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00a3d20bf3
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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bugpoint | ||
bugpoint-passes | ||
dsymutil | ||
gold | ||
llc | ||
lli | ||
llvm-ar | ||
llvm-as | ||
llvm-as-fuzzer | ||
llvm-bcanalyzer | ||
llvm-c-test | ||
llvm-cat | ||
llvm-cfi-verify | ||
llvm-config | ||
llvm-cov | ||
llvm-cvtres | ||
llvm-cxxdump | ||
llvm-cxxfilt | ||
llvm-cxxmap | ||
llvm-diff | ||
llvm-dis | ||
llvm-dwarfdump | ||
llvm-dwp | ||
llvm-elfabi | ||
llvm-exegesis | ||
llvm-extract | ||
llvm-go | ||
llvm-gsymutil | ||
llvm-ifs | ||
llvm-isel-fuzzer | ||
llvm-itanium-demangle-fuzzer | ||
llvm-jitlink | ||
llvm-jitlistener | ||
llvm-libtool-darwin | ||
llvm-link | ||
llvm-lipo | ||
llvm-lto | ||
llvm-lto2 | ||
llvm-mc | ||
llvm-mc-assemble-fuzzer | ||
llvm-mc-disassemble-fuzzer | ||
llvm-mca | ||
llvm-microsoft-demangle-fuzzer | ||
llvm-ml | ||
llvm-modextract | ||
llvm-mt | ||
llvm-nm | ||
llvm-objcopy | ||
llvm-objdump | ||
llvm-opt-fuzzer | ||
llvm-opt-report | ||
llvm-pdbutil | ||
llvm-profdata | ||
llvm-profgen | ||
llvm-rc | ||
llvm-readobj | ||
llvm-reduce | ||
llvm-rtdyld | ||
llvm-rust-demangle-fuzzer | ||
llvm-shlib | ||
llvm-size | ||
llvm-special-case-list-fuzzer | ||
llvm-split | ||
llvm-stress | ||
llvm-strings | ||
llvm-symbolizer | ||
llvm-undname | ||
llvm-xray | ||
llvm-yaml-numeric-parser-fuzzer | ||
llvm-yaml-parser-fuzzer | ||
lto | ||
msbuild | ||
obj2yaml | ||
opt | ||
opt-viewer | ||
remarks-shlib | ||
sancov | ||
sanstats | ||
split-file | ||
verify-uselistorder | ||
vfabi-demangle-fuzzer | ||
xcode-toolchain | ||
yaml2obj | ||
CMakeLists.txt |