forked from OSchip/llvm-project
4f05f4c8e6
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 |
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| clang-shlib.cpp | ||