[Fuchsia][cmake] Avoid referencing cxx_shared in compiler-rt

[ilovepi]

After #80007 Fuchsia builds are
now always building cxx_shared for arm64 and x64 Linux. Ultimately, this
is because the LIBCXX_ENABLE_SHARED is not used in compiler-rt to select
the correct libc++ target, and because cxx_shared is now always defined,
it is selected as a dependency when building runtimes tests.

[llvmbot]

@llvm/pr-subscribers-compiler-rt-sanitizer

Author: Paul Kirth (ilovepi)

Changes

After #80007 Fuchsia builds are
now always building cxx_shared for arm64 and x64 Linux. Ultimately, this
is because the LIBCXX_ENABLE_SHARED is not used in compiler-rt to select
the correct libc++ target, and because cxx_shared is now always defined,
it is selected as a dependency when building runtimes tests.

---

Full diff: https://github.com/llvm/llvm-project/pull/112257.diff

2 Files Affected:

• (modified) compiler-rt/CMakeLists.txt (+3-3)
• (modified) compiler-rt/test/fuzzer/CMakeLists.txt (+3-4)

diff --git a/compiler-rt/CMakeLists.txt b/compiler-rt/CMakeLists.txt
index deb6994f481854..98c28556755525 100644
--- a/compiler-rt/CMakeLists.txt
+++ b/compiler-rt/CMakeLists.txt
@@ -616,7 +616,7 @@ if(COMPILER_RT_USE_LLVM_UNWINDER)
   if (COMPILER_RT_ENABLE_STATIC_UNWINDER)
     list(APPEND COMPILER_RT_UNWINDER_LINK_LIBS "$<TARGET_LINKER_FILE:unwind_static>")
   else()
-    list(APPEND COMPILER_RT_UNWINDER_LINK_LIBS "$<TARGET_LINKER_FILE:$<IF:$<TARGET_EXISTS:unwind_shared>,unwind_shared,unwind_static>>")
+    list(APPEND COMPILER_RT_UNWINDER_LINK_LIBS "$<TARGET_LINKER_FILE:$<IF:$<BOOL:${LIBUNWIND_ENABLE_SHARED}>,unwind_shared,unwind_static>>")
   endif()
 endif()
 
@@ -629,7 +629,7 @@ if (COMPILER_RT_CXX_LIBRARY STREQUAL "libcxx")
   if (COMPILER_RT_STATIC_CXX_LIBRARY)
     set(COMPILER_RT_CXX_LINK_LIBS "$<TARGET_LINKER_FILE:cxx_static>")
   else()
-    set(COMPILER_RT_CXX_LINK_LIBS "$<TARGET_LINKER_FILE:$<IF:$<TARGET_EXISTS:cxx_shared>,cxx_shared,cxx_static>>")
+    set(COMPILER_RT_CXX_LINK_LIBS "$<TARGET_LINKER_FILE:$<IF:$<BOOL:${LIBCXX_ENABLE_SHARED}>,cxx_shared,cxx_static>>")
   endif()
 elseif (COMPILER_RT_CXX_LIBRARY STREQUAL "none")
   # We aren't using any C++ standard library so avoid including the default one.
@@ -671,7 +671,7 @@ if (SANITIZER_TEST_CXX_LIBNAME STREQUAL "libc++")
     if (SANITIZER_USE_STATIC_TEST_CXX)
       list(APPEND SANITIZER_TEST_CXX_LIBRARIES "$<TARGET_LINKER_FILE:cxx_static>")
     else()
-      list(APPEND SANITIZER_TEST_CXX_LIBRARIES "$<TARGET_LINKER_FILE:$<IF:$<TARGET_EXISTS:cxx_shared>,cxx_shared,cxx_static>>")
+      list(APPEND SANITIZER_TEST_CXX_LIBRARIES "$<TARGET_LINKER_FILE:$<IF:$<BOOL:${LIBCXX_ENABLE_SHARED}>,cxx_shared,cxx_static>>")
     endif()
     # We are using the in tree libc++ so avoid including the default one.
     append_list_if(COMPILER_RT_HAS_NOSTDINCXX_FLAG -nostdinc++ COMPILER_RT_UNITTEST_CFLAGS)
diff --git a/compiler-rt/test/fuzzer/CMakeLists.txt b/compiler-rt/test/fuzzer/CMakeLists.txt
index b19f52e591fc0f..982225f294d238 100644
--- a/compiler-rt/test/fuzzer/CMakeLists.txt
+++ b/compiler-rt/test/fuzzer/CMakeLists.txt
@@ -73,11 +73,10 @@ endmacro()
 
 test_fuzzer("default")
 if("${CMAKE_SYSTEM_NAME}" STREQUAL "Linux")
-  if(TARGET cxx_shared)
-    test_fuzzer("libc++" DEPS cxx_shared)
-  endif()
-  if(TARGET cxx_static)
+  if(LIBCXX_ENABLE_STATIC)
     test_fuzzer("static-libc++" DEPS cxx_static)
+  elseif(LIBCXX_ENABLE_SHARED)
+    test_fuzzer("libc++" DEPS cxx_shared)
   endif()
 endif()
 

[ldionne]

Does someone understand how the layering works for compiler-rt? Libc++ links against compiler-rt itself, so how can we depend on libc++ from compiler-rt?

[ilovepi]

Does someone understand how the layering works for compiler-rt? Libc++ links against compiler-rt itself, so how can we depend on libc++ from compiler-rt?

Aren't the builtins the only dependency? IIRC builtins are special in the runtimes build, and is handled that way to break the cyclic dependency in the build graph. @petrhosek can speak more to that though.

[petrhosek]

Does someone understand how the layering works for compiler-rt? Libc++ links against compiler-rt itself, so how can we depend on libc++ from compiler-rt?

Aren't the builtins the only dependency? IIRC builtins are special in the runtimes build, and is handled that way to break the cyclic dependency in the build graph. @petrhosek can speak more to that though.

That's correct, we build builtins separately by using https://github.com/llvm/llvm-project/tree/main/compiler-rt/lib/builtins as the top-level CMake build directory, and then build the rest of compiler-rt as part of the runtimes build.

libc++ is currently only used by libFuzzer, ORC and XRay plus all unit tests (since it's a dependency of GoogleTest), the other runtimes in compiler-rt like sanitizers only depend on libc++abi. libFuzzer is a bit special in that it build its own copy of libc++ (in a subbuild) which is statically linked into a relocatable object.

[ldionne]

Thanks for the explanation!

That's correct, we build builtins separately by using https://github.com/llvm/llvm-project/tree/main/compiler-rt/lib/builtins as the top-level CMake build directory

Would it make sense to separate this out of compiler-rt then? This sounds like a slightly brittle setup dependency-wise.

libc++ is currently only used by libFuzzer, ORC and XRay plus all unit tests (since it's a dependency of GoogleTest)

Is there a reason why libFuzzer, ORC and XRay need to explicitly mention the just-built libc++? Couldn't they use whatever standard library is provided by the compiler, which in the case of a bootstrapping build would be the just-built libc++? That would be cleaner than having subparts of compiler-rt depending directly on libc++ targets.

the other runtimes in compiler-rt like sanitizers only depend on libc++abi

Do you know what they depend on libc++abi for?