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From 4f05f4c8e66bc76b1d94f5283494404382e3bacd Mon Sep 17 00:00:00 2001
From: Fangrui Song <i@maskray.me>
Date: Thu, 13 May 2021 13:44:57 -0700
Subject: [PATCH] [CMake][ELF] Link libLLVM.so and libclang-cpp.so with
-Bsymbolic-functions
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
---
llvm/tools/llvm-shlib/CMakeLists.txt | 5 +++++
1 file changed, 5 insertions(+)
diff --git a/llvm/tools/llvm-shlib/CMakeLists.txt b/llvm/tools/llvm-shlib/CMakeLists.txt
index b0ee19049e6f..03e1383ec8b4 100644
--- a/llvm/tools/llvm-shlib/CMakeLists.txt
+++ b/llvm/tools/llvm-shlib/CMakeLists.txt
@@ -50,6 +50,11 @@ if(LLVM_BUILD_LLVM_DYLIB)
# Solaris ld does not accept global: *; so there is no way to version *all* global symbols
set(LIB_NAMES -Wl,--version-script,${LLVM_LIBRARY_DIR}/tools/llvm-shlib/simple_version_script.map ${LIB_NAMES})
endif()
+ # Optimize function calls for default visibility definitions to avoid PLT and
+ # reduce dynamic relocations.
+ # Note: for -fno-pic default, the address of a function may be different from
+ # inside and outside libLLVM.so.
+ target_link_options(LLVM PRIVATE LINKER:-Bsymbolic-functions)
elseif("${CMAKE_SYSTEM_NAME}" STREQUAL "Darwin")
set(LIB_NAMES -Wl,-all_load ${LIB_NAMES})
endif()
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