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686fd925f4
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 |
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| README | ||
| unwind.h | ||
See docs/CMake.html for instructions on how to build LLVM with CMake.