Author: Fangrui Song Date: 2021-05-12T10:34:31-07:00 New Revision: 3bf1acab5b454ad7fb2074b34663108b53620695
URL: https://github.com/llvm/llvm-project/commit/3bf1acab5b454ad7fb2074b34663108b53620695 DIFF: https://github.com/llvm/llvm-project/commit/3bf1acab5b454ad7fb2074b34663108b53620695.diff LOG: [CMake][ELF] Add -fno-semantic-interposition and -Bsymbolic-functions llvm-dev message: https://lists.llvm.org/pipermail/llvm-dev/2021-May/150465.html In an ELF shared object, a default visibility defined symbol is preemptible by default. This creates some missed optimization opportunities. -fno-semantic-interposition can optimize -fPIC: * in Clang: avoid GOT/PLT cost for variable access/function calls to external linkage definition in the same TU * in GCC: enable interprocedural optimizations (including inlining) and avoid PLT See https://gist.github.com/MaskRay/2d4dfcfc897341163f734afb59f689c6 for more information. -Bsymbolic-functions is more aggressive than -fvisibility-inlines-hidden (present since 2012) as it applies to all function definitions. It can * avoid PLT for cross-TU function calls && reduce dynamic symbol lookup * reduce dynamic symbol lookup for taking function addresses and optimize out GOT/TOC on x86-64/ppc64 With both options, the libLLVM.so and libclang-cpp.so performance should be closer to PIE binary linking against `libLLVM*.a` and `libclang*.a` (In a -DLLVM_TARGETS_TO_BUILD=X86 build, the number of JUMP_SLOT decreases from 12716 to 1628, and the number of GLOB_DAT decreases from 1918 to 1313 The built clang with `-DLLVM_LINK_LLVM_DYLIB=on -DCLANG_LINK_CLANG_DYLIB=on` is significantly faster. See the Linux kernel build result https://bugs.archlinux.org/task/70697 ) Some implication: Interposing a subset of functions is no longer supported. (This is fragile anyway and cannot really be supported. For Mach-O we don't use `ld -interpose`, so interposition is not supported on Mach-O at all.) Compiling a program which takes the address of any LLVM function with `{gcc,clang} -fno-pic` and expects the address to equal to the address taken from libLLVM.so or libclang-cpp.so is unsupported. I am fairly confident that llvm-project shouldn't have different behaviors depending on such pointer equality (as we've been using -fvisibility-inlines-hidden which applies to inline functions for a long time), but if we accidentally do, users should be aware that they should not make assumption on pointer equality in `-fno-pic` mode. Reviewed By: phosek Differential Revision: https://reviews.llvm.org/D102090 Added: Modified: clang/tools/clang-shlib/CMakeLists.txt llvm/cmake/modules/HandleLLVMOptions.cmake llvm/tools/llvm-shlib/CMakeLists.txt Removed: ################################################################################ diff --git a/clang/tools/clang-shlib/CMakeLists.txt b/clang/tools/clang-shlib/CMakeLists.txt index 5949223fc8e34..73e806c495556 100644 --- a/clang/tools/clang-shlib/CMakeLists.txt +++ b/clang/tools/clang-shlib/CMakeLists.txt @@ -48,3 +48,8 @@ add_clang_library(clang-cpp ${_OBJECTS} LINK_LIBS ${_DEPS}) +# Optimize function calls and global variable access for default visibility +# definitions to avoid PLT and reduce dynamic relocations. +if (NOT APPLE) + target_link_options(clang-cpp PRIVATE LINKER:-Bsymbolic-functions) +endif() diff --git a/llvm/cmake/modules/HandleLLVMOptions.cmake b/llvm/cmake/modules/HandleLLVMOptions.cmake index a64644a08e599..81f46cf116cf6 100644 --- a/llvm/cmake/modules/HandleLLVMOptions.cmake +++ b/llvm/cmake/modules/HandleLLVMOptions.cmake @@ -305,6 +305,7 @@ if( LLVM_ENABLE_PIC ) # On Windows all code is PIC. MinGW warns if -fPIC is used. else() add_flag_or_print_warning("-fPIC" FPIC) + add_flag_if_supported("-fno-semantic-interposition" FNO_SEMANTIC_INTERPOSITION) endif() # GCC for MIPS can miscompile LLVM due to PR37701. if(CMAKE_COMPILER_IS_GNUCXX AND LLVM_NATIVE_ARCH STREQUAL "Mips" AND diff --git a/llvm/tools/llvm-shlib/CMakeLists.txt b/llvm/tools/llvm-shlib/CMakeLists.txt index b0ee19049e6fa..7259ba2636581 100644 --- a/llvm/tools/llvm-shlib/CMakeLists.txt +++ b/llvm/tools/llvm-shlib/CMakeLists.txt @@ -50,6 +50,9 @@ if(LLVM_BUILD_LLVM_DYLIB) # Solaris ld does not accept global: *; so there is no way to version *all* global symbols set(LIB_NAMES -Wl,--version-script,${LLVM_LIBRARY_DIR}/tools/llvm-shlib/simple_version_script.map ${LIB_NAMES}) endif() + # Optimize function calls and global variable access for default visibility + # definitions to avoid PLT and reduce dynamic relocations. + target_link_options(LLVM PRIVATE LINKER:-Bsymbolic-functions) elseif("${CMAKE_SYSTEM_NAME}" STREQUAL "Darwin") set(LIB_NAMES -Wl,-all_load ${LIB_NAMES}) endif() _______________________________________________ cfe-commits mailing list cfe-commits@lists.llvm.org https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits
