[clang] 4f05f4c - [CMake][ELF] Link libLLVM.so and libclang-cpp.so with -Bsymbolic-functions

[Fangrui Song via cfe-commits]

Author: Fangrui Song
Date: 2021-05-13T13:44:57-07:00
New Revision: 4f05f4c8e66bc76b1d94f5283494404382e3bacd

URL: https://github.com/llvm/llvm-project/commit/4f05f4c8e66bc76b1d94f5283494404382e3bacd
DIFF: https://github.com/llvm/llvm-project/commit/4f05f4c8e66bc76b1d94f5283494404382e3bacd.diff

LOG: [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

Added: 
    

Modified: 
    clang/tools/clang-shlib/CMakeLists.txt
    llvm/tools/llvm-shlib/CMakeLists.txt

Removed: 
    


################################################################################
diff  --git a/clang/tools/clang-shlib/CMakeLists.txt b/clang/tools/clang-shlib/CMakeLists.txt
index 5949223fc8e34..d08cf89383282 100644
--- a/clang/tools/clang-shlib/CMakeLists.txt
+++ b/clang/tools/clang-shlib/CMakeLists.txt
@@ -48,3 +48,8 @@ add_clang_library(clang-cpp
                   ${_OBJECTS}
                   LINK_LIBS
                   ${_DEPS})
+# Optimize function calls for default visibility definitions to avoid PLT and
+# reduce dynamic relocations.
+if (NOT APPLE)
+  target_link_options(clang-cpp PRIVATE LINKER:-Bsymbolic-functions)
+endif()

diff  --git a/llvm/tools/llvm-shlib/CMakeLists.txt b/llvm/tools/llvm-shlib/CMakeLists.txt
index b0ee19049e6fa..03e1383ec8b4f 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 ${LIB_NAMES})
     endif()
+    # Optimize function calls for default visibility definitions to avoid PLT and
+    # reduce dynamic relocations.
+    # Note: for -fno-pic default, the address of a function may be 
diff erent from
+    # inside and outside libLLVM.so.
+    target_link_options(LLVM PRIVATE LINKER:-Bsymbolic-functions)
   elseif("${CMAKE_SYSTEM_NAME}" STREQUAL "Darwin")
     set(LIB_NAMES -Wl,-all_load ${LIB_NAMES})
   endif()