From 43fc921795bd130a325c013d60f209b5c6128fc7 Mon Sep 17 00:00:00 2001 From: Paul Kirth Date: Fri, 15 Mar 2024 16:35:06 -0700 Subject: [PATCH] [CMAKE] Enable FatLTO as a build option for LLVM (#80480) Since LLVM supports `-ffat-lto-objects` we should enable this as an option in the LLVM build. FatLTO should improve the time it takes to build tests for LTO enabled builds of the compiler by not linking w/ the bitcode portion of the object files, which should speed up build times for LTO builds without disabling optimizations. --- clang/cmake/caches/Fuchsia-stage2.cmake | 1 + llvm/cmake/modules/AddLLVM.cmake | 10 ++++++++-- llvm/cmake/modules/HandleLLVMOptions.cmake | 9 +++++++++ 3 files changed, 18 insertions(+), 2 deletions(-) diff --git a/clang/cmake/caches/Fuchsia-stage2.cmake b/clang/cmake/caches/Fuchsia-stage2.cmake index db7430b3344c3..d5546e20873b3 100644 --- a/clang/cmake/caches/Fuchsia-stage2.cmake +++ b/clang/cmake/caches/Fuchsia-stage2.cmake @@ -11,6 +11,7 @@ set(LLVM_ENABLE_RUNTIMES "compiler-rt;libcxx;libcxxabi;libunwind" CACHE STRING " set(LLVM_ENABLE_BACKTRACES OFF CACHE BOOL "") set(LLVM_ENABLE_DIA_SDK OFF CACHE BOOL "") +set(LLVM_ENABLE_FATLTO ON CACHE BOOL "") set(LLVM_ENABLE_HTTPLIB ON CACHE BOOL "") set(LLVM_ENABLE_LIBCXX ON CACHE BOOL "") set(LLVM_ENABLE_LIBEDIT OFF CACHE BOOL "") diff --git a/llvm/cmake/modules/AddLLVM.cmake b/llvm/cmake/modules/AddLLVM.cmake index 5610880da7107..d84d9d7cca68c 100644 --- a/llvm/cmake/modules/AddLLVM.cmake +++ b/llvm/cmake/modules/AddLLVM.cmake @@ -1637,8 +1637,14 @@ function(add_unittest test_suite test_name) # The runtime benefits of LTO don't outweight the compile time costs for tests. if(LLVM_ENABLE_LTO) if((UNIX OR MINGW) AND LINKER_IS_LLD) - set_property(TARGET ${test_name} APPEND_STRING PROPERTY - LINK_FLAGS " -Wl,--lto-O0") + if(LLVM_ENABLE_FATLTO) + # When using FatLTO, just use relocatable linking. + set_property(TARGET ${test_name} APPEND_STRING PROPERTY + LINK_FLAGS " -Wl,--no-fat-lto-objects") + else() + set_property(TARGET ${test_name} APPEND_STRING PROPERTY + LINK_FLAGS " -Wl,--lto-O0") + endif() elseif(LINKER_IS_LLD_LINK) set_property(TARGET ${test_name} APPEND_STRING PROPERTY LINK_FLAGS " /opt:lldlto=0") diff --git a/llvm/cmake/modules/HandleLLVMOptions.cmake b/llvm/cmake/modules/HandleLLVMOptions.cmake index 745a8354f1189..92fa9839db652 100644 --- a/llvm/cmake/modules/HandleLLVMOptions.cmake +++ b/llvm/cmake/modules/HandleLLVMOptions.cmake @@ -32,6 +32,8 @@ endif() set(LLVM_ENABLE_LTO OFF CACHE STRING "Build LLVM with LTO. May be specified as Thin or Full to use a particular kind of LTO") string(TOUPPER "${LLVM_ENABLE_LTO}" uppercase_LLVM_ENABLE_LTO) +option(LLVM_ENABLE_FATLTO "Build LLVM with -ffat-lto-objects." OFF) + # Ninja Job Pool support # The following only works with the Ninja generator in CMake >= 3.0. set(LLVM_PARALLEL_COMPILE_JOBS "" CACHE STRING @@ -1280,6 +1282,13 @@ elseif(LLVM_ENABLE_LTO) endif() endif() +if(LLVM_ENABLE_FATLTO AND (FUCHSIA OR UNIX)) + append("-ffat-lto-objects" CMAKE_C_FLAGS CMAKE_CXX_FLAGS) + if(NOT LINKER_IS_LLD_LINK) + append("-ffat-lto-objects" CMAKE_EXE_LINKER_FLAGS CMAKE_SHARED_LINKER_FLAGS CMAKE_MODULE_LINKER_FLAGS) + endif() +endif() + # Set an AIX default for LLVM_EXPORT_SYMBOLS_FOR_PLUGINS based on whether we are # doing dynamic linking (see below). set(LLVM_EXPORT_SYMBOLS_FOR_PLUGINS_AIX_default OFF)