[NFC][libc] Disable benchmarks when the LLVM benchmark target is not available

[gchatelet]

Summary

Fixes https://github.com/llvm/llvm-project/issues/53686

[lgxbslgx]

I tried this patch locally and got the error message like https://discourse.llvm.org/t/error-when-building-llvm-libc-with-gcc/4166 instead of The target "benchmark" does not exist..
It means this patch can solve the issue.

But I am still confused: why the conditional statement if(LLVM_INCLUDE_BENCHMARKS) can solve this issue?
Accoding to the following document and configuration, the LLVM_INCLUDE_BENCHMARKS is default to ON.
The LLVM_INCLUDE_BENCHMARKS is ON, so the if(LLVM_INCLUDE_BENCHMARKS) seems not neccesary.
I am a newbie in llvm project and may miss something. Thanks for your help.

doc: llvm/docs/CMake.rst

LLVM_INCLUDE_BENCHMARKS:BOOL
Generate build targets for the LLVM benchmarks. Defaults to ON.

config: llvm/CMakeLists.txt

option(LLVM_INCLUDE_BENCHMARKS "Generate benchmark targets. If OFF, benchmarks can't be built." ON)

[gchatelet]

• Guard libc benchmarks behind a flag
• Use ExternalProject as the benchmark target is not always available

[gchatelet]

Thx for challenging my patch @lgxbslgx.

I intended to use LIBC_INCLUDE_BENCHMARKS and not LLVM_INCLUDE_BENCHMARKS.
The fact that it "works" was puzzling to me so I ran a bunch of experiments to understand what's going on.

When libc is part of the LLVM_ENABLE_RUNTIMES list, the CMake configuration is created in such a way that it does not inherit from llvm/CMakeLists.txt. So LLVM_INCLUDE_BENCHMARKS is not defined. This explains why my patch "works".
Similarly, the benchmark target was not available when libc is instantiated as part of LLVM_ENABLE_RUNTIMES, but it is when it is part of LLVM_ENABLE_PROJECTS which explains the bug in https://github.com/llvm/llvm-project/issues/53686.

Because the benchmark target is not necessarily defined when libc is instantiated (it depends whether libc is part of LLVM_ENABLE_RUNTIMES or LLVM_ENABLE_PROJECTS) I have to make sure it is defined in libc/benchmarks/CMakeLists.txt, at this point it is not possible to use the add_subdirectory directive as it may result in duplicate target names. That's why I reverted to the original implementation using ExternalProject.

Additionally, it was not wise to refer to Google Benchmark by the benchmark identifier as CMake is "smart" and if it does not find a target it infers that it is a system wide library and appends -lbenchmark to the linker which also happens to work depending on your local environment. To fix this I now use the benchmark::benchmark alias that cannot be conflated with precompiled libraries.

I believe this patch now solves all the use cases. Let me know if it looks good to you and I'll submit it.

[lgxbslgx]

Thanks for the detailed analysis. I tried the new patch locally and the libc was built successfully.
But I am not an official reviewer(committer). Maybe it needs other developers to review again.

[gchatelet]

rebase

[gchatelet]

rebase with all the diffs

libc/CMakeLists.txt

 # and libraries potentially draw from the components present in all
 # of the other directories.
 add_subdirectory(lib)
 if(LLVM_INCLUDE_TESTS)
   add_subdirectory(test)
   add_subdirectory(fuzzing)
 endif()
 
+if(LIBC_INCLUDE_BENCHMARKS)
   add_subdirectory(benchmarks)
+endif()
 
 if (LIBC_INCLUDE_DOCS)
   add_subdirectory(docs)
 endif()

libc/benchmarks/CMakeLists.txt

function(add_libc_benchmark_unittest target_name)
   )
   add_dependencies(libc-benchmark-util-tests ${target_name})
 endfunction()
 
 #==============================================================================
 # Build Google Benchmark for libc
 #==============================================================================
 
+include(ExternalProject)
+ExternalProject_Add(google-benchmark-libc
+        EXCLUDE_FROM_ALL ON
+        PREFIX google-benchmark-libc
+        SOURCE_DIR ${LLVM_THIRD_PARTY_DIR}/benchmark
+        INSTALL_DIR ${CMAKE_CURRENT_BINARY_DIR}/google-benchmark-libc
+        CMAKE_CACHE_ARGS
+          -DBENCHMARK_ENABLE_EXCEPTIONS:BOOL=OFF
+          -DBENCHMARK_ENABLE_LTO:BOOL=OFF
+          -DBENCHMARK_ENABLE_TESTING:BOOL=OFF
+          -DBENCHMARK_ENABLE_WERROR:BOOL=${LLVM_ENABLE_WERROR}
+          -DBENCHMARK_FORCE_WERROR:BOOL=OFF
+          -DBENCHMARK_USE_LIBCXX:BOOL=OFF
+          -DCMAKE_BUILD_TYPE:STRING=RELEASE
+          -DCMAKE_C_COMPILER:STRING=${CMAKE_C_COMPILER}
+          -DCMAKE_CXX_COMPILER:STRING=${CMAKE_CXX_COMPILER}
+          -DCMAKE_CXX_FLAGS:STRING=${BENCHMARK_LIBC_COMPILE_FLAGS}
+          -DCMAKE_CXX_STANDARD:STRING=14
+          -DCMAKE_INSTALL_PREFIX:PATH=<INSTALL_DIR>
+        )
+
 add_custom_target(libc-benchmark-util-tests)
 
 # libc-benchmark
 add_library(libc-benchmark
     STATIC
     EXCLUDE_FROM_ALL
     LibcBenchmark.cpp
     LibcBenchmark.h
 )
-add_dependencies(libc-benchmark benchmark)
+
 target_link_libraries(libc-benchmark
     PUBLIC
-    benchmark
+    benchmark::benchmark
     LLVMSupport
     Threads::Threads
 )
+add_dependencies(libc-benchmark google-benchmark-libc)
 llvm_update_compile_flags(libc-benchmark)
 
 add_libc_benchmark_unittest(libc-benchmark-test
     SRCS LibcBenchmarkTest.cpp
     DEPENDS libc-benchmark
 )
 
 # libc-memory-benchmark