diff options
Diffstat (limited to 'extra/llvm12/llvm-link-with-Bsymbolic-functions.patch')
| -rw-r--r-- | extra/llvm12/llvm-link-with-Bsymbolic-functions.patch | 66 |
1 files changed, 66 insertions, 0 deletions
diff --git a/extra/llvm12/llvm-link-with-Bsymbolic-functions.patch b/extra/llvm12/llvm-link-with-Bsymbolic-functions.patch new file mode 100644 index 00000000..b8e2e40e --- /dev/null +++ b/extra/llvm12/llvm-link-with-Bsymbolic-functions.patch @@ -0,0 +1,66 @@ +From 4f05f4c8e66bc76b1d94f5283494404382e3bacd Mon Sep 17 00:00:00 2001 +From: Fangrui Song <i@maskray.me> +Date: Thu, 13 May 2021 13:44:57 -0700 +Subject: [PATCH] [CMake][ELF] Link libLLVM.so and libclang-cpp.so with + -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. +-Bsymbolic-functions is more aggressive than our current -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 + +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 + +Note: the performance of -fno-semantic-interposition -Bsymbolic-functions +libLLVM.so and libclang-cpp.so is close to a PIE binary linking against +`libLLVM*.a` and `libclang*.a`. When the host compiler is Clang, +-Bsymbolic-functions is the major contributor. On x86-64 (with GOTPCRELX) and +ppc64 ELFv2, the GOT/TOC relocations can be optimized. + +Some implication: + +Interposing a subset of functions is no longer supported. +(This is fragile on ELF and unsupported on Mach-O at all. For Mach-O we don't +use `ld -interpose` or `-flat_namespace`) + +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. + +See more on https://maskray.me/blog/2021-05-09-fno-semantic-interposition + +Reviewed By: phosek + +Differential Revision: https://reviews.llvm.org/D102090 +--- + llvm/tools/llvm-shlib/CMakeLists.txt | 5 +++++ + 1 file changed, 5 insertions(+) + +diff --git a/llvm/tools/llvm-shlib/CMakeLists.txt b/llvm/tools/llvm-shlib/CMakeLists.txt +index b0ee19049e6f..03e1383ec8b4 100644 +--- a/llvm/tools/llvm-shlib/CMakeLists.txt ++++ b/llvm/tools/llvm-shlib/CMakeLists.txt +@@ -50,6 +50,11 @@ 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 ${LIB_NAMES}) + endif() ++ # Optimize function calls for default visibility definitions to avoid PLT and ++ # reduce dynamic relocations. ++ # Note: for -fno-pic default, the address of a function may be different from ++ # inside and outside libLLVM.so. ++ target_link_options(LLVM PRIVATE LINKER:-Bsymbolic-functions) + elseif("${CMAKE_SYSTEM_NAME}" STREQUAL "Darwin") + set(LIB_NAMES -Wl,-all_load ${LIB_NAMES}) + endif() |