llvm12/trunk/llvm-link-with-Bsymbolic-functions.patch

   1 From 4f05f4c8e66bc76b1d94f5283494404382e3bacd Mon Sep 17 00:00:00 2001
   2 From: Fangrui Song <i@maskray.me>
   3 Date: Thu, 13 May 2021 13:44:57 -0700
   4 Subject: [PATCH] [CMake][ELF] Link libLLVM.so and libclang-cpp.so with
   5  -Bsymbolic-functions
   6
   7 llvm-dev message: https://lists.llvm.org/pipermail/llvm-dev/2021-May/150465.html
   8
   9 In an ELF shared object, a default visibility defined symbol is preemptible by
  10 default. This creates some missed optimization opportunities.
  11 -Bsymbolic-functions is more aggressive than our current -fvisibility-inlines-hidden
  12 (present since 2012) as it applies to all function definitions.  It can
  13
  14 * avoid PLT for cross-TU function calls && reduce dynamic symbol lookup
  15 * reduce dynamic symbol lookup for taking function addresses and optimize out GOT/TOC on x86-64/ppc64
  16
  17 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
  18 The built clang with `-DLLVM_LINK_LLVM_DYLIB=on -DCLANG_LINK_CLANG_DYLIB=on` is significantly faster.
  19 See the Linux kernel build result https://bugs.archlinux.org/task/70697
  20
  21 Note: the performance of -fno-semantic-interposition -Bsymbolic-functions
  22 libLLVM.so and libclang-cpp.so is close to a PIE binary linking against
  23 `libLLVM*.a` and `libclang*.a`. When the host compiler is Clang,
  24 -Bsymbolic-functions is the major contributor.  On x86-64 (with GOTPCRELX) and
  25 ppc64 ELFv2, the GOT/TOC relocations can be optimized.
  26
  27 Some implication:
  28
  29 Interposing a subset of functions is no longer supported.
  30 (This is fragile on ELF and unsupported on Mach-O at all. For Mach-O we don't
  31 use `ld -interpose` or `-flat_namespace`)
  32
  33 Compiling a program which takes the address of any LLVM function with
  34 `{gcc,clang} -fno-pic` and expects the address to equal to the address taken
  35 from libLLVM.so or libclang-cpp.so is unsupported. I am fairly confident that
  36 llvm-project shouldn't have different behaviors depending on such pointer
  37 equality (as we've been using -fvisibility-inlines-hidden which applies to
  38 inline functions for a long time), but if we accidentally do, users should be
  39 aware that they should not make assumption on pointer equality in `-fno-pic`
  40 mode.
  41
  42 See more on https://maskray.me/blog/2021-05-09-fno-semantic-interposition
  43
  44 Reviewed By: phosek
  45
  46 Differential Revision: https://reviews.llvm.org/D102090
  47 ---
  48  llvm/tools/llvm-shlib/CMakeLists.txt | 5 +++++
  49  1 file changed, 5 insertions(+)
  50
  51 diff --git a/llvm/tools/llvm-shlib/CMakeLists.txt b/llvm/tools/llvm-shlib/CMakeLists.txt
  52 index b0ee19049e6f..03e1383ec8b4 100644
  53 --- a/llvm/tools/llvm-shlib/CMakeLists.txt
  54 +++ b/llvm/tools/llvm-shlib/CMakeLists.txt
  55 @@ -50,6 +50,11 @@ if(LLVM_BUILD_LLVM_DYLIB)
  56        # Solaris ld does not accept global: *; so there is no way to version *all* global symbols
  57        set(LIB_NAMES -Wl,--version-script,${LLVM_LIBRARY_DIR}/tools/llvm-shlib/simple_version_script.map ${LIB_NAMES})
  58      endif()
  59 +    # Optimize function calls for default visibility definitions to avoid PLT and
  60 +    # reduce dynamic relocations.
  61 +    # Note: for -fno-pic default, the address of a function may be different from
  62 +    # inside and outside libLLVM.so.
  63 +    target_link_options(LLVM PRIVATE LINKER:-Bsymbolic-functions)
  64    elseif("${CMAKE_SYSTEM_NAME}" STREQUAL "Darwin")
  65      set(LIB_NAMES -Wl,-all_load ${LIB_NAMES})
  66    endif()