Idris Kalp
67 lines
3.3 KiB
Diff
67 lines
3.3 KiB
Diff
From 4f05f4c8e66bc76b1d94f5283494404382e3bacd Mon Sep 17 00:00:00 2001
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From: Fangrui Song <i@maskray.me>
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Date: Thu, 13 May 2021 13:44:57 -0700
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Subject: [PATCH] [CMake][ELF] Link libLLVM.so and libclang-cpp.so with
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-Bsymbolic-functions
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llvm-dev message: https://lists.llvm.org/pipermail/llvm-dev/2021-May/150465.html
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In an ELF shared object, a default visibility defined symbol is preemptible by
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default. This creates some missed optimization opportunities.
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-Bsymbolic-functions is more aggressive than our current -fvisibility-inlines-hidden
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(present since 2012) as it applies to all function definitions. It can
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* avoid PLT for cross-TU function calls && reduce dynamic symbol lookup
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* reduce dynamic symbol lookup for taking function addresses and optimize out GOT/TOC on x86-64/ppc64
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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
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The built clang with `-DLLVM_LINK_LLVM_DYLIB=on -DCLANG_LINK_CLANG_DYLIB=on` is significantly faster.
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See the Linux kernel build result https://bugs.archlinux.org/task/70697
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Note: the performance of -fno-semantic-interposition -Bsymbolic-functions
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libLLVM.so and libclang-cpp.so is close to a PIE binary linking against
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`libLLVM*.a` and `libclang*.a`. When the host compiler is Clang,
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-Bsymbolic-functions is the major contributor. On x86-64 (with GOTPCRELX) and
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ppc64 ELFv2, the GOT/TOC relocations can be optimized.
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Some implication:
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Interposing a subset of functions is no longer supported.
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(This is fragile on ELF and unsupported on Mach-O at all. For Mach-O we don't
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use `ld -interpose` or `-flat_namespace`)
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Compiling a program which takes the address of any LLVM function with
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`{gcc,clang} -fno-pic` and expects the address to equal to the address taken
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from libLLVM.so or libclang-cpp.so is unsupported. I am fairly confident that
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llvm-project shouldn't have different behaviors depending on such pointer
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equality (as we've been using -fvisibility-inlines-hidden which applies to
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inline functions for a long time), but if we accidentally do, users should be
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aware that they should not make assumption on pointer equality in `-fno-pic`
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mode.
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See more on https://maskray.me/blog/2021-05-09-fno-semantic-interposition
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Reviewed By: phosek
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Differential Revision: https://reviews.llvm.org/D102090
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---
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llvm/tools/llvm-shlib/CMakeLists.txt | 5 +++++
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1 file changed, 5 insertions(+)
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diff --git a/llvm/tools/llvm-shlib/CMakeLists.txt b/llvm/tools/llvm-shlib/CMakeLists.txt
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index b0ee19049e6f..03e1383ec8b4 100644
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--- a/llvm/tools/llvm-shlib/CMakeLists.txt
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+++ b/llvm/tools/llvm-shlib/CMakeLists.txt
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@@ -50,6 +50,11 @@ if(LLVM_BUILD_LLVM_DYLIB)
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# Solaris ld does not accept global: *; so there is no way to version *all* global symbols
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set(LIB_NAMES -Wl,--version-script,${LLVM_LIBRARY_DIR}/tools/llvm-shlib/simple_version_script.map ${LIB_NAMES})
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endif()
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+ # Optimize function calls for default visibility definitions to avoid PLT and
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+ # reduce dynamic relocations.
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+ # Note: for -fno-pic default, the address of a function may be different from
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+ # inside and outside libLLVM.so.
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+ target_link_options(LLVM PRIVATE LINKER:-Bsymbolic-functions)
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elseif("${CMAKE_SYSTEM_NAME}" STREQUAL "Darwin")
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set(LIB_NAMES -Wl,-all_load ${LIB_NAMES})
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endif()
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