd like to build runtimes using FatLTO (see
https://llvm.org/docs/FatLTO.html for details). This gives us more
control over how libc++ can be consumed by users of our toolchain, like
the Fuchsia SDK.

[ilovepi]

@petrhosek originally I had used a helper function to do add these flags in the libcxx top level cmake, but that didn't work (similar to this

llvm-project/libcxx/CMakeLists.txt

Line 567 in d0d54fa

# RTTI flags ==================================================================

). I'm pretty confused why that approach failed, even when use add_flags directly, or appending to the list, vs. doing something here.

[petrhosek]

Why cannot we just set LLVM_ENABLE_FATLTO in our cache files?

[ilovepi]

Why cannot we just set LLVM_ENABLE_FATLTO in our cache files?

I don't think we want to couple building libc++ with FATLTO to whether you want to build the compiler that way, do we? Isn't how libc++ built orthogonal to the other build options? Once we have a way to spell that in the build, then yes, we should update the cache file, and we'll probably key that off of LLVM_ENABLE_FATLTO.

But even if we do change the spelling to reuse LLVM_ENABLE_FATLTO, I don't understand is why I couldn't meaningfully change the build flags for libc++ by following the example linked above and adding a call to the new function in cxx_add_common_build_flags(). The fact that I could only get it to work this way seems wrong, and kind of hacky.

[ilovepi]

I think I understand what you’re asking, now. I had the values default to ON for testing, but I’ll update them to default to OFF, and update the cache file so that they can be on in our build.

[petrhosek]

We use HandleLLVMOptions in the runtimes build and support existing LLVM options. That's why there's no LIBCXX_USE_SANITIZER (and LIBCXXABI_USE_SANITIZER or LIBUNWIND_USE_SANITIZER), instead we simply reuse LLVM_USE_SANITIZER. That's why LLVM_ENABLE_FATLTO should already work and we shouldn't be introducing new options like LIBCXX_ENABLE_FATLTO.

It should be sufficient to simply set LLVM_ENABLE_FATLTO in the cache file like https://github.com/llvm/llvm-project/blob/e55869ae8a4ef1ae2e898ff5cd66fb8ae6e099b8/clang/cmake/caches/Fuchsia-stage2.cmake and most importantly test that everything is working as expected (and address any issues as needed).

[ilovepi]

Oh, I finally see what you mean. In the per target runtimes, none of the top level flags get propagated unless you specify them w/ the correct target prefix. I was so confused about why things worked this way.

[llvmbot]

@llvm/pr-subscribers-clang

Author: Paul Kirth (ilovepi)

Changes

We'd like to build libc++ using FatLTO (see https://llvm.org/docs/FatLTO.html
for details). This gives us more control over how libc++ can be consumed
by users of our toolchain, like the Fuchsia SDK.

---

Full diff: https://github.com/llvm/llvm-project/pull/112277.diff

1 Files Affected:

• (modified) clang/cmake/caches/Fuchsia-stage2.cmake (+8)

diff --git a/clang/cmake/caches/Fuchsia-stage2.cmake b/clang/cmake/caches/Fuchsia-stage2.cmake
index 5af98c7b3b3fba..e62f29ecbe6f45 100644
--- a/clang/cmake/caches/Fuchsia-stage2.cmake
+++ b/clang/cmake/caches/Fuchsia-stage2.cmake
@@ -192,6 +192,10 @@ foreach(target aarch64-unknown-linux-gnu;armv7-unknown-linux-gnueabihf;i386-unkn
     set(RUNTIMES_${target}_LLVM_TOOLS_DIR "${CMAKE_BINARY_DIR}/bin" CACHE BOOL "")
     set(RUNTIMES_${target}_LLVM_ENABLE_RUNTIMES "compiler-rt;libcxx;libcxxabi;libunwind" CACHE STRING "")
 
+    # Enable FatLTO for Linux and baremetal runtimes
+    set(RUNTIMES_${target}_LLVM_ENABLE_LTO ON CACHE BOOL "")
+    set(RUNTIMES_${target}_LLVM_ENABLE_FATLTO ON CACHE BOOL "")
+
     # Use .build-id link.
     list(APPEND RUNTIME_BUILD_ID_LINK "${target}")
   endif()
@@ -274,6 +278,10 @@ if(FUCHSIA_SDK)
     set(RUNTIMES_${target}+asan+noexcept_LIBCXXABI_ENABLE_EXCEPTIONS OFF CACHE BOOL "")
     set(RUNTIMES_${target}+asan+noexcept_LIBCXX_ENABLE_EXCEPTIONS OFF CACHE BOOL "")
 
+    # Enable FatLTO for Fuchsia runtimes
+    set(RUNTIMES_${target}_LLVM_ENABLE_LTO ON CACHE BOOL "")
+    set(RUNTIMES_${target}_LLVM_ENABLE_FATLTO ON CACHE BOOL "")
+
     # Use .build-id link.
     list(APPEND RUNTIME_BUILD_ID_LINK "${target}")
   endforeach()

[ilovepi]

ping.

[ilovepi]

I'm now getting an error w/ ToT. Not sure why I'm running into it now, but it happens with any clean build, so its deterministic.

The Relevant bits:

ld.lld: error: input file '/usr/local/google/home/paulkirth/fuchsia-idk/arch/arm64/sysroot/lib/libzircon.so' added after LTO
ld.lld: error: input file '/usr/local/google/home/paulkirth/fuchsia-idk/arch/arm64/sysroot/lib/libzircon.so' added after LTO

Full Error Log:

[1761/1802] Linking CXX shared library /usr/local/google/home/paulkirth/llvm-fork/build/lib/aarch64-unknown-fuchsia/libc++abi.so.1.0
FAILED: /usr/local/google/home/paulkirth/llvm-fork/build/lib/aarch64-unknown-fuchsia/libc++abi.so.1.0
: && /usr/local/google/home/paulkirth/llvm-fork/build/./bin/clang++ --target=aarch64-unknown-fuchsia --sysroot=/usr/local/google/home/paulkirth/fuchsia-idk/arch/arm64/sysroot -fPIC --target=aarch64-unknown-fuchsia -I/usr/local/google/home/paulkirth/fuchsia-idk/pkg/sync/include -I/usr/local/google/home/paulkirth/fuchsia-idk/pkg/fdio/include -fPIC -fno-semantic-interposition -fvisibility-inlines-hidden -Werror=date-time -Werror=unguarded-availability-new -Wall -Wextra -Wno-unused-parameter -Wwrite-strings -Wcast-qual -Wmissing-field-initializers -Wimplicit-fallthrough -Wcovered-switch-default -Wno-noexcept-type -Wnon-virtual-dtor -Wdelete-non-virtual-dtor -Wsuggest-override -Wstring-conversion -Wmisleading-indentation -Wctad-maybe-unsupported -ffunction-sections -fdata-sections -flto -ffat-lto-objects  -O2 -g -DNDEBUG  -L/usr/local/google/home/paulkirth/fuchsia-idk/arch/arm64/lib -Wl,-z,defs -fuse-ld=lld -flto -ffat-lto-objects  -nostdlib++ --unwindlib=none -shared -Wl,-soname,libc++abi.so.1 -o /usr/local/google/home/paulkirth/llvm-fork/build/lib/aarch64-unknown-fuchsia/libc++abi.so.1.0 libcxxabi/src/CMakeFiles/cxxabi_shared_objects.dir/cxa_aux_runtime.cpp.obj libcxxabi/src/CMakeFiles/cxxabi_shared_objects.dir/cxa_default_handlers.cpp.obj libcxxabi/src/CMakeFiles/cxxabi_shared_objects.dir/cxa_demangle.cpp.obj libcxxabi/src/CMakeFiles/cxxabi_shared_objects.dir/cxa_exception_storage.cpp.obj libcxxabi/src/CMakeFiles/cxxabi_shared_objects.dir/cxa_guard.cpp.obj libcxxabi/src/CMakeFiles/cxxabi_shared_objects.dir/cxa_handlers.cpp.obj libcxxabi/src/CMakeFiles/cxxabi_shared_objects.dir/cxa_vector.cpp.obj libcxxabi/src/CMakeFiles/cxxabi_shared_objects.dir/cxa_virtual.cpp.obj libcxxabi/src/CMakeFiles/cxxabi_shared_objects.dir/stdlib_exception.cpp.obj libcxxabi/src/CMakeFiles/cxxabi_shared_objects.dir/stdlib_stdexcept.cpp.obj libcxxabi/src/CMakeFiles/cxxabi_shared_objects.dir/stdlib_typeinfo.cpp.obj libcxxabi/src/CMakeFiles/cxxabi_shared_objects.dir/abort_message.cpp.obj libcxxabi/src/CMakeFiles/cxxabi_shared_objects.dir/fallback_malloc.cpp.obj libcxxabi/src/CMakeFiles/cxxabi_shared_objects.dir/private_typeinfo.cpp.obj libcxxabi/src/CMakeFiles/cxxabi_shared_objects.dir/stdlib_new_delete.cpp.obj libcxxabi/src/CMakeFiles/cxxabi_shared_objects.dir/cxa_exception.cpp.obj libcxxabi/src/CMakeFiles/cxxabi_shared_objects.dir/cxa_personality.cpp.obj libcxxabi/src/CMakeFiles/cxxabi_shared_objects.dir/cxa_thread_atexit.cpp.obj  -lc  /usr/local/google/home/paulkirth/llvm-fork/build/lib/aarch64-unknown-fuchsia/libunwind.so.1.0 && :
ld.lld: error: input file '/usr/local/google/home/paulkirth/fuchsia-idk/arch/arm64/sysroot/lib/libzircon.so' added after LTO
ld.lld: error: input file '/usr/local/google/home/paulkirth/fuchsia-idk/arch/arm64/sysroot/lib/libzircon.so' added after LTO
ld.lld: error: undefined symbol: _zx_system_get_features
>>> referenced by fuchsia.inc:9 (/usr/local/google/home/paulkirth/llvm-fork/compiler-rt/lib/builtins/cpu_model/aarch64/lse_atomics/fuchsia.inc:9)
>>>               aarch64.c.obj:(init_have_lse_atomics) in archive /usr/local/google/home/paulkirth/llvm-fork/build/lib/clang/20/lib/aarch64-unknown-fuchsia/libclang_rt.builtins.a
>>> referenced by fuchsia.inc:12 (/usr/local/google/home/paulkirth/llvm-fork/compiler-rt/lib/builtins/cpu_model/aarch64/fmv/fuchsia.inc:12)
>>>               aarch64.c.obj:(__init_cpu_features_resolver) in archive /usr/local/google/home/paulkirth/llvm-fork/build/lib/clang/20/lib/aarch64-unknown-fuchsia/libclang_rt.builtins.a
clang++: error: ld.lld command failed with exit code 1 (use -v to see invocation)
[1763/1802] Building CXX object libcxx/src/CMakeFiles/cxx_static.dir/locale.cpp.obj
ninja: build stopped: subcommand failed.